Gas-saturated frozen rocks as an object of geocryology study

Khimenkov Aleksandr Nikolaevich PhD in Geology and Mineralogy Leading Scientific Associate, the Institute of Geoecology of the Russian Academy of Sciences 101000, Russia, Moskva oblast', g. Moscow, ul. Ulanskii Proezd, 13, stroenie 2 a_khimenkov@mail.ru Other publications by this author     Koshurnikov Andrei Viktorovich PhD in Geology and Mineralogy Leading Scientific Associate, Faculty of Geology, Department of Geocryology, M. V. Lomonosov Moscow State University 119991,, Russia, g. Moscow, ul. Leninskie Gory, 1 msu-geophysics@mail.ru Other publications by this author

Dernova Elena Olegovna Researcher, Institute of Geoecology RAS 101000, Russia, Moscow, Ulansky Pereulok str.,, 13 building2 dernova.eo@gmail.com

IntroductionCurrently, such indicators as: iciness, cryogenic structure, salinity, material composition, temperature, etc. are used to isolate various types of frozen rocks.

At the same time, such an indicator as gas saturation is completely absent, which leaves a significant group of frozen rocks with their specific properties and processes outside the scope of special study. The structure and properties of frozen rocks are caused by the interaction of solid, liquid and gaseous components. The main solid component is mineral particles and ice, liquid - various categories of water (free, loose and strongly bound), gaseous – the whole set of gases and water vapor contained in frozen rocks. The first two components have been studied quite well, quantitative characteristics have been identified, showing their influence on the formation, structure and properties of frozen rocks. On their basis, various classifications have been developed, studies of mechanical properties have been carried out, fixed in regulatory documents, forecasts of the behavior of rocks under various scenarios of climate change are being developed. The gas component, its influence on the structure and properties of frozen rocks is considered much weaker. When assessing the properties of frozen soils, the gas component is practically not taken into account, its role is not considered when carrying out design and geocryological engineering works or developing forecasts for the development of adverse processes when external conditions change. This topic of scientific research seemed insignificant and of only academic interest. The attitude to the role of the gas component in frozen soils can be illustrated by the words of N. A. Tsytovich: "As for gases, their role in frozen soils is reduced only to the formation of soil porosity and, in the presence of closed gas vacuoles, to an increase in elasticity." ^[1]. This position is quite understandable, since it corresponds to the weak knowledge of this problem in the middle of the 20th century. But at present it is impossible to agree with her. By the end of the 20th and at the beginning of the 21st centuries, enough data had already accumulated to consider the significant influence of the gas component on the formation of frozen rocks, their properties and the processes occurring in them. It turned out that the presence of the gas component of frozen rocks worsens their strength and deformation characteristics ^[2,3,4]. It was found that when their temperature rises to values close to the temperatures of phase transitions, but remaining in the region of negative values, gas filtration is possible in even icy rocks ^[5]. It has been revealed that methane emissions from frozen rocks can reach levels capable of causing significant and even catastrophic warming of our planet ^[6]. It has been recorded that when drilling wells in frozen rocks, especially within hydrocarbon deposits, numerous gas phenomena are observed, expressed in the release of drilling tools, sludge, washing liquid, etc. The range of depths from which most emissions occur is from 10 to 100 m ^[7,8,9,10]. In recent years, craters tens of meters deep have been discovered on the territory of Western Siberia, associated with natural emissions of underground gases from frozen rocks ^{[11,12,13, etc.]}. There are reasonable assumptions that underground gases, moving from great depths to the surface, are cooled due to adiabatic expansion. At the same time, their temperature decreases to negative values, which in turn leads to freezing of precipitation and the formation of permafrost over gas fields in the north of Western Siberia ^[14]. Yu.K. Vasilchuk has done a lot of work to study the isotopic composition of gases in frozen rocks, providing the possibility of direct chronological comparison of isotopic records of underground and terrestrial ice and their reliable age binding for a period of more than 40 thousand years ^[15]. Yu. B. Badu considered the general patterns of the manifestation of cryolithogenesis of rocks over a gas deposit, where sedimentation and freezing of sediments was accompanied by their gas saturation.  They showed that the effect of a gas deposit on the overlying rock strata leads to the development of a cryogenic geosystem, defined as "Cryogenic strata in a gas-bearing structure"^[16]. Intra-frozen gases in frozen rocks are found not only in the free state, but also in the form of gas hydrates, which can be formed during epigenetic freezing of gas-saturated sediments.  The conditions of their preservation in the upper horizons of frozen rocks in a metastable state have been established. The pressures created in the soil mass during the decomposition of various gases and their mixtures are determined ^{[17,18,19,20, etc.]}

 It should be noted that the existing ideas about the insignificant role of ground gases in the formation of frozen rocks and their properties exclude from the theory of cryolithogenesis a significant amount of processes associated with the presence of a gas component. In practical terms, this complicates the development of regulatory documents and technologies for engineering and geological work in the territories of the distribution of gas-saturated frozen rocks. Currently, the available data on the accumulation of the gas phase and its effect on the processes occurring in unfrozen, freezing, frozen and thawing rocks are fragmentary and limited. Their complex analysis is necessary, taking into account the occurrence of the gas component, its concentration and localization, the heterogeneity of thermobaric fields and the formation of various kinds of deformations in thawed and frozen massifs. This is the subject of the proposed article.

Sources of gas supply and reasons for the formation of gas-saturated zonesBiochemical processes in precipitation

The most common cause of gas formation in sedimentary rocks are biochemical processes associated with the decomposition of organic matter as a result of the vital activity of bacteria. The intensity of natural biochemical gas generation, which is a source of biogenic gases, depends on the facies conditions of sedimentation. The concentration of biogenic gas in local zones is possible due to the bubble migration mechanism, which is realized when, under anaerobic conditions, microorganisms emit gases that do not have time to be diverted from the generation sites into the atmosphere by means of a diffusion mechanism. In this case, the concentration of the gas becomes so high that it exceeds its solubility, and therefore a gas phase occurs, forming in the form of a gas bubble. Finding the paths of least resistance and making oscillatory movements, the gas bubble moves to the area of lower pressures, evenly distributed in the thickness of precipitation. Thus, the regional background of hydrocarbon gases is formed. If gas removal is difficult, then gas-saturated zones may form, confined to the lenses of permeable rocks or to the arches of anticlinal structures covered with gas-tight rocks (lithological traps). In these cases, the gas pressure will continuously increase until it reaches values at which the integrity of the roof will be violated ^[21]. In case of man-made violations, this leads to gas emissions. Let's look at some examples. On the territory of the St. Petersburg region, when drilling wells at depths of 8-40 m, dozens of gas-mud emissions were recorded, sometimes accompanied by gas ignition. The period of prolonged release of gases lasted from several minutes to 6 hours ^[22]. Late Quaternary shallow biogenic gas reservoirs have been discovered and are being exploited for industrial purposes in the coastal area of Hangzhou Bay, northern Zhejiang Province, eastern China. Here the cut-in valleys were filled with river sand deposits and then in the post-glacial period were blocked by marine sediments. (Fig. 1). All commercial gas deposits are confined to sand layers with a capacity from 3.0 to 7.0 m, lying at a depth of 30 – 60 m, which are covered with gas-tight clays. The rapid deposition of overlying sediments of shallow marine sediments rich in organic matter provided not only abundant sources of gas, but also good conditions for its accumulation ^[23].

Fig. 1. Schematic section of a buried paleodoline with inclusion of local gas accumulations in sand lenses ^[23].

It is obvious that the formation of gas-saturated zones with increased pressure occurs in the upper soil strata at a depth of the first tens of meters.

Gas flow due to tectonic disturbancesDispersed biogenic gases formed in bottom sediments, released into bottom water, form in most cases only a regional background of hydrocarbon gases.

