1 Introduction

Towards the end of the 20th century, coal was losing its position as the world’s most prolific energy source. A number of coal mines were closed, including, for example, the Pyramiden on the archipelago of Svalbard, Norway, and the Seredeiskaya mine in the Moscow basin. However, coal has regained its position as a key energy supplier due to its three advantages over oil and gas, namely lower price per energy unit, different geopolitical distribution of reserves, and a higher reserves-to-production ratio (Kavalov and Peteves, 2007). Unfortunately, the mining and burning of coal is not environmentally friendly and much of the coal in the ground is either too deep or too low in quality to be mined economically (Walter, 2007).

Underground coal gasification (UCG) is a solution at least to the deep and poor coal seams. It is one of the physical-chemical-geotechnical methods of coal mining. The method is not restricted to purely burning coal; some successful experiments on underground sulphur and shale burning have also been conducted (Miller, 1964; Arens, 1986).

UCG has several advantages over traditional mining. Its benefits include applicability in areas where conventional mining methods are not suitable and that it reduces or even eliminates human work underground. In the simplest scheme, only two boreholes are required—one for oxygen ignition and the other for production. The product of coal gasification, synthesis gas or syngas, is easy to handle and can be used as fuel. Moreover, the method can be coupled with carbon capture and storage (CCS) by injection of CO2 in the void left after UCG. According to MacDonald (2010), UCG is the cheapest way to produce electricity in comparison with traditional mining.

2 Surface subsidence

Ali et al. (2012) emphasised that ground subsidence is probably the most important single obstacle to the commercialisation of UCG. This phenomenon may cause swamping the territory. Ground deformation caused by UCG has the potential for large-scale detrimental effects, including initiation of flow paths between underground aquifers and damage to surface structures and buried infrastructure. Zhukov (1963a) pointed out the importance of the knowledge of surface subsidence for gas generator design and technology of the gasification. For example, Zhukov et al. (1963) argued that wells located in the middle of the trough have less possibility to be damaged.

The magnitude and form of subsidence depends on multiple aspects, such as a seam depth, its thickness and dip angle, physical-mechanical properties of the geomaterials above and under the seam, the initial stress conditions, in-situ fractures and groundwater. Skafa (1960) indicated four types of surface subsidence behaviour after UCG: no ground surface movements, smooth bending, bending with fractures, and crater (sink hole).

There are several aspects which differentiate surface subsidence during UCG from the conventional mining methods. During UCG, rocks are subject to one or both of mechanical and thermal loads. In addition, because coal burning occurs from the bottom to the top of the seam, vertical displacements are observed to occur at a slower rate compared with conventional mining (Turchaninov, 1957a). As a result, the bulking factor of the overburden rock is smaller (Turchaninov, 1957a), hence, greater surface subsidence is expected. However, during UCG the void is filled with slag and ash which can also mitigate the surface sag.

It can be concluded that subsidence during UCG is a complicated process which deserves further investigation. Tian (2013) pointed out the need for broader knowledge on high-temperature mechanical behaviour of coal, the underground temperature distribution during UCG, and field measured surface subsidence data. Zamzow (2010) argued that the subsidence behaviour from industrial scale projects was not clear. The overview on the Soviet UCG projects presented here extends the knowledge on these issues.

3 Soviet UCG projects

Gregg et al. (1976) summarised the Soviet experience at that time and came to the conclusion that “the amount of UCG research effort expended by the Soviets far exceeds the summation of research efforts by other nations”. The detailed history of the gas and coal industry in the Soviet Union was described by Gregg et al. (1976), Antonova et al. (1990), Klimenko (2009), Matveichuk and Evdoshenko (2011), and Kopytov (2012). The history of UCG in the Soviet Union began quite early and one of the first scientists to mention the possibility of coal extraction without conventional mining was Dmitri I. MENDELEEV in the early 1880s (Mendeleev, 1939). Kuprin (1971), the famous Russian writer, mentioned the UCG process in one of his stories in 1899. This idea was accepted with great enthusiasm by Lenin (1973) and this was one of the decisive factors that drove UCG development in the Soviet Union. The Krutovskaya station was the first UCG project in the Soviet Union which was unsuccessfully conducted in 1932 (Kolesnikov, 1935; Gregg et al., 1976). Later efforts were more successful. The experience was not limited by one horizontal coal seam, but included steeply dipped coal seams (Kazak, 1965; Kreinin, 2010) and several interleaved coal seams (Lazarenko et al., 2006). The effect of permafrost on UCG has also been studied (Gusyatnikov, 1940).

