REGULARITIES OF PERMAFROST INTERACTION WITH GAS AND GAS HYDRATE DEPOSITS

Transcription

1 REGULARITIES OF PERMAFROST INTERACTION WITH GAS AND GAS HYDRATE DEPOSITS Nikolai N. Romanovskii 1, Genadi S. Tipenko 2 1. Department of Geocryology, Faculty of Geology, nromanovsky@glas.apc.org 2. Department of Mathematical Analysis, Faculty of Mathematics and Mechanic Moscow State University, Leninskie Gory, Moscow, , Russia Abstract Geothermal anomalies and anomalies of permafrost thickness are known to occur above gas deposits in the northern gas and oil fields of Russia (Baulin, 1985). A two-dimensional mathematical model was used to investigate the interaction between permafrost and gas/ gas hydrate deposits under the influence of long-term (periods of 100 and 40 Kyr) climatic fluctuations. Gas deposits at depths in excess of 800 m are affected by the longer 100 Kyr variation but exhibit only minor anomalies. In contrast, gas reservoirs at depths of about 400 m, i.e. near the base of permafrost, form anomalies comparable with those in nature. Positive and negative geothermal anomalies and anomalies of permafrost thickness appear during both its aggradation and degradation. Analysis suggests that these anomalies may be associated with the presence of secondary gas/ gas hydrate deposits formed above the primary gas deposits situated at depths of about 1000 m. Introduction In the gas and oil fields of northern Eurasia the position of the lower boundary of permafrost has long been known to be dependent on the character of local structures of the platform cover (Kudryavtsev,195). The thickness of permafrost was found to be smaller above the axial parts of anticlinal folds than above their limbs and above gently sloping strata. In contrast, in the central parts of geosynclinal folds the permafrost thickness was usually greater than above their limbs. Horizontally bedded strata of rocks did not perturb the subhorizontal position of the lower permafrost boundary (Fig. 1-A). In numerous instances, above the anticlinal structures the temperatures were higher and the permafrost thickness was reduced by 10-0% compared to areas of subhorizontal rocks; that is, the change in thickness ranged from a few tens of m to m. The occurrence of such positive geothermal anomalies is explained by the non-horizontal distribution of rocks differing in their composition and properties (as well as by the anisotropic properties of layered rocks in the normal and parallel directions relative to stratification). The differing properties induced redistribution of the geothermal flux such that it was enhanced in the central parts of structures and decreases in their limbs. Initially, the results of permafrost studies in wells drilled during gas and oil prospecting in the late 1950s - early 1970s seemed to confirm this regularity. However, subsequent data does not fit the above scheme. Above anticlinal gas-bearing structures, the permafrost boundary in some cases was found to be subhorizontal. In other cases, it was lower relative to the anticlinal limbs and formed a negative anomaly in its thickness (Fig.1-B). In the anticlinal folds of platforms, the gas deposits are usually located at axial depths of m. The thickness of permafrost at their axes reaches m, whereas permafrost is m thinner in the limbs. In this case the geothermal gradients and heat fluxes under the axial part of anticlines are lower than on the limbs. Similar negative geothermal anomalies above gas traps are also present outside of permafrost areas. In this latter instance, the negative geothermal anomalies are explained by ascending gas fluxes which expand and cool rocks above the gas deposit as an adiabatic effect; however, this explanation is unsatisfactory for areas of continuous permafrost. Ice-bonded permafrost forms cryogenic water- and gas-tight beds. Therefore, the flow of gases above primary gas structures should form secondary deposits of gases below the permafrost boundary. The causes and mechanisms of formation of secondary gas deposits were investigated by Istomin and Yakushev (1992). The concentration of gas hydrates (GH) in the pore spaces of secondary deposits was found to be higher than in the primary gas structures and to account for as much as 85% of this Nikolai N. Romanovskii, Genadi S. Tipenko 961