When gas flows from deep horizons, including from oil and gas deposits, zones with its abnormal content are formed in the upper part of the sections and the bottom layer of water. They arise in cases when fractured zones develop over the forming gas deposit in the rocks overlying it and there is an influx of hydrocarbon gases. For example, the background values of methane content in samples of bottom sediments of the Kara Sea are 0.001 ml/kg, and the modal values of the content of this gas within the gas anomalies (usually confined to the zones of discontinuous disturbances) exceed the background by more than 100 times ^[24]. A.I. Obzhirov, studying the distribution of natural gases in the sediments of the Far Eastern moray ^[25,26], came to the conclusion that biogenic methane, formed i n situ in bottom sediments, forms only a regional background of hydrocarbon gases, which has a positive correlation with the amount of organic matter in sediments. At the same time, abnormal concentrations of hydrocarbon gases are associated with the inflow from external sources due to the migration of deep gas. In zones of tectonic disturbances, gas migrates to the surface from the underlying horizons through a system of cracks and faults. In this case, areas of increased gas saturation may form, in which the concentration of gases may far exceed the background values. In the Far Eastern seas, background concentrations of methane in the bottom layer of water are usually 0.00007-0.00009 ml/l. Above oil and gas fields, the methane content is 10-100 times higher than the background values. In the bottom water above the Lunskaya structure (Sea of Okhotsk), the methane content in the central part of the structure is 0.011 ml/l, which is two orders of magnitude higher than the background. Such a high anomaly is associated with a strong disturbance of the structure - the upper part of the sedimentary cover. In the area of the gas outlet in the core of bottom silty sediments, voids and inclusions of gas hydrates are found.^{[25, 26]}

In the pores of subaqueous sediments, the gas is contained in a free (in the form of bubbles), adsorbed, pinched and dissolved state. Approximate estimates indicate that the gas content in clay soils does not exceed 5%. In sandy soils (in gas pockets) it can reach much larger values. In marine sediments, the gas component is mainly represented by methane, carbon dioxide, hydrogen sulfide, oxygen, nitrogen. ^[27]

Cryogenic factor of the concentration of in-ground gasAlong with the reasons for gas accumulation discussed above (biochemical processes and gas supply from the underlying horizons), cryogenic gas concentration is a powerful factor in the formation of gas accumulations.

This mechanism is due to the squeezing of impurities, including gas from the forming ice crystals. So-called cryogenic pressures are created, causing a significant redistribution of gas concentration in frozen rocks. At the same time, the pressure in the gas will increase. Concentrating in favorable zones of accumulation of free gas or in the form of gas hydrates can exist for a long time ^[28]. These patterns of gas redistribution during freezing do not depend on the genesis of deposits can be traced at all structural levels of the cryolithosphere.

G.N. Krayev and colleagues ^[29] during laboratory modeling of gas migration during unilateral freezing of methane-saturated loamy and sandy soils, it was found that in loams, methane during freezing was concentrated in the upper part of the sample due to the migration of moisture (and gas with it) to the freezing front. The water turns into ice, and the released gas fills the pores in the ground. In the lower part of the samples, the moisture content of clay soils and, accordingly, the dissolved gas content decreased. In the sands, where the amount of bound water capable of migration is insignificant, gas was squeezed out of the ice formation zone to the gas-tight sole of the sample. The results of the experiment allowed the authors to conclude that the distribution of methane in the experiment is the result of a specific cryogenic migration of gas together with a pore solution.

The results obtained in laboratory experiments were confirmed during the study of the dynamics of cryogenic methane concentration during uneven freezing of the active layer. In the seasonally frozen layer, despite its insignificant vertical dimensions and short duration of existence, local zones of increased gas content are formed.  The researchers found that the average values of methane concentration at the sole of the STS – about 0.002 ^cm3 / g are formed due to biogenic production. At the same time, the local maxima observed in places are many times higher than the average values, which can be explained only if the possibility of gas redistribution along the lateral is assumed. This occurs when the irregularities of the freezing front are formed due to inhomogeneities of surface conditions, or differences in moisture and material composition of sediments. In the concave parts of the sole of the seasonal freezing layer, peculiar gas micro traps are formed. For example, to form a concentration of 0.015 ^cm3/g at one of the studied sites, biogenic methane was required not only from the freezing overlying model soil column with a volume of 5 ^dm3, but also from 11-12 neighboring ones ^[29].

The same patterns were found in the study of epigenetically frozen sediments of ancient lakes. Freezing begins after the lake basin is filled with precipitation or drained during drainage. The formation of a gas-tight frozen layer leads to an increase in gas pressure in the melt zone. At the same time, under a layer of frozen soil, in deeper horizons of thawed precipitation, methanogenesis continues to develop.  Directed freezing displaces most of the methane down into coarse-grained layers with a significant volume of free pores, where it accumulates in a free form. It has been established that the gas contained in thawed sediments can shift down tens of meters during epigenetic freezing of lake sediments and accumulate in lithological pockets ^[30]. It should be added that pockets in which thawed sediments can accumulate during freezing can be caused not only by the lithology, but also by the morphology of the freezing front.

With frontal epigenetic freezing of large territories (for example, those that have emerged from under the Arctic ocean), the front of freezing and ice formation squeezes both free and released from the water-dissolved state down the section, deep into the freezing massif. 

The highest gas content in frozen rocks is observed when several heterogeneous factors are combined:  tectonic, associated with the formation of local tectonic structures that cause the movement of gas to the surface and its accumulation in domes; hydrological, associated with the movement of powerful flows of ground gas-saturated waters to the surface; geological, associated with the formation of a powerful water-saturated, layered, multi-age, polygenetic strata; cryogenic, caused epigenetic freezing of lithologically heterogeneous water and gas-saturated strata as a result, a complexly constructed paragenesis of cryogenic formations is formed, including formation ice, icy frozen rocks, cryopags, horizons of gas hydrates and pockets of pressure free gases ^[31]. Such a combination of heterogeneous factors is observed in the Yamal gas condensate fields (Bovanenkovskoye gas condensate field, Kharasaveyskoye gas condensate field, etc.)

The zones of local increased gas saturation due to cryogenic gas concentration also include long-term heaving mounds (hydrolaccolites). These formations are formed during epigenetic freezing of lake sediments (most often thermokarst). Under these conditions, freezing occurs according to the type of "closed systems", the characteristic features of which are the occurrence of cryogenic pressure. It leads to the concentration of water and dissolved gas in the local area under the growing hillock. The apical parts of the mounds are enriched with large air inclusions, due to the release of air during ice formation, as a result of which extensive air cavities are formed here. Overlapping gas-saturated zones, low-temperature icy frozen rocks do not allow gases under pressure to enter the atmosphere.

Distribution of gas-saturated rocks in the cryolithozoneGas-saturated rocks are found throughout the cryolithozone region, while the gas content in them is very diverse.

There is a significant variation in the values of the gas content in frozen rocks, even within the same genetic type. The differences between the minimum and maximum values may differ by an order of magnitude. In the research materials of A. A. Vasiliev, I. D. Streletskaya, V. P. Melnikov, G. E. Oblogov, the values of methane content in frozen rocks and ice at the key Marre Sale site located on the western coast of Yamal are given (Tables 1, 2).^[32]

Table 1. Methane concentration (ppmV) in frozen rocks ^[32]

Table 2. Methane concentration (ppmV) in re-vein (PL) and reservoir (PL) ice ^[32]

Type 1 submarines are confined to the contact of the marine and continental strata. The ice contains many sandy and sandy loam inclusions scattered in the ice thickness. Type 2 PL lies inside the loamy strata of marine genesis. Here the ice is layered, represented by an alternation of pure glassy ice and interlayers enriched with inclusions of clay material.