Unfortunately, access to the UCG material is complicated because the papers are almost unavailable as electronic copies and not presented in the international journals because the Soviet science was mostly conducted with the scientists in solitude (Kapitsa, 2010). However, there is some evidence of collaboration on UCG between the Soviet Union and the USA; for example, a licence agreement with Licensintorg (the international technology exchange enterprise) of the Soviet Union and the American company ‘Texas Utilities’ for technical documentation and assistance in UCG (Clements, 1977). Clements (1977) reported that they had obtained documentation with data on UCG in various types of coal deposits and visited two sites in different geological basins but the outcomes could not be disclosed. There is a summary of the Soviet studies focused on surface subsidence in English done by Gregg (1979), but the author was limited by the availability of the translations from the Russian language and this makes a present review of the Soviet experience valuable.

The Soviet UCG activities were mainly focused in four basins at Angren, Moscow, Donetsk, and Kuznetsk. The first three letters of their names and the word “basin” with double “s” due to the Russian language pronunciation constitute alternative second names for the latter three basins—the Mosbass, Donbass, and Kuzbass. In the literature available in English, both names are used. Table 1 presents the UCG stations with the seam characteristics in these basins.

Table 1 Soviet UCG stations
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4 Measurements at the sites

Almost the same field measurements were organized for all sites. For example, in the Moscow basin, the initial vertical fractures were measured in the rock outcrops. They were generally non-uniform; however, there was some regularity of the directions (Vinogradov, 1963). Vinogradov (1963) quoted V.G. VALENTSOV based on their personal communication that the production well and the injection well connected better in the directions of the fractures. Thus, the rates of coal burning in different directions could be a hint of the orientation of the fractures. The control of surface and underground space deformation and observation of the underground space after UCG was also conducted. In the Podmoskovnaya station in 1949, a square geodetic net of reference points were established with a spacing of 5–10 m that covered a gasified area of 70 000 m2 along with deep reference points installed in boreholes which measured vertical deformations of different layers (Fokin, 1954). In 1952–1954, the strata were studied by new boreholes or shafts dug into the used UCG reactors (Semenenko and Turchaninov, 1957; Kazak and Semenenko, 1960). Ovchinnikov et al. (1966) reported about a geodetic net and deep reference points in five boreholes as well as excavating the gas generator after 60% coal gasification at the Yuzhno-Abinsk station. The results of the measurements will be discussed and analysed in the following sections.

5 Strata deformations

5.1 Overview

The reported magnitudes of surface subsidence resulting from UCG in different basins range from 0.5 m to 10 m (Table 2) due to different seam depths, seam inclinations (from 0° to 80°), seam heights, seam widths, ash content in the coal, and different geological profiles. These factors will be discussed further in subsequent sections.

Table 2 Maximum subsidence in the different basins
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Horizontal deformations are also important to study together with settlement depths. Horizontal strata movements reduce the maximum subsidence depth but increase the size of the affected area. At the Kuznetsk basin, the tensile horizontal deformations were +220 mm/m and compressive horizontal deformations were −160 mm/m (Ovchinnikov et al., 1966). After a field study of the damaged strata using the exploitation boreholes at the Angren station, it was noticed that the horizontal displacements played a crucial role in the distortion of the boreholes (Zhukov et al., 1963). It also should be highlighted that horizontal deformations can impact on the measurements of surface settlements.

5.2 Role of coal seam inclination in subsidence

The seam inclination plays an important role in the type of the surface subsidence. For horizontal deposits, the bending mechanism of subsidence is typical, whereas for synclined deposits, a crater type subsidence is generally observed. According to the description of the subsidence by Turchaninov and Zabrovsky (1958) and Ovchinnikov et al. (1966), it can be concluded that the Yuzhno-Abinsk station with a 70° dipped seam had a crater type subsidence with fractures propagating up to the surface. Ovchinnikov et al. (1966) reported shear fractures at the ground surface and Zabrovsky (1959) observed gas on the surface which indicated that the fractures spread to the surface. Opposed to this, Turchaninov and Sazonov (1958) observed that at the Shatskaya station with a horizontal seam, fractures did not propagate to the surface. According to the contour maps of the subsidence by Turchaninov and Sazonov (1958), it can be concluded that the Shaskaya could be characterised by the second (smooth bending) and third (bending with fractures) types of the subsidence. At the same basin, the Moscow basin, the Podmoskovnaya station has the same type of the subsidence according to the contour maps of the subsidence by Skafa (1960). The contour maps of the subsidence by Zhukov and Orlov (1964) show that the Angren station with a seam dipped at a small angle of 5° also showed the second and third types of the subsidence.