2 Figure 1. Typical position of the lower boundary of ice-bonded permafrost in geological structures of the platform cover (A) in the absence and (B) in the presence of gas deposits: 1 - stratification of deposits; 2 - gas deposit in anticlinal structure; - permafrost and its boundary. space. Secondary accumulations of gases and their hydrates may be located at relatively shallow depths both within the limits of actual permafrost and near its base. Gases often occur in the zone of gas hydrate stability (ZGHS) (Chersky et al.., 198) and are consequently capable of interaction with permafrost under the influence of external conditions. The interaction of gas deposits with permafrost is a relatively recent topic in geocryology (Romanovskii, 1986), associated with the concept of a common cause of the formation of permafrost and a zone of gas hydrate stability (Chersky et al.., 198) and the similarity of processes occurring during water freezing and ground ice thawing, on the one hand, and formation and decomposition of gas hydrates, on the other. This similarity includes the release/absorption of latent heat (potential energy) and changes in the thermophysical properties of earth material. However, the phase transition water <-> ice is virtually independent of pressure (P), whereas the transition (gas + water) <-> GH is determined by thermobaric conditions (Byk et al.., 1980; Makagon, 1974). At a permafrost thickness of m and more, the upper boundary of ZGHS of methane lies within permafrost, whereas the lower boundary lies below the 0 C geoisotherm. Dynamic changes in the upper part of the geothermal field of terrestrial permafrost occur subject to recurrent variations of the surface temperature (Kudryavtsev, 1978). These include aggradation and degradation of permafrost and ZGHS as well as modification of their boundaries. In local accumulations of hydrocarbons, i.e. gas deposits localized in the range of depths where the dynamics of the geothermal field induce the appearance and degradation of ZGHS, formation/decomposition of gas hydrates occurs accompanied by the above-mentioned processes. These processes are in turn conducive to disturbance of the geothermal field and changes in the configuration of the bottom surface of the permafrost. This process was discussed previously (Romanovskii, 1986) and subsequently confirmed by the results of test modeling (Tipenko et al.., 1990). Purpose of the study The current work investigates the interaction of terrestrial permafrost with gas (gas hydrate) deposits under 962 The 7th International Permafrost Conference

3 long-term temperature fluctuations at the earth's surface using mathematical simulations. As the dynamics of permafrost and changes in the position of ZGHS occur over lengthy intervals of geologic time, at present we can see only the resultant effects of such processes. Their dynamics can be studied only by means of mathematical models. The results of numerous studies carried out in the oil and gas fields of West Siberian lowland and of the Siberian platform are indicative of a rather high gas saturation of the upper part of sedimentary cover, including ice-bonded permafrost. In such areas, drilling and geophysical studies of boreholes are widely associated with the release of gases indicative of gas deposits. Secondary gas deposits are considered of no industrial importance at present. Although, they are ignored during gas and oil prospecting, they may in the future become non-conventional (non-traditional) sources of hydrocarbons. Choice of model and input data An anticlinal fold was simulated as the most typical gas-bearing structure in the sedimentary cover of platforms. The gas deposit may be localized at depths where the formation of ZGHS is possible and which are under the effect of long-term oscillations of temperature influencing formation of permafrost several hundred meters thick. These include climatic oscillations with periods (T) of 40 Kyr and 100 Kyr. In the Late Cenozoic the oscillations led to both aggradation and degradation of permafrost outside of glaciated areas. In southern regions permafrost was totally thawed, whereas in northern areas temperatures increased with concomitant reduction of permafrost thickness (Kudryavtsev, 1978). Climatic oscillations with shorter periods are effective at shallower depths and have little impact on the temperature field of the upper horizons of the main bulk of the permafrost. A necessary condition for the interaction of permafrost and natural gas deposits is the localization of such deposits at depths where changes of ZGHS boundaries take place. This range of depths is dependent on: the period of temperature oscillations (T); the period-average temperature of ground surface (t 0 ); the amplitude of temperature variation on the earth's surface (A 0 ); the geothermal gradient (g) and the gas composition. The gas composition determines the character of the equilibrium curve of hydrate formation (Byk et al.., 1980). Note that changes in t 0 allow simulation of geocryological zonality in models, whereas A 0 permits imitation of the range of changes in freezing conditions over a chosen time interval. These considerations determined the choice of input data. Computations were based on: g ranging from to 0.0 C /m, values that are characteristic of West Siberian lowland and other regions with both permafrost and a ZGHS; a range of t0 from -4.4 to -11 C reflecting contemporary conditions in the northern geocryological zone where the permafrost is believed to be of Pleistocene age. Values of A 0 were selected from literature for estimated ranges of Late Cenozoic climate variation in the abovementioned regions. The t 0 /A 0 ratio was used to model a theoretically known case: within the period of oscillations, permafrost will never thaw from the surface at /t 0 / > A 0 ; but it thaws from the surface at /t 0 / < A 0, while being preserved during part of the period as relict permafrost (Kudryavtsev, 1978). Computations were performed using a two-dimensional subsurface grid (a rectangle with Lx = 800 m, Ly = 1000 m) in which a gas deposit was localized at different depths in the form of the upper quarter of an ellipse with a horizontal semi-axis: ax = 500 m, and a vertical semi-axis: ay having variable values from 80 to 160 m. The variation of ay values made it possible to investigate the effect of the gas deposit shape on the specifics of the interaction under study. The upper part of the subsurface grid freezes and thaws under the effect of harmonic temperature oscillations with T = 100 Kyr and 40 Kyr under varying values of t 0 and A 0. Between the frozen and thawed (unfrozen) parts of models there is a mobile interface-i 1. At lower temperatures, the gas trapped in the deposit may become gas hydrate with mobile upper and lower interfaces. Heat transfer is by conduction alone. Our model omits possible processes of water and gas filtration, mechanical deformation of the earth, changes in the deposit size during reciprocal transitions (water + gas)<-> GH, and the kinematics of these processes. The last is fully justified because the time of mutual transitions (water + gas)< -> GH is within the range of s.(byk et al.., 1980). This time interval is incommensurably small compared to the geological time scale (Kyr). The model assumes that the gas is represented by methane, pressure- P is constant in the deposit and equal to the hydrostatic pressure of fresh ground water, and the temperature of its phase transition at the freezing front is 0 C. Given the above assumptions, the process of GH formation/dissociation in earth materials is formally described by analogy with the processes of freezing/thawing of water-saturated coarsely dispersed rocks (two-zone model). At interfaces ]{(gas+water)<->gh} phase transformations of the first kind occur similar to water <-> ice transitions. In this case the dependence of GH formation/ dissociation Nikolai N. Romanovskii, Genadi S. Tipenko 96