The minimum concentrations of methane are typical for sands and average 70-200 ppmV. The maximum concentrations of methane are inherent in sandy loam and clay deposits, and the average concentrations in them reach 1000– 3000 ppmV. An increase in the concentration of methane was found with an increase in the content of organic carbon, which limits methanogenesis in the host sediments. The minimum concentration of methane is inherent in Holocene PLL, on average less than 100 ppmV. In neo-Pleistocene PPL, the concentration is on average up to 700 ppmV. The authors explain the high concentrations of methane in the fuel cell by its production directly into the fuel cell or by ingestion with melt water. In the formation ice of both types, the concentration of methane is an order of magnitude higher than in the PLL – up to 10,000 ppmV. The researchers conclude that such high concentrations of methane cannot be explained by the methanogenesis of anaerobic bacteria in ice. Its high content in the PL is explained by migration from the host rocks and concentration in the ice body. The most probable cause of migration may be the cryogenic concentration of groundwater in the ice body formation zone, together with the gases contained in them during epigenetic freezing.

M. Y.Cherbunina, who studied the gas content in the frozen sediments of central Yakutia, obtained similar results. According to its data, the distribution of methane in frozen rocks and ice is characterized by great variability in depth and in plan, both between different geological layers and within the same horizon, and the spread of values increases with increasing average concentration. There is a significant variation in the values of the gas content in frozen rocks of the same genesis, for example, in the alasic deposits of the Lena River, the methane content ranges from 17.2 ppmv (0.0017%) to 234,346 ppmv (23.4%), that is, it differs by more than 4 orders of magnitude. The observed uneven distribution of gas may be partly related to the conditions of methanogenesis, but the main reason for the accumulation of deposits with a high content of methane is its significant cryogenic redistribution. After freezing, the redistribution of methane did not occur, despite the emerging gradients of its concentration.^[33]

A. I., Gresov and A.V. Yatsuk, who studied the gas content of frozen rocks in Yakutia, identified a regional generation-accumulative “storage” of methane, carbon dioxide, hydrocarbon gases, hydrogen, confined to the Lena coal-bearing basin. The characteristic features of coal-methane basins include the formation of abnormal concentrations of methane in the near-surface horizon: from 0.5 to 12.5% in the depth range of 8-16 m. In the depth range of 20-40 m, gas occurrences with methane concentrations up to 24-47% and gas flow rate up to 0.1 ^m3/min were recorded in wells.^[34]

Gas-saturated frozen rocks within oil and gas fieldsThe greatest gas saturation of frozen rocks is observed in areas of oil and gas structures.

Geophysical studies of the Arctic shelf have shown a direct spatial relationship of the location of gas-saturated zones, including in the form of gas hydrates, with anticline uplifts in the sedimentary cover. Seismoacoustic studies conducted by specialists of JSC MAGE, in the waters of the West-Svalbard continental margin of the Barents Sea showed a direct spatial relationship of the location of the zone of gas-saturated precipitation (acoustically transparent body) with anticlinal uplifts (sharp interruption of the correlation of reflecting horizons, domed "bulges" of overlying horizons, the presence of characteristic interference, "migrating" through the layers, up the slope, this is further confirmed by the results of gas-hydrochemical survey, and with zones of increased concentration of dissolved hydrocarbons.^[35]

The same pattern of distribution of gas-saturated frozen rocks can be traced in subaerial conditions. The highest gas content in permafrost rocks of the north of Western Siberia is confined to elevated blocks in the form of tectonic shafts and arches.  In the sedimentary cover of the north of Western Siberia, these arches and megawalls form structures of tens and hundreds of kilometers in size and with an amplitude of hundreds of meters.^[36]

In this case , gas saturation is caused by the combined action of several factors:

- tectonic, associated with the deformation of rocks confined to local tectonic structures, which causes the movement of deep gas to the surface;

- hydrological, associated with the movement of flows of ground gas-saturated water to the surface;

- cryogenic, which caused epigenetic freezing of lithologically heterogeneous, water- and gas-saturated strata, as a result of which a complexly constructed paragenesis of cryogenic formations is formed, including formation ice, icy frozen rocks, cryopags, horizons of gas hydrates and "pockets" of pressure free gases.

In the course of engineering and geological works on the route of the Ob-Bovanenkovo railway on the section of the bridge crossing over the Yuribey River (southern Yamal), more than 200 wells with a depth of 10 to 53 m were drilled in permafrost rocks. In most wells, gas emissions were observed, confined to fine-grained sands of coastal-marine genesis, overlain by clay sediments.  In wells where geothermal observations were carried out, gas emission was recorded throughout the year.^[37] Sometimes gas emissions were accompanied by gorenje, the duration of which in one of the wells lasted about a month. Calculations carried out by F. E. Are have shown that the observed gas porosity in loams averages 5-7%, reaching 10%, and in some cases up to 50%. In the sands, the average gas porosity was 0.5%. The study of the gas component of frozen rocks allowed the researcher to draw a number of important conclusions: that the upper layers of permafrost rocks (up to 100 m) above oil and gas fields contain a significant amount of free gas under excessive pressure; zones of increased gas micro- and macroporosity can serve as a reservoir of free gas in permafrost rocks; prolonged gas emissions from wells drilled in monolithic permafrost rocks, indicate their significant gas permeability and the possibility of filtration through them. ^[7]

Separately, we will consider the gas saturation of frozen rocks in the area of one of the most studied hydrocarbon deposits - the Bovanenkovsky NGCM. This deposit is located in the central part of the Yamal Peninsula and is associated with the Bovanenkov local uplift.  Permafrost rocks are mainly represented by clays with sand layers of varying thickness. Almost all gas manifestations at the Bovanenkovskoye gas field are confined to detached dusty sands occurring in the section of the frozen strata to depths of about 130 m. Gas manifestations of varying intensity and duration are confined to the sand layers and lenses. The gas is in free form or in the form of gas hydrates. By composition, the gas consists of 99% methane with an insignificant admixture of nitrogen, carbon dioxide. According to the gas content at the Bovanenkovskoye NGCM, two types of frozen rocks are distinguished. In the first, corresponding to the horizons where no gas manifestations are observed, the gas content is insignificant and is 0.005 ^cm3 / g. The degree of filling of the pore space (the specific porosity of these rocks, free from ice and unfrozen water) slightly exceeds the gas content and ranges from 0.86 to 0.95%, which indicates the free form of gas inclusions. The second type is confined to the horizons of gas phenomena. The gas content here reaches 0.5 ^cm3/g. Given the high degree of filling of pores with moisture (more than 98%), this is two orders of magnitude higher than the specific active porosity. The flow rates of gas emissions range from 50 to 14,000 m3/day, with average values of about 800-1000 ^m3/day. Such flow rates and the size of deposits indicate that a powerful network of channels exists or is being formed in frozen rocks, through which gas is filtered to the well. Gas volumes reach significant sizes, In one of the wells (64-P-2) in the range of 72-80 m, the gas reserve was estimated at 490 thousand ^m3, and the area of the gas deposit at 80 thousand ^m2. More than 85% of gas occurrences were recorded at depths of about 60-80 m. This horizon is sustained over an area of about 120 ^km2 and is confined to the rocks of the Yamal series. ^{[38, 8]}

Gas in frozen rocks can be contained in solid form in the form of gas hydrates. During epigenetic freezing of water-saturated deposits, free gas is squeezed out of the ice formation zone. This causes its accumulation in the irregularities of the formed gas-tight freezing front. Further freezing will lead to concentration and compression of the gas, and, when the necessary thermobaric conditions are reached, to hydrate formation.^[10] As a result of the interaction of these factors, the most pronounced gas-saturated frozen rocks are formed. V. S. Yakushev's study of the gas content of the samples taken at the Bovanenkovskoye GCM showed that the volume of gas released during thawing of the cores of rocks with increased gas content significantly exceeded the volume of free pore space. In cores extracted from sediments without gas manifestations, the gas content during thawing either corresponded to the free pore space, or was significantly less (Table. 3) Gas release was active in the form of large bubbles up to 2-3 mm in diameter and numerous small bubbles up to 0.5 mm forming chains and swarms.^[19]

Table 3. Results of determination of gas content during thawing of samples of frozen core of undisturbed composition from the cryolithozone at the Bovanenkovskoye GCM.^[10]

This gives every reason to believe that at least some of the samples studied contain relic gas hydrates in the pore space.