5.3 Role of coal seam thickness in subsidence

The coal seam thickness influences the subsidence depth directly. This is illustrated by considering the Kuznetsk basin, where Table 1 shows that the thickness of the coal seam is the largest (9.2–9.8 m) and Table 2 shows that this basin also has the largest subsidence (2.2 m).

The coal seam thickness impacts on the height of the distressed zone that is a combination of fractured and caved zones above the gas generator, which is one of the factors affecting subsidence behaviour. Kazak and Semenenko (1960) suggested that if the coal seam is 0.4–0.5 m, the vertical deformation of the seam roof is smooth, without failure. Skafa (1960) postulated that the height of the distressed zone for the Lisichansk station was ten times the coal seam thickness. For this station, Kazak and Semenenko (1960) reported almost the same values: the distressed zone was six to eight times the coal thickness and no failure was observed.

After laboratory experiments Zhukov (1963b) came to the conclusion that the thickness of the coal seam plays an important role in fracture opening within the caved zone. A 1:100 scale model of the Angren station showed that gasification of a coal seam up to 4 m thick caused fractures 15–20 m up from the seam, with small openings observed. For a 4–6 m thick coal seam, a net of fractures, sometimes with wide openings was observed, and for a 6–8 m thick coal seam, fractures with wide openings were observed. Kazak and Semenenko (1960) reported the absence of the fracture net and through fractures at the Podmoskovnaya station with a 2.5 m thick coal seam.

5.4 Role of strata in subsidence

5.4.1 Role of weak strata in subsidence

The existence of a weak strata decreases the time of the response of the ground surface to the UCG. At the Shatskaya station with weak strata (refer to the borehole log in Table 3), the first surface subsidence was observed on the 34th day after ignition (Turchaninov and Sazonov, 1958). At the Yuzhno-Abinsk station with mostly rock material profile, the first surface deformation was noticed eight months after ignition (Ovchinnikov et al., 1966).

Table 3 Borehole log at the Shatskaya station (Turchaninov and Sazonov, 1958)
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5.4.2 Role of sand in subsidence

The gasified area at the Shatskaya station was further spread from the production wells due to the presence of sand in the roof and floor of the seam, which conducted oxygen and increased the area of the burn (Turchaninov and Sazonov, 1958). This distance was wider at the Shatskaya station (15 m) than that at the Podmoskovnaya station (6–8 m) (Turchaninov and Sazonov, 1958). However, the depths of the surface subsidence for both of these stations (in the Moscow basin) do not differ significantly.

5.4.3 Role of limestone in subsidence

A layer of limestone (being relatively strong) above the burn tends to smoothen the subsidence trough; however, the state of the limestone is also important. The Shatskaya and Podmoskovnaya stations are in the same basin. The profile of the Shatskaya station (see the borehole log in Table 3) includes more limestone (24% of the vertical profile) whereas the Podmoskovnaya station has a vertical profile that includes only 10% limestone (Turchaninov and Sazonov, 1958). The schematic borehole log given by Semenenko (1965) shows that the locations of the limestone are at mean depths between 19.0 m and 30.0 m, which are 18 m and 29 m above the coal seam. Turchaninov (1957b) did not notice any significant difference between these sites and concluded that the limestone did not influence the trough development because it was weakened by fractures. For the Lisichansk station, Kazak (1965) presented three roof borehole logs from the Donetsk basin (Table 4): one was before UCG and the other two were after. Limestone is presented along the whole profile and contributes to the shallow subsidence of 0.5 m (Semenenko and Turchaninov, 1957). According to the contour map of the subsidence by Semenenko and Turchaninov (1957), a smooth subsidence trough was observed. The difference between Boreholes 2 and 3 will be discussed later.

Table 4 Bottom height of the layers above the coal seam at the Lisichansk station (Kazak, 1965)
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5.5 Rate of surface subsidence

The rate at which surface subsidence occurs (typically in mm/day) is important because it can provide a hint to how the UCG process has advanced and organize mitigation measures to minimize the effect of subsidence on near-surface structures and infrastructure. The subsidence velocity can also give an idea of the overburden strata’s bulking factors.