4 temperature (T ) on pressure (P) is taken into account. The curves reflecting equilibrium conditions of hydrate formation fit the equation T = a lnp + b (Tipenko et al.., 1990). The earth materials in the upper part of the geological section are represented by sandy loam and loam typical of the West Siberian lowland. The moisture content of the deposits is equal to their total water-absorbing capacity. This corresponds to the heat of phase transitions Qp = 97,200 kj/m. Thermophysical characteristics of frozen and thawed (unfrozen) sediments were taken from Ershov (1984). Thermophysical parameters of rocks saturated in GH and a vapour-gas mixture were as reported by Groisman (1985), while values of QGH which is energy of GH <-> (water + methane) formation/dissociation follow Tipenko et al.. (1990). In computations QGH was assumed to be 400 kj/kg. At a present reservoir porosity of 0%, QGH in a unit deposit volume was found to be 144,700 and 202,720 kj/m, respectively. The mathematical model using the above parameters and with the outlined limitations was described by Tipenko et al.. (1990). Numerical computation was performed using an implicit absolutely stable locally unidimensional difference scheme with an automated choice of time step. The initial distribution of temperatures was assumed to be a field with zero temperature on the surface and a linear variation with depth. Computations were started -4 periods before the onset of steady recurrent changes in the temperature field over the entire computational area. Modeling was performed for 16 variants, using the input data presented in Table 1. Table 1. Input data used in the mathematical model Results and discussion The computational results of mathematical modeling were used to create integral schemes of permafrost dynamics and behavior of gas/gh in the deposit (Figure 2). Analysis of these results leads to the following: 1. Computations demonstrated the occurrence of interactions manifested by distortions of the geothermal field and by the configuration of permafrost above the gas deposit. Positive and negative geothermal anomalies, including those accompanied by changes in the permafrost thickness, occur at different stages of the simulation of surface temperature variation, i.e. in both the permafrost aggradation and degradation phases. 2. At oscillations with T = 100 Kyr the deviations of the temperature field and the bottom surface of permafrost are substantially smaller than those induced by oscillations with T = 40 Kyr. At T = 100 Kyr, the differences in the permafrost thickness above (X = 0) and outside (X = 800) the deposit did not exceed m, i.e. they made up a few percent of the maximum thickness ( m). At T = 40 Kyr such differences amounted to m accounting for 10-25% of the ma-ximum permafrost thickness ( m).. The above considerations suggest that the interaction is most pronounced in shallow gas deposits at depths of m and with surface oscillations of T = 40 Kyr. Variant A-1 A-2 A- B-1 B-2 B-2* B- B-4 B-4* B-5 B-5* B-6 B-6* B-7 B-7* N¼Ê: t 0 ŸC A 0 ŸC g ŸC/m T, Kyr Depth of deposit foot, m Central height of deposit, m * Computations were done at Q GH =202,720 kj/m : computational results at same model parameters, except Q GH = kj/m, were reported by Tipenko et al. (1990). ** At depths of m a rock layer without phase transitions of water was simulated in deposit framing. 964 The 7th International Permafrost Conference

5 Figure 2. Diagram showing the modelled interaction between ice-bonded permafrost and gas/gas hydrate (GH) deposit; T = 40 Kyr: I-case, if the base of permafrost, at maximum thickness, does not reach the gas deposit top (/t0/ > A0,); II-case, if the geoisotherm 0 C does not reach the gas deposit at maximum thickness of permafrost (/t0/ > A0); III-case, with the gas- and gas hydrate deposit at /t0/ < A0 /; note the characteristic behavior of relict permafrost above the gas/gas hydrate deposit. Legend: (1) stratified ground composed of sandy loam; (2) loam, () clay and (4) sand; (5) deposit with gas hydrate and (6) gas; (7) permafrost and (8) its boundary. Letters on the earth surface temperature curve designate the times for which the various figures with ratios of permafrost to gas and GH-deposit are shown. Nikolai N. Romanovskii, Genadi S. Tipenko 965

