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The application of basin modeling of oil and gas systems based on the capillary-gravity concept

The application of basin modeling of oil and gas systems based on the capillary-gravity concept GEOLOGY, ECOLOGY, AND LANDSCAPES INWASCON https://doi.org/10.1080/24749508.2022.2085852 RESEARCH ARTICLE The application of basin modeling of oil and gas systems based on the capillary-gravity concept E.Y. Neyolova and A.A. Ponomarev Department of Oil and Gas Deposits Geology, Tyumen Industrial University, Tyumen, Russia ANNOTATION ARTICLE HISTORY Received 1 April 2022 Oil and gas reservoirs exhibiting low reservoir properties often have significant deviations in Accepted 31 May 2022 structure of hydrocarbon reservoirs they contain, based on anticlinal theory of oil and gas accu- mulation. First, these deviations are related to sharp fluctuations found in water-oil and water- KEYWORDS gas interfaces and failure to subordinate the shape of deposits to structural factor. When dealing Oil; gas; reservoir; capillary with such objects, geologists often have to assign unreasonable structural deflections, various pressure; interfacial tension; impermeable seals of tectonic or sedimentary origin whose presence is not always confirmed by capillary barriers seismic and drilling data. The reason for such difficulties is that anticlinal theory does not consider capillary forces counteracting hydrocarbon migration to be key to reservoir formation. Any oil and gas reservoir is a multiphase pore system with countless contacts, both between different fluids and in host pore space. As per molecular physics laws, the major role in oil and gas water distribution in such systems is controlled primarily by capillary