A strong overburden stratum can reduce the subsidence velocity. In the Moscow basin with weak strata, the subsidence velocity was the highest when compared with the other three basins. At the Shatskaya station, the mean subsidence velocity was 25 mm/day with a maximum of 40 mm/day (Turchaninov and Sazonov, 1958). In the Kuznetsk basin, the maximum subsidence velocity reached the mean velocity in the Moscow basin—25 mm/day (Ovchinnikov et al., 1966). In both basins the depths of the coal seams were approximately 50 m below the surface but the Kuznetsk basin has stronger strata than the Moscow basin.

The subsidence rate also reduces with an increase in depth of the coal seam. At the Angren site, the coal seam is at a depth of 110–120 m and the subsidence was slower than that in the basins described above, with a maximum of 5 mm/day (Zhukov et al., 1963). In the Donetsk basin, the subsidence velocity was the slowest, 1 mm/day (Semenenko and Turchaninov, 1957). Possibly, this is because it has the deepest coal seam (up to 400 m).

The presence of the strong limestone in the profile causes a constant subsidence velocity over time. Table 5 presents two borehole logs which are located at a distance of 450 m from each other in the Angren basin. Table 5 shows that the location of the limestone is far away from the failure zone and near the surface in the Angren basin. However, the process of the surface subsidence had a constant velocity, and this is believed to be due to the limestone layer (Zhukov et al., 1963).

Table 5 Two borehole logs in the Angren basin (Zhukov et al., 1963)
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6 Impact of coal burning in-situ

6.1 Thermal geomaterial conductivity

The main differences between UCG and conventional coal mining are exposure of the geomaterial to the high temperatures and the products of burning that are left in the void. According to Turchaninov (1956), the temperature was more than 1500 °C in the generators in the Donetsk basin. However, Turchaninov (1956) believed the temperature was lower in the Moscow basin due to the coal’s lower heat conductivity because the air was injected instead of oxygen.

The high temperature impact of UCG is a complicated process. The information about thermal geomaterial conductivity, calculation of heat losses for heating wet soil, and strata physical-mechanical properties under thermal conditions will be discussed. The ash properties will be also considered.

Semenenko and Turchaninov (1957) claimed UCG heats rock and soil over only a relatively small distance away from the UCG generator. Russo and Kazak (1958) agreed with this fact but pointed out that the spread of the heat mainly occurs due to the convection of hot gas through the fractures that appear near the generator.

The energy conductivity of the coal in the seam is very small (Kolesnikov, 1935), but the real conductivity could be much higher due to fractures. Kolesnikov (1935) reported a temperature decrease of 10 °C/m at the Moscow basin. Kazak and Semenenko (1960) gave some data on soil heating at the Lisichansk UCG station.

Fig. 1 shows that the temperature reduces to less than 100 °C at a distance of 3 m above the seam and 4 m below (the lowest depth where measurements were conducted). At a distance of 10 m above the seam, the thermal effects from UCG were not observed.

Fig. 1
figure 1

Distribution of soil temperature after UCG at the Lisichansk station (modified from (Kazak and Semenenko, 1960))

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Kazak et al. (1990) observed at the Yuzhno-Abinskaya station that the temperature dropped sharply from 1000 °C to 400 °C in the lower, non-structured, part of the caved zone, then the geomaterial fell from the roof to the bottom of the seam, and the temperature does not change significantly for the rest of the caved zone. Kazak et al. (1990) explained that the geomaterial of the lower part falling from the roof piece by piece is exposed to direct burning but the other material is subsequently heated due to conduction. Kazak et al. (1990) suggested an equation for heat loss during conduction:

$${{{Q_1} + {Q_2}} \over Q} = {{350} \over {{q_{\rm{y}}}}} \cdot {{{m_{\rm{c}}}} \over m} + {{100.7\sqrt l } \over {m\sqrt v {q_{\rm{y}}}}},$$
(1)

where Q1 represents the convective heat losses (J), Q2 represents the conductive heat losses (J), Q represents the general heat produced by UCG (J), mc is the thickness of the isothermic area (m), l is the width of the isothermic area (m), m is the thickness of the gasified area (m), v is the velocity of the face development (m/day), and qy is the heat of the coal burn (J/m3).