This assumption makes it possible to explain a number of contradictory facts regarding the gas component of the frozen strata of the Bovanenkovsky GCM. As is known, epigenetic freezing of gas-saturated dispersed rocks causes cryogenic gas concentration and a significant increase in pore pressure in the freezing massif.^{[19,30,39,40]}. It should have been expected that increased pore pressure would have been observed in the frozen rocks of the Bovanenkovsky GCM. But for them, as noted in ^[8], low values of reservoir pressure with a deficit relative to hydrostatic from 0.92 to 0.21 are characteristic. In only five tested objects, the reservoir pressure exceeded the hydrostatic pressure (1.03—1.14). This contradiction can be explained precisely by the formation of gas hydrates during epigenetic freezing of gas-saturated precipitation. V. S. Yakushev proposed the following scheme. The freezing front of the gas-saturated sediment column is ahead of the hydrate formation front. A closed system is formed in the frozen rock. The removal of gas from the frozen rock stops. A cryogenic concentration of gas occurs, which leads to an increase in pressure in the formed gas pockets and the transition of part of the gas to a hydrated state. At the same time, the pore pressure drops.^[19]

The increased gas content is observed in the uppermost horizons of frozen rocks. Geochemical studies of dispersed hydrocarbon gases conducted on the territory of the Bovanenkovsky and Kharasaveysky oil and gas condensate fields (NGCM) have shown the unevenness of their concentration in the upper horizons of permafrost rocks. In wells up to 3 m deep located in the contour of deposits, the values of hydrocarbon gas concentrations were more than 2 times higher than outside the contours (Table. 4) ^[41]. Here, high-pressure small accumulations of hydrocarbons are widespread in the clay cover, within the contour of the deposit. When moving away from the arch, the anomaly of reservoir pressure in these accumulations decreases, and gas manifestations in the tire disappear behind the contour of the deposit ^[4]. This distribution of hydrocarbon gas concentration values reflects the general patterns of gas saturation of frozen rocks in the areas of gas fields. Probably, this distribution of gas concentration is due to the fact that even before freezing in non-frozen subaqueous sediments, an increased gas content is observed over tectonic gas-bearing sediments, which was discussed above. The freezing that began only fixed this.

Table 4. Average values of gas-chemical parameters of surface deposits of Bovanenkovsky and Kharasaveysky GCM.^[41]

The materials discussed above have shown that frozen rocks are diverse in terms of the degree of gas saturation and the form of the gas component. Based on the materials discussed above, it is possible to preliminarily distinguish several groups of frozen rocks according to the gas content, the pressure in it20 and the processes associated with the redistribution of gas.  The first includes deposits in which there is a background distribution of gas before freezing. In clay-loamy deposits, free gas is contained in a dispersed form (in the form of separate bubbles in the pore space). There is no significant redistribution of the gas component during freezing and its cryogenic concentration. The gas saturation of frozen rocks is about 5% (5-7% ^[7], 4-5% ^[4], and the gas content is about 0.002 – 0.005 ^cm3 / g. The gas is in free form in the form of trapped in ground pores and capillaries. The gas pressure corresponds to the hydrostatic pressure. With an increase in the temperature of frozen rocks above 5-7 ° C, it is possible to redistribute the gas without load due to filtration through capillaries. At loads, gas filtration begins at lower temperatures. The second group includes frozen rocks in which the size of gas cavities exceeds the size of natural pores. The gas content is 0.2 – 0.5 ^cm3/g or more. and the amount of gas can exceed the free porosity by 2-3 orders of magnitude ^[8], while the porosity index can reach 50% ^[7].

The second group includes frozen soils in which the pressure of the gas component exceeds the hydrostatic. In that case, the size of the gas bubbles will exceed the thickness of the capillaries, while the continuity of the soil mass is preserved. A further increase in pressure will cause the ground particles to be squeezed out of the boundary of the gas bubble, which will lead to the formation of cavities larger than the thickness of the capillaries. The free gas will be under pressure, the parameters of which are determined by many factors (the amount of organic matter, the water content of precipitation, the geological structure, the proximity to gas channels, the intensity of cryogenic concentration, etc.) and significantly exceed the hydrostatic. The gas saturation of sediments can reach 50% or more. Under the conditions considered, it is freely dispersed over the soil massif or concentrated in "gas pockets". In frozen rocks, the gas pressure in frozen rocks reaches the highest when the gas hydrates contained in them decompose. It is about 2.5 MPa ^[20]. When gas-saturated horizons are opened during drilling, gas emissions occur. When a certain ratio is reached between the strength of the rock and the gas pressure in natural conditions, a pneumatic rupture of the soil mass can occur and the formation of a gas layer (lens) that perceives the weight of the overlying strata. 

The third group includes gas located in frozen rocks in solid form (gas hydrates). It is formed when a certain ratio is reached between the pressure of the in-ground gas and the temperature. At low temperatures, the values of in-ground gas pressures decrease, for example, for the formation of methane gas hydrates at temperatures of 0 - -5 ° C, the pressure is about 2.5 MPa necessary for hydrate formation ^[20]. The corresponding pressures could be formed under conditions of creating cryogenic pressures during epigenetic freezing, under glaciers, or at sufficient sea depths. The gas hydrates formed in frozen rocks are similar in their properties to underground ice.

Forms of finding gas inclusions in thawed and frozen soils

In clay-loamy deposits, free gas is contained in a dispersed form (in the form of separate bubbles in a porous space) or in thin permeable lenses and interlayers inside weakly permeable clay strata. In sandy formations, it occurs in the form of localized clusters concentrated in mini-traps (small anticalinal structures or in the head parts of obliquely overlying low-permeable inner layers of clay composition) ^[43]. A. Andersons co-authors (1998) identified three types of gas bubbles in subaqueous sediments (Fig. 2) ^[44]:

- interstitial bubbles – very small bubbles in undeformed interstitial pore spaces of sediment;

- reservoir bubbles – a gas bubble occupying an area of the undeformable solid frame of the sediment larger than the normal pore space (a bubble is a gas–filled, liquid-free area of the solid frame of the sediment);

- gas voids – "a cavity that is larger than the normal interstitial space of the sediment contains only gas and is surrounded by sediment, either undisturbed or slightly distorted by the expansion of the cavity due to the formation of a bubble of free gas."

Fig. 2. Three different types of gas bubbles in the sediment: Type 1 – interstitial bubbles; 2–reservoir bubbles; bubbles displacing sediment, 3 – gas voids.^[44]

Most gas bubbles are gas voids, the size of which varies from 0.5 mm (spheres with a volume equal to the volume of the bubble) to 50 mm. Most of them have not spherical, but "coin-shaped" flattened shape, while their planes are oriented subvertically, which implies vertical gas migration ^[44]. A study by M.Y. Tokarev and co-authors ^[45] of the sediments of the Kandalaksha Bay of the White Sea (sampling depth up to 3 m) showed the presence of gas-saturated sediments here. The computed tomography data of the cores showed the presence of numerous hollow fractures of subvertical orientation in the sediment thickness due to sediment degassing. Gas inclusions are represented by isometric, rounded cavities with a diameter of about 1 mm and curved cracks up to 5 mm long and up to 2 mm thick elongated in the subvertical direction (Fig. 3).

Fig. 3. Fragment of the X-ray plane section. Gray — sediment, black — cracks caused by sediment degassing (depth 100-120 cm) ^[45]

In sandy sediments, gas bubbles have a predominantly isometric shape (Fig. 4)

Fig. 4. A sample of a core, slightly silty sand of the Aberdeen Ground formation, taken from a depth of 59 m below the seabed, on which gas voids are visible. The well was drilled in the area of South Fladen, North Sea ^[46].

According to F.E. Are, who studied gas-saturated frozen rocks near the route of the Ob-Bovanenkovo railway, horizontally oriented macro-voids of lentil-shaped shape up to 12 mm thick were recorded in the cores of many engineering-geological wells. Sometimes clusters of small subhorizontal broken voids up to 1 mm thick were observed ^[7]. A distinctive feature of the gas-saturated frozen rocks on the territory of the Bovanenkovsky NGCM is the presence of isometric caverns with a diameter of 5-8 mm and a depth of up to 7 mm. The walls of the caverns are smooth, the caverns themselves are often filled with fir-like snow ^[38,8].