6 4. The effect of gas deposit and deposit-based processes on the geothermal fields and on the permafrost thickness has been shown to increase when the freezing front approximates the upper deposit surface. This is most distinctly pronounced at times when phase transition (water + gas)< -> GH occurs within the deposit. Geothermal anomalies are enhanced with the increasing deposit thickness and with higher values of potential energy in the GH formation in a specific vo-lume of gas-bearing deposits. In the latter case variant B-1 (see Table 1) was compared with the results reported by Tipenko et al.. (1990). At greater depths a delay of temperature oscillations occurs which leads at certain stages to the appearance of two GH fronts in the deposit moving in opposite directions (Figure 2-III). 5. The largest absolute values of positive and negative anomalies at the permafrost base, amounting to 0-70 m at the deposit centre (X = 0) or 10-25% of the thickness of m, proved to be smaller than the maximum values observed under natural conditions (Baulin, 1985). This seems to be indicative of the existence of additional factors (for instance, layered rocks and anisotropy of their properties in normal and parallel directions) responsible for large anomalies in permafrost thickness above the gas-bearing structures. 6. When the base of permafrost reaches the GH interface and moves downward, the 0 C isotherm descends rapidly forming a large (0-45 m) negative temperature anomaly (Figure 2-II). This occurs due to the absence of heat absorption or phase transitions in the deposit with GH, whereas outside the deposit the propagation of the freezing front boundaries is relatively slow because of the occurrence of phase transitions. 8. At t 0 < A 0, at the stage of surface temperatures above 0 C the permafrost thaws from above forming a relict permafrost layer. This layer disappears completely primarily under the gas deposit when the latter is in the GH state because GH-bearing rocks have a higher heat conductivity than the unfrozen rocks in its framing. Above the deposit a thawed "gap" forms in the relict ice-bonded permafrost (Figure 2-III). Conclusions The authors assume that most of the gas deposits affecting the geothermal field and the configuration of permafrost are situated at depths of m, near the lower permafrost boundary. These gas deposits are of secondary origin and form as a result of gas emission from reservoir structures localized at greater depths. Under conditions of ice-bonded permafrost, the ascending gas fluxes are blocked, and their constituent gases pass into the GH state. Therefore, anomalies in permafrost temperature and thickness may be used as an indicator in prospecting for secondary gas/gh (subpermafrost) deposits and may also be indicative of the presence of deeply located primary deposits. Acknowledgments This study was partly supported by the Russian Foundation for Basic Research. The authors would like to express sincere gratitude to our reviewers: Dr. Keith A. Kvenvolden and Dr. Al Taylor who helped to improve the language and contents of the paper. 7. Degradation of GH occurs in deposits both at the bottom and on the two sides: on the side of deposit top and on the side of its base; both fronts are often curvilinear leading to a complex shape of the GH portion for the deposit (Figure 2-III). References Baulin, V. V. (1985). Perennially Frozen Rocks in Gas and Oil Fields of the USSR. Nedra, Moscow, 176 pp. (in Russian). Byk, S. Sh., Makagon, Yu. F., and Fomina V. I. (1980). Gas Hydrates. Chimera, Moscow, 296 pp. (in Russian). Chersky, N. V., Tsarev, V. P., and Nikitin, S. P. (198). Study and Prediction of Conditions of Gas Resource Accumulation in Gas Hydrate Fields. Yakusk, 156 pp. (in Russian). Groisman, A. G. (1985). Thermophysical Properties of Hydrates. Nauka, Novosibirsk, 94 pp. (in Russian). Ershov, E. D. (1984). (Ed.). Thermophysical Properties of Rocks. Moscow State University Press, 20 pp. (in Russian). Istomin, V. A. and Yakushev, V. S. (1992). Gas Hydrates under Natural Conditions. Nedra, Moscow, 26 pp. (in Russian). Kudryavtsev, V. A. (195). Temperatures of Upper Horizons of Permafrost in the USSR. USSR Academy of Sciences Press, Moscow, 18 pp. (in Russian). Kudryavtsev, V. A. (Ed.) (1978). General Geocryology. Moscow State University Press, 462 pp. (in Russian). Makagon, Yu. F. (1974). Hydrates of Natural Gases. Nedra, Moscow, 208 pp. (in Russian). Romanovskii, N. N. (1986). Gas Hydrate of Underground Hydrosphere of Permafrost Area.Vestn. MGU, Ser. 4, Geology, No., 7-17 (in Russian). Tipenko G. S., Sergeeva N., Komarov I., Romanovskii N.N. (1990). Modeling of Permafrost and Gas/ Gas Hydrate Deposit Interaction. Vestnik MGU, ser.4á Geology, No. 4, 7-84 (in Russian). 966 The 7th International Permafrost Conference