6.2 Calculation of heat loss due to evaporation

The magnitude of heat loss due to evaporation in wet ground during UCG has been considered. Based on the assumption by Stefan (Riemann and Weber, 1927), Lykov and Pomerantchev (1935) analytically showed that the evaporation surface would expand into the soil according to the equation:

$$s = \alpha \sqrt t,$$
(2)

where t represents the time (h), and α represents a coefficient which depends on the thermal heat conductivity coefficient, dry soil density, absolute soil moisture, soil surface temperature, soil temperature, and vaporisation temperature.

After modification, it is possible to obtain Eq. (3) to determine the amount of the evaporated water (kg/m2).

$$w = \alpha {\rho _1}{W_{\rm{a}}}\sqrt t,$$
(3)

where ρ1 represents the dry soil density (kg/m3), and Wa represents the absolute soil moisture.

In two calculations involving heat loss in soils, α was taken as 0.0455 and 0.0480. These values were estimated based on mathematical calculations using the parameters characterising the heating technique and for the prescribed physical constants.

6.3 Strata under thermal impact

During UCG, the strata are subject to both mechanical and thermal loads. Gerdov (1940) argued that the thermal impact on different strata could be very different and each case needs to be studied individually.

Under high temperatures, the strength of the geomaterial can either increase or decrease. Generally, over the range of the UCG temperatures, the laboratory tests show that the geomaterial strength increases. The uniaxial compression strength of shale at the Lisichansk station was shown to increase from 7.7 MPa to 40.7 MPa after UCG (Russo and Kazak, 1958). According to Russo and Kazak (1958), the strength of a sample of shale with high SiO2 from the Lisichansk station increased from 24.2 MPa at 0 °C to 87.0 MPa at 900 °C.

Fig. 2 shows the relationship between temperature and strength of clay at the Mosbass, and rock of the Donbass and Kuzbass. The common trend is that mainly the strength increases with the temperature until a particular temperature, for example, 800 °C for the sandy clay and 1000 °C for the organic clay with high coal content, so-called coaly clay. After these temperatures, the strength decreases due to the agglomeration of the soil particles (Semenenko and Turchaninov, 1957). The strength of the sandy clay increases more rapidly until 400 °C due to water evaporation (Semenenko and Turchaninov, 1957). In Fig. 2, the rocks increase in strength less than the soils; however, the coaly clay has the opposite behaviour. At the beginning of burning, the strength decreases until 400 °C is reached because the coal particles burn and fracture (Semenenko and Turchaninov, 1957). This agrees with Ruschinsky (1952) who concluded that the compressive strength of the Moscow basin coal reduces from 1.61–2.02 MPa to 0.70–0.75 MPa under the thermal impact and after coal burning, and the left ash has strength of only 0.02–0.04 MPa. Fig. 2 also shows the strength of the clay increases almost linearly with temperature. Semenenko et al. (1952) pointed out that the clay lost its plastic properties under high temperatures.

Fig. 2
figure 2

Compressive strength under different temperatures (for soils modified from (Semenenko and Turchaninov, 1957), the unit of compressive strength is MPa; for rock modified from (Antonova et al., 1990), the unit of compressive strength is × 10 MPa)

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6.4 Change in volume

The other effect of the thermal impact on the strata is volume change. This expansion or contraction in the strata volume can markedly reduce or increase surface subsidence. Table 5 shows two different borehole logs after a UCG event. Borehole 2 has 0.75 m of slag, and borehole 3 has 1.4 m of slag. The deformation of the strata over borehole 3 was insignificant and failure was not observed (Kazak, 1965). Kreinin and Kogan (1963) observed that for the coal, the highest rate of the increase in volume was at 350–450 °C.

Gerdov (1940) conducted several thermal experiments on the strata samples from the Donetsk and the Moscow basins. The cylindrical samples, 50 mm long and 35 mm in diameter, were kept in a stove and under no load as well as a constant load of 0.5 MPa. Gerdov (1940) came to the following conclusions:

  1. 1.

    The Donetsk basin limestone. The 600 mm× 45 mm×55 mm sample starts sagging while set on two supports without load at 1295 °C. At 1365 °C sagging reaches 50 mm. The sample becomes powder (CaO) at 1395 °C and loses about 50% volume.

  2. 2.

    The Moscow basin clay. The melting temperature is quite high at 1730 °C and an initial soil increase in volume is observed at 600–800 °C.