Significant gas saturation is noted for underground ice. The most common are the ices of perennial heave mounds In this case, gas-saturated ices are formed when the lens of water lying at the base of the growing heave hill is comprehensively frozen.

Sometimes gas-saturated ice-saturated rocks are formed with the participation of gas coming from great depths. Below are the materials from the report of A. S. Smirnov "Surface manifestations of fluid dynamic processes of the Earth at the seminar of the Community of Young Permafrost Scientists of Russia (PYRNR), dedicated to gas emission funnels, held at the Institute of Geoecology named after E.M. Sergeev (IGE RAS) on 28.11.2014. Fig. 5. shows the structure of the ice core of a long-term heaving hillock at the Pestsovoye deposit of the Tazovsky Peninsula, formed by gas-saturated ice with an abnormally high hydrogen content (depth 16 m). Gas voids of isometric shape up to 10 mm across are observed in the ice. In this case, it is possible to assume the arrival of deep gas.

Fig. 5. Gas-saturated ice from the horizon with a high hydrogen content (depth of 16 m) in a long-term heaving mound at the Pestsovoye field of the Tazovsky Peninsula. Photo by A. S. Smirnov.

            A wide variety of gas inclusions is observed in the gas emission funnels discovered in 2014 in the valley of the Yerkuta Yakha River. Figure 6 shows the structure of gas-saturated ice, broken by cracks into separate blocks. The gas is distributed in separate clusters of small air bubbles. In this case, the most likely process of gas saturation is the flow of gas under pressure into the previously formed ice.

Fig. 6. Gas clusters in the form of clusters, ice in the wall of a gas discharge funnel on the Yerkutayakha River (Southern Yamal) Photo by Yu. Stanilovskaya.

A well drilled near the Yamal crater (central Yamal), at a depth of 5.8 – 6.3 m, a layer of dense gas-saturated (milk ice) broken by vertical and horizontal cracks and winding channels of subvertical orientation was uncovered (Fig. 7).

Fig.7. Gas–saturated (milk) ice near the gas discharge funnel depth of 5.8 - 6.3 m (Yamal crater Photo by V. Hilimonyuk ^[47].

Blocks of ice ejected from a funnel located in the area of the Se Yakh region in the north-east of the Yamal Peninsula consist of alternating layers of white gas-saturated ice and ice ground (Fig. 8, 9)

Fig. 8. Blocks of gas-saturated ice ejected during the formation of a gas discharge funnel in the area of the Seyakha River (northeast of Yamal Peninsula) Photo by A. I. Sinitsky ^[48].

Fig. 9. A fragment of gas-saturated porous ice ejected during the formation of a gas discharge funnel in the area of the Seyakha River (northeast of Yamal Peninsula) Photo by A. I. Sinitsky.

On the vertical surfaces of the gas discharge funnel, called the Yamal crater, gas inclusions of various sizes and morphologies are observed, forming elongated chains or isometric clusters (Fig. 10).

Fig. 10. Gas-saturated frozen rocks composing the walls of the Yamal crater.   July 2014. Photo by V. I. Bogoyavlensky^[49].

The gas hydrates lying in the ground strata do not differ in appearance from ice formations.

Gas pressure in frozen ground massifsThe gas component is the most mobile part of frozen rocks.

The gas reacts to the slightest change in temperature and pressure by increasing or decreasing its volume. Having a high compressibility, gas can accumulate in existing pores and ground cavities. Let's consider some estimates of the pressure values recorded in frozen rocks and possible reasons for their formation.

J. R. McKie, who studied long-term heaving mounds (pingo) on the Arctic coast of Canada, measured the pressure in water lenses lying at the base of the mounds. The obtained values of the hydrostatic pressure did not exceed 0.35 MPa, with the thickness of the frozen thickness of about 25 m. This pressure was sufficient for the deformation of the frozen roof and the formation of pingo, but not enough for the development of the explosive process.^[50]

Accumulations of free gas with pressure up to 0.2–0.3 MPa are often recorded in permafrost coal-bearing rocks of the Anadyr, Bering, Arkagalinsky, Lena and other basins and frozen sediments overlapping them ^[34].

In 1986-1990 . On the Yamal Peninsula, when drilling an array of frozen soil in the area of the bridge crossing over the Yuribey River from a depth of 26 m, an ejection of a drilling tool weighing about 150 kg to a height of 12 m was noted. According to the calculations of R.G. Kalbergenov (oral report), the required pressure was 2.5 MPa. According to the calculations of V.I. Bogoyavlensky and I.A. Garagash ^[11], a pressure of 1.25 MPa is sufficient to destroy the frozen tire of the Yamal Crater gas discharge funnel with a capacity of 8 m. Similar values (1.74 MPa) were obtained by V.P. Merzlyakov ^[31]. According to S.N. Buldovich ^[51], the pressure of 1.5 MPa and temperatures below -1.4 ° C are sufficient for the formation of stable carbon dioxide hydrates. Accordingly, an increase in temperature above these values will lead to the decomposition of gas hydrate and the formation of appropriate carbon dioxide pressures in the frozen rock thickness. For methane at temperatures ranging from -5 °C to -0 °C, the equilibrium pressure in the gas—water (ice)— hydrate system is in the range of 2.2-2.6 MPa ^[20]. That is, the maximum possible pressure during the decomposition of methane gas hydrate will not exceed 2.6 MPa. For the Bovanenkovsky NGCM, it was found that the volume of released gases in the horizons of active gas release exceeds the space in the pores of rocks by 2-3 orders of magnitude, which the gas could occupy in free form ^[52]. Using the known ratios of pressures and volumes of gas (Boyle Marriott's law), it can be determined that the gas pressure in the pores should reach values of 2-3 MPa, this is quite consistent with the assumptions that it was in clathrate form. In the undisturbed state, the gas in solid form corresponds to ice formations of the frozen strata, abnormal pressures are not observed. When the pressure is removed or the temperature of frozen rocks increases, the decomposition of gas hydrates begins. At the same time, significant pressures are created, which lead to deformation of frozen rocks and filtration of gas in them. When the amount of gas increases to certain values, its removal will not be provided by filtration, which causes an increase in pressure in the local zone to values that ensure a violation of the continuity of frozen rocks. This process can be defined as a pneumatic rupture. The term pneumatic fracturing is widely used in engineering geology and is used when using technical means to rupture rock with compressed gas ^[53]. With the formation of significant gas pressures, pneumatic rupture will also be observed in natural conditions. In addition to increasing the temperature and relieving pressure, the decomposition of gas hydrates can be observed with an increase in the mineralization of pore solutions of frozen rocks. When the temperature of frozen rocks increases, the thickness of the films of bound water with a lower concentration of salts increases, salt ions migrate into the frozen ground due to diffusion ^{[54, 55]}. That is, gas hydrates will begin to decompose and release free gas under high pressure even before the frozen rock thaws.

There is a certain relationship between the amount of gases, the pressure in the gas component and the processes in the soil mass. In the group with a low content (0.002 – 0.005 ^cm3/g) and pressure (about 4 kg/^cm2), the main mechanism for the redistribution of the gas component is the filtration of gas through capillaries by pushing (squeezing) bound water. If the gas content and pressure in it are higher, then the processes that ensure the redistribution of the gas component will largely be determined by the strength characteristics of frozen soils. In the case when the adhesion forces will ensure the monolithic state of the frozen massif, and its minor deformations, the filtration of gas weakens as it dissipates. In this case, a stage of damped creep will be observed, occurring at stresses not exceeding the long-term strength of the soil and characterized by a gradual decrease in the rate of irreversible deformations, in the limit tending to zero.  

When the pressure increases to values at which the resistance of the frozen soil will be less than the shear stress, then the pneumatic rupture of the soil will occur, cracks will begin to develop. In this case, plastic or discontinuous deformations in the frozen rock mass will be the main cause of gas filtration. Under these conditions, there is a stage of undamped creep characteristic of the shear phase, when the level of acting stresses exceeds the long-term strength of the soil and, in turn, is divided into three stages. The origin and development of creep is caused by the development of microcracks, the destruction of particle aggregates and the growth of other defects in the structure of frozen soil under load ^[56]. In this case, the deformation-filtration mechanism of gas redistribution used in the construction of a phenomenological model of the formation of gas discharge funnels is implemented ^[13]. If this mechanism is implemented, the frozen array retains its integrity, only local deformations are observed in it, which do not change the overall structural pattern. With a further increase in pressure, conditions for unsteady creep may arise. The rate of deformation, in which, unlike damped creep, tends not to zero, but to some constant value. The second stage is steady—state creep, or ductile-viscous flow with almost constant deformation rate. When deformations of a certain magnitude are reached, it passes into the third stage of the progressive flow. At the stage of progressive flow, irreversible deformations reach the limit value. There is an accelerating development of microcracks, the emergence of new microcracks with their transition into macro-cracks, which causes the decompression of frozen soil ^[1] and the development of local zones of unsteady flow in the frozen soil massif. The movement of the resulting fluid mass will be directed in the direction of least resistance, usually upwards. When the roof is destroyed, a pneumatic explosion may occur, the release of gas together with gas-saturated ice-ground material and the formation of a funnel. As a justification for the transition of the steady motion of the gas-saturated ice ground into an explosive process, let us consider the data on the growth rate of some mounds preceding explosions forming gas discharge funnels described in the literature. The hillock on the site of the Yamal crater has been developing for about 60 years, its height is about 6 m, the average growth rate of the hillock is about 0.1 m/year ^[57]. The height of the Seyakhinsky crater is 1.7 m, the formation dates are 2015-2017.^[58] The growth rate is about 0.8 m/year. The hillock on the site of the Yerkutinsky crater (the valley of the Yerkuta River in the south of Yamal) was formed 1-2 years ago, it was discovered by a group of biologists led by A. Sokolov in 2016. The explosion occurred in the spring of 2017. The height of the hillock is 2-3 m ^[59]. The growth rate is 1-1.5 m/year. For comparison, the growth rate of large heave mounds is usually 0.015 — 0.025 cm per year ^[60]. According to E. D. Ershov's estimates, in the initial stage of the growth of perennial mounds, the maximum is 0.1 - 0.3 m/year, and then, as the permafrost core grows and the mound itself increases, it decreases to 0.01-0.02 m/year ^[61]. The rate of growth of the heave mounds associated immediately before the explosion reflects the stage at which the roof under the influence of pressure created by the in-ground gas begins to deform intensively and eventually ruptures. The materials considered are fragmentary and few in number, since there is currently no general theory considering the various stages of gas-dynamic processes in frozen rocks.

Study of the processes of gas redistribution in a soil massif in laboratory conditionsThe study of the properties of gas-saturated frozen soils in natural conditions is currently not carried out due to the lack of appropriate techniques and poor knowledge of the object itself.

In recent years, the study of this problem in the laboratory has begun to develop. The work is being carried out in different directions. This includes the study of the influence of the gas component of frozen soils on their mechanical properties, and the study of gas filtration in frozen soils, and modeling the development of gas cavities due to pneumatic rupture of the soil mass.

Employees of the Soil Science Laboratory of the Institute of Geoecology of the Russian Academy of Sciences conducted a study of the dynamics of gas release from frozen soils. The research was carried out on the soils of the southern part of the Yamal Peninsula. The tests were carried out on hard and refractory loams, as well as sands. The gas content in the samples (dissolved, adsorbed and free) reached 4-5%. During the tests, it was found out that even minimal external loads have a significant effect on the process of gas release in frozen soils with an increase in temperature (Fig.11). In sandy soils, the temperature of the beginning of the release of gas inclusions with an increase in the volume compressive pressure gradually decreases to -8 ° C (at a pressure of 0.2 MPa). The greatest intensity of gas emission occurs at a temperature of 0 °With a pressure of 0.05 MPa, -1 ° C at 0.1 MPa and -1.5 ° C at 0.2 MPa. At the same time, the dynamics of gas release under the action of compressive loads has a more pronounced peak of the maximum intensity of gas release, and gas release stops at a lower temperature. In loam, the temperature of the beginning of gas release gradually decreases with an increase in the volume compressive pressure from -7 ° C at a pressure of 0.05 MPa to -8 ° C (at a pressure of 0.2 MPa). The greatest intensity of gas emission occurs at a temperature of -3 ° C at a pressure of 0.05 MPa, -4 ° C at 0.1 and 0.2 MPa. The action of external loads reduces the temperature of the beginning of gas release and affects its dynamics when the temperature changes. At the same time, the maximum volume of gases released increases with increasing compressive pressure ^[3].

Fig. 11. Dependence of the volume of gas released under the action of

compressive loads (1 - 0.001 MPa, 2 - 0.05 MPa, 3 - 0.1 MPa,

4 - 0.2 MPa) of temperature: a) dusty sand; b) loam ^[3].

It was found that the change in the strength of frozen gas-containing soils under the influence of the temperature factor occurs naturally, in accordance with the nature of gas release in them. With free gas release, strength reduction begins already at negative temperatures (-5 ° C), and strength indicators reach their minimum values when the thawing temperature is reached. At the same time, the greater the applied load, the faster the soil begins to react and change properties, losing its bearing capacity. It is established that the decrease in the strength properties of soils depends on the number of gas inclusions in them (Table 5). From the data given, it follows that under the same temperature conditions (soil temperature -5 ° C), the strength properties of the same soil depend on the gas content in them. The decrease in the strength of soils is due to a change in their structure, which occurs as a result of gas release, begins already at 2% content of the gas component, with its increase to 8% it proceeds most intensively ^{[3, 4]}.

Table 5. Changes in the strength properties of frozen loams at a temperature of -5 ° C, depending on the content of the gas component in them.^[62]Employees of the IGE RAS and the Department of Geocryology of Moscow State University conducted laboratory studies of gas filtration through frozen samples.

The gas was supplied through a fitting frozen into the sample, at a pressure of about 0.3 MPa. The tests were carried out on ice samples of different salinity and kaolin with an initial humidity close to the upper limit of plasticity, which ensured high iciness after freezing. The temperature of the samples increased from -9°C to values close to 0 °C. At a temperature of about - 1 ° C, gas filtration was observed in samples of fresh ice and icy kaolin. A detailed description of the observed processes is given in a number of publications ^{[5, 31, etc.]}, here we will also consider some processes in the contact zone of frozen samples with the gas supply channel. In the lower part of the sample, a fan-shaped gas-saturated zone with a height of about 2 cm and a width of 3-4 cm was formed at the contact with the gas supply fitting, to which radially divergent cracks were confined (Fig. 12).

Fig.12 . The lower part of the sample is adjacent to the gas supply fitting. Shooting in reflected light.  Photo by A.N. Khimenkov ^[5]

The survey of the ice structure of the lower ice layer in polarized light showed the presence of numerous traces of plastic and discontinuous deformations. The most significant ice deformations are observed in the area adjacent to the gas supply nozzle (Fig. 13). An ice deformation zone with strongly dislocated, crushed and fragmented crystals has formed here. Plastic deformations of crystals, wave-like boundaries, indentation of crystals into each other, cracks and crushing zones are visible, chains of air inclusions at the crystal contacts.

Fig. 13. Ice structure of the ice zone directly adjacent to the gas supply connection. Shooting in polarized light. Photo by A.N. Khimenkov ^[5]

Similar processes were also observed when compressed gas was exposed to a frozen sample of kaolin clay. In the lower part of the gas-saturated sample, near the gas supply nozzle, the general pattern of ice slots is preserved.  The gas supplied to the lower part of the sample acts on the ground (ice) at the beginning as a kind of stamp. In the soil zone adjacent to the gas supply channel, the frozen soil is significantly deformed by plastic and discontinuous deformations dividing the soil massif into separate blocks; channels elongated in the subvertical direction, partially filled with ice (Fig. 14).