  3. 3.

    The Donetsk basin shale starts deforming at 860–940 °C under a constant load of 0.5 MPa with plastic deformation starting at 1000–1140 °C. The deformation ends at 1030–1250 °C, and at 1470–1580 °C the rock melts. The shale of the Moscow basin starts deforming, increasing in volume at almost the same temperature of 970 °C. Fractures appear at 970–1100 °C without exfoliation, and melting starts earlier at 1000 °C.

The laboratory experiments by Russo and Kazak (1958) showed that the coefficient of the volume increase, the so-called swelling coefficient, for the shale of the Lisichansk station rises non-linearly over 1000–1200 °C (Fig. 3) and the plastic state is reached at 1200 °C. The swelling coefficient can be as high as 2.2. Moreover, the in-situ volume increase is greater than the theoretical extrapolation of this value because of the increase of fractures and porosity in the bulk material (Russo and Kazak, 1958).

Fig. 3
figure 3

Swelling coefficient under different temperatures (modified from (Russo and Kazak, 1958))

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Porosity is partly responsible for the volume change in a geomaterial which is an important factor of soil deformation (Chen et al., 2014). Fig. 4 shows that within the sandy clay (the Moscow basin) porosity does not change greatly with increasing temperature. However, the clay porosity increases at low temperatures (200–600 °C) and decreases at high temperatures (600–1200 °C).

Fig. 4
figure 4

Changes of porosity under different temperatures (modified from (Semenenko and Turchaninov, 1957))

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6.5 Role of the ash in subsidence

One more feature of UCG is that the void is partly filled with ash after the underground burn of the coal. Turchaninov (1956) pointed out that the physical-mechanical ash properties can have an impact on the ground surface subsidence. According to Gregg et al. (1976), the coal in the Moscow basin has the highest ash content of up to 60%, whereas the others have approximately 10% ash content. Turchaninov (1956) gave ash shrinkage vs. pressure curves (Fig. 5) for two samples of 14.1% and 21.0% ash contents taken at the Podmoskovnaya station and the Moscow basin.

Fig. 5
figure 5

Ash shrinkage vs. pressure (modified from (Turchaninov, 1956))

Sample 1: Ac=21.0%; Sample 2: Ac=14.1%

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Fig. 5 shows that ash shrinkage decreases with pressure, and at pressures greater than 1 MPa very little change in volume occurs. Turchaninov (1956) provided Eq. (4) to determine the volume of remaining ash (Vash) after the UCG burn based on experimental results.

$${V_{{\rm{ash}}}} = 0.014{A_{\rm{c}}}{V_{\rm{y}}},$$
(4)

where Ac is the ash content in the coal (%), and Vy is the volume of the gasified coal (m3).

The pressure on the goaf increases with the distance from the face during conventional mining. In Fig. 6, there is dependence between the pressure and the distance from the face after failure at the Moscow basin. Turchaninov (1956) suggested that it is the same for UCG but the transition should be smoother.

Fig. 6
figure 6

Pressure vs. the distance from the face (modified from (Ruschinsky, 1952))

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6.6 Groundwater

The temperature and mineralization of the groundwater can be a key to the UCG thermal effect and it should be investigated thoroughly (Kreinin et al., 1991). According to Kreinin et al. (1991), the UCG area has an abnormally high water temperature and this can be seen 20 years after the burn at the Yuzhno-Abinsk UCG station. Moreover, higher groundwater mineralization was noticed near the UCG reactor (Kreinin et al., 1991).

7 Conclusions

This paper has provided a review of some literatures describing the Soviet experience of the UCG, with the main emphasis on ground movements. The main source of the literatures was the National Library of Russia in Saint Petersburg. The papers reviewed were rather old, some being issued before World War II, and most of them are only available as hard copies. Two additional libraries could be useful sources to obtain further information: the Russian State Library in Moscow and the Library of the Russian Academy of Sciences in Saint Petersburg (access is limited). In this paper, the focus is on early work on UCG because more recent developments have shifted from the countries of the former Soviet Union to other regions mainly due to discovery of the large natural gas deposits in the Soviet Union. Today, there is only one station, Angren station in Uzbekistan still operated by Yerostigaz, a subsidiary of Linc Energy, and recently it has been announced that Coal Bed Methane Partners, a subsidiary of Red Mountain Energy, launched the first UCG project in Russia for many years.