Fig. 14. The lower part of the gas-saturated sample. Deformation formations of soil near the gas supply fitting: 1 - plastic deformations, 2- cracks, 3-soil blocks separated by cracks, 4-the place of contact of the gas supply channel with the soil. Photo by A.N. Khimenkov ^[5]

The frozen soil near the gas supply channel is saturated with gas inclusions with a diameter of about 1 mm. The abundance of gas inclusions leads to the fact that in local zones, the frozen soil acquires a honeycomb structure (Fig. 15).

15. Distribution of gas inclusions in the lower part of the sample adjacent to the gas supply channel. Photo by A.N. Khimenkov^[5]

A gradually increasing area of continuous filtration flow of general orientation towards the upper part of the sample is formed above the penetration zone in the sample. In this case, a network of fan-shaped channels diverging from the center (gas supply fitting) is formed. A continuous gas flow in this zone is transformed and enters the overlying layers in the form of small branching fluids through a system of cracks and deformations in the soil. The gas supplied to the lower part of the frozen soil sample, due to the tortuosity of the pore channels and the different area of their real cross-section, will dissipate, look for the weakest zones and push through them. Layered cryotextures located above the insertion zone were torn by numerous, winding channels with a thickness of fractions of mm. Elongated gas bubbles are confined to local deformations of the primary cryogenic structure, without disturbing the general position of the ice slots (Fig. 16) ^[31].

Fig. 16. Filtration channels that break through the primary layered. The photo was taken in reflected light. The light stripes are kaolin clay, the dark ones are ice slivers. The arrows indicate the places where the continuity of the ice slots is broken. Photo by A.N. Khimenkov ^[31].The conducted studies allow us to draw some preliminary conclusions.

The supply of gas under pressure leads to the occurrence of local deformations, through which the scattered gas flows in the form of small bubbles (in fractions of mm) diverge from the center, where the fitting is located, to the edge parts of the sample. From the formed system of sub-vertical branching channels, gas bubbles spread throughout the sample array. The movement of gas bubbles is represented in the form of chaotic oscillations, contributing to the selection of the most weakened zones. Therefore, the formed channels have a curved, worm-like shape. In high-temperature conditions (sample temperature is about -0.5 °C), ice is a harder component compared to frozen soil, therefore, the primary pattern of ice slots has generally been preserved, although the primary ice elements have been partially deformed. At the same time, an increase in the number of gas cells is observed at the contacts of ice slots with the ground

The mechanism of gas flow distribution in frozen soil is similar to the mechanism of hydraulic dispersion, the process of dispersion of matter in a porous or fractured medium at the boundary of liquids with different concentrations ^[63]. Both processes are caused by the heterogeneity of the velocity field of matter due to the tortuosity of the pore channels in the rock and the different area of their real cross-section (Fig. 17.). It can be assumed that when the gas moves through frozen samples, similar processes will develop.

Fig.17. Gas scattering in a frozen soil sample,

supplied from a local source (using materials.

According to the ideas of V.M. Goldberg and N.P. Skvortsova, bound water in clays fills the entire volume of small pores and most of the large ones. Under the influence of the applied pressure, the bound water is "forced through". Such penetration begins in large pores, i.e. the part of the water that is least connected to a solid surface. As the pressure drop increases, more bound water in small pores will also be involved in the movement ^[64]. An increase in temperature in finely dispersed frozen soil, to values close to phase transitions, will lead to a sharp increase in unfrozen water in them. As a result, local gas jets under pressure act on loosely bound water and ice sluices, deforming them  

Chinese researchers conducted experiments to study the processes associated with the pressure effect of gas on the ground layer ^[65]. An array consisting of a layer of sand covered with fine-grained silt was formed under the water layer. Air was supplied to the sand layer under pressure. As a result of the pneumatic rupture, a gas cavity (gas bag) was formed at the boundary of sand and silt. The experimenters divided the gas bag formation process into three stages: initial, late and final. The formation of a gas bag begins with the appearance of a transverse crack (Fig. 18), confined to the boundary of sand and silt. Then the crack begins to grow in the longitudinal direction, which causes the cavity to expand and the upper fine-grained layer to rise, and the excess pressure remains almost unchanged (Fig. 19). As the volume of gas increases, precipitation will be displaced. In the muddy soil, the formation of plastic deformations is observed, the layers shift relative to each other. At the final stage of the development of the gas bag, discontinuous deformations begin to develop in the form of vertically and obliquely oriented cracks. Through them, gas is discharged into the water from the gas bag, in which the excess pressure is dissipated.

Under natural conditions, when abnormally high reservoir pressures are formed due to gases contained in frozen rocks, similar processes occur. A region of plastic and discontinuous deformations is also formed above the gas-saturated zone. As a result, the gas is either gradually filtered into the atmosphere, or in the form of a pneumatic explosion throws the frozen roof to the surface, forming a funnel of gas emission. The processes accompanying the formation of these formations are discussed in more detail in a number of publications.^{[11, 12, 13, 31, etc.]}

Fig.18. Formation of a longitudinal crack ^[65].

Fig. 19. The gas bag (in the lower part of the photo) and the plastic and discontinuous deformations of the soil caused by its formation ^[65].

Evaluating in general the results of laboratory modeling of the pressure effect of gas on thawed and frozen soils, some common signs can be noted. First of all, it is the formation of a local area of increased pressure. Depending on the ratio of gas pressure and the strength of the soil mass, a series of paragenetic processes develops in it. These include plastic deformations without breaking continuity and pneumatic fractures, leading to the development of a series of cracks and even the formation of cavities filled with gas.

Discussion

            In the previous sections, it was shown that the distribution of gas in frozen soils, its manifestations, forms of location, and pressure are very diverse. This makes it difficult to study and understand the role of gas saturation, as a cryolithological factor in general, and in solving particular problems: patterns of formation of gas accumulations in frozen strata, determination of physical and mechanical properties of gas-saturated frozen rocks, processes in freezing, frozen and thawing rocks, structural features of gas-saturated frozen rocks, etc. It would seem that these problems should be solved within the framework of geocryology. Nevertheless, there is no separate section dedicated to the gas component within the framework of the scientific direction studying the part of the lithosphere with a negative temperature.

To date, the only example of generalization and systematization of the gas component of frozen rocks is the classification of the gas component of cryolithozone rocks proposed by E. M. Chuvilin and co-authors ^[66].

They distinguish three main forms of finding gas by phase state sorbed, (adsorbed and absorbed) dissolved, free (pinched and mobile).

Sorbed (adsorbed) gas confined to the surface of ice and the organomineral skeleton. Its quantity is small.

Gas hydrates belong to the absorbed gases, is a crystalline substance (clathrate compound) in which gas molecules enter the cavities of the ice frame. Violation of thermodynamic conditions (temperature rise, pressure relief) can lead to decomposition of gas hydrates with the release of gas, the volume of which is 2-3 orders of magnitude larger than the volume of pores.

Sorbed coal gas is characteristic of permafrost coals, its amount depends on the stage of their metamorphism and can reach 90% of the total volume of coal gas.

The dissolved gas is contained in the pore solution of cooled rocks, as well as in films of unfrozen water, and can reach the first tens of percent of the volume of pore water.

The free gas is located in the pores of the frozen ground, not occupied by ice and unfrozen water. It is present in rocks before freezing and its content increases significantly due to cryogenic concentration. This form of gas is present in 2 types, pinched gas and mobile. The redistribution of the latter is limited by the reservoir properties of frozen rocks.

As the materials in the previous sections have shown, free gas has the greatest influence on the properties of frozen rocks. This is the most mobile part of the gas component, which is extremely sensitive even to small changes in temperature and pressure in frozen strata. In addition, the free gas is compressed. That is, without changing the initial volume of pores and the structure of the soil layer itself, its amount can increase with a simultaneous increase in pressure. Or decrease, when creating conditions for filtering. Trapped gases in the soil pores are often under pressure created by various reasons, for example, the movement of the freezing front, phase transitions in the freezing rock (crystallization pressure), an increase in hydrostatic pressure, an increase in temperature, etc. In the equilibrium state, this pressure is equal to the pressure in the liquid phase in contact with the gas and corresponds to the pore pressure. The amount of pore pressure affects many physical and mechanical properties of soils, in particular their strength and compressibility under load. Shrinking, gas bubbles reduce their volume and, with a certain ratio of the diameter and size of the pores, they can move from a pinched state to a free one, which can be accompanied by a sharp breakthrough of gases from the pores of the soil and a release of pore pressure ^[67]. It is more correct to call the free gas moving in the ground bulk mobile, since the trapped gas also refers to the free form.

A trapped gas bubble in a capillary or pore can be considered a closed system, since in this state there is no mass exchange with neighboring areas of frozen soil, with groundwater, or with the atmosphere. In the case when the gas bubble gets the opportunity to move in the ground mass, the gas-ground system becomes open. At the same time, the physical and mechanical properties of the soil layer itself change dramatically. Back in the 50s of the last century, in the works of F. Gassmann, it was proposed to introduce two limiting states of the medium when considering deformations of porous media (soil massifs).  In the first of them, the porous medium is an "open system", the hydrostatic pressure in the pores is always unchanged. In the second state, the medium behaves as a "closed" system, the relative motion of a liquid or gas is excluded ^{[68, 69]}. During this transition, the structure and properties of the rocks will change. In this case, the structural connections that ensure the solidity and strength of the array are violated.  Deformations and cracks develop. Streams of gas filtered through it will form in the previously gas-tight frozen thickness. If the gas flow is sufficiently intense, then the process of its expiration will acquire an explosive character. Hydrate-containing frozen rocks can also be considered as closed systems. In case of violation of equilibrium thermodynamic conditions, during heating or pressure relief, decomposition of gas hydrates will begin with the release of large volumes of gas under high pressure. If the host rocks are strong enough and the gas is not removed, then the dissociation process quickly fades. When the host rocks are deformed and gas is filtered into the surrounding space, the closed system becomes open. Conditions for unsteady creep may arise in the soil massif, up to the stage of undamped creep and even progressive flow.

It is known from the theory of filtration consolidation that even a slight gas saturation leads to capillary overlap, which predetermines an increase in pore pressure and a slowdown in the consolidation process under load. This leads to the weighing of the soil, reduction of friction forces, weakening of the bonds between the soil particles. For example, methane formed in swamps, as well as in marine and lake sediments, can accumulate in the form of small gas bubbles, increasing pore pressure and reducing the permeability of sediments. This is largely due to the low density of young sediments ^[70]. S. And Rokos analyzed changes in the physical and mechanical properties of gas-saturated sediments of the Arctic seas with abnormally high intra-stratum pressure. It was found that the increased reservoir pressure reduces the lithostatic (household) and affects the stress-strain state of the soil mass. This is expressed in a decrease in strength and density with a simultaneous increase in fluidity, compressibility and porosity, which makes gas-saturated soils with excessive intra-layer pressure unstable to external dynamic loads and serves as one of the causes of numerous natural deformations. This happens in the case of tires that overlap the gas-saturated thickness. With a decrease in external pressure or an increase in temperature, the gas accumulated in the "mini-traps" expands. This causes crumpling and squeezing to the surface of the enclosing and overlying deposits. When the pressure in the gas-saturated horizon reaches a certain critical value, probably comparable to the resistance to the undrained shift of sediments of this thickness (about 5-30 kPa), deformation occurs. When opening such intervals, the gas evaporates, the reservoir pressure drops, which causes soil softening. In gas-saturated horizons not shielded by weakly permeable layers, the gas pressure does not significantly differ from the hydrostatic one. When these intervals are opened by wells, there is no significant pressure reduction and, accordingly, the properties of soils change only depending on the material composition and the degree of diagenetic transformations ^[43].

Under conditions of increasing temperature of frozen soils, the pressure of "trapped gas" in the pores will increase, which in turn leads to their decompression, a decrease in bulk weight and a corresponding decrease in strength.^[71,72]. A system of paragenetically related processes is beginning to develop: an increase in the content of unfrozen water, the appearance of brine cells, the formation of microcracks due to uneven thermal deformations of the mineral skeleton and ice, volumetric changes in gas inclusions.^[73,74] In the region of negative temperatures close to the values of intense phase transitions, the amount of loosely bound water in the capillaries increases sharply, some of it can pass into free.  Gas inclusions, expanding, form a system of filtration channels among themselves. This leads to an increase in gas permeability, as a result of which gas can be released from impermeable frozen soils ^[3].

The mechanism of gas filtration, in this case, is as follows. In gas-saturated rocks with a high water content retained due to capillary forces, the pore channels are blocked by water barriers, and the gas is in a dissipated state. Until a certain threshold limit value of the pressure drop is reached, gas movement through the porous rock does not occur. When the pressure drop exceeds the threshold, the parts of the barriers represented by loosely bound water deform and open part of the pore channels for filtration ^[75]. In frozen rocks, the barrier is the overlap of pores with ice formed when water freezes in the capillaries of the rock. When the temperature rises, part of the ice covering the capillaries passes into loosely bound water. It creates the possibility of a free gas under pressure, pushing and squeezing it to migrate to a region of lower pressures.

 These processes can have an impact on the stability of engineering structures. At some gas wells of the Bovanenkovsky and Kharasaveysky NGCM, there is an increase in the compressibility of permafrost rocks with an increase in their temperature without crossing the thawing boundary. This led to the development of deformations of the soil base with the risk of transition to an unacceptable state. It is noted that mining enterprises in the Arctic region have not previously encountered this process ^[76]. It can be reasonably assumed that the occurrence of subsidence with an increase in the temperature of gas-saturated frozen rocks, widespread in these territories, may be due to the squeezing of free gas from the zone of impact of engineering structures. Its filtration into the atmosphere leads to the release of greenhouse gases, to emissions in wells and pneumatic explosions that form gas emission funnels. The effect of free gas is especially enhanced during the decomposition of gas hydrates. In this case, a huge amount of gas is released inside the frozen strata. At the same time, the continuity of the rock is broken due to plastic and discontinuous deformations.

The materials considered in the article show the importance of taking into account the influence of the gas component on the properties of frozen gas-saturated soils. It is extremely necessary to develop approaches to assessing the influence of the gas component, especially in the form of free gas, on the physical and mechanical properties of frozen soils. Nevertheless, this issue is poorly reflected in both scientific publications and regulatory documents. In the Rulebook 11 – 114 – 2004 "Engineering surveys on the continental shelf for the construction of offshore oil and gas facilities"^[77] it is only noted that during engineering and geological surveys in the area of oil and gas fields, the presence of soils with accumulation of gases and abnormally high reservoir pressure (AVPD) should be established in the bottom sediments, In SP 11-102-97 "Engineering and environmental surveys for construction" ^[78] only issues related to the toxicity and explosiveness of biogas are considered, entering the basements of structures. The complex effect of gas-saturated soils on their physical and mechanical properties of soils, including frozen ones, is not mentioned anywhere else in Russian regulatory documents. A. S. Mashtakov, assessing the current state of this problem, notes that due to the lack of standards in Russia regulating laboratory dynamic tests of gas-saturated soils, as well as the use of their results in the design of engineering structures, the methodology and composition of the obtained research results is determined only by the technical specification of the customer. At the same time, the terms of reference for assessing the effect of free gas on soils are not compiled, and these studies are not carried out, no assessment of possible risks is given, there are no methodological recommendations for the study of gas-saturated soils ^[79].

- development of regulatory documents regulating the conduct of field and laboratory studies of the effect of gas saturation on the mechanical properties of frozen soils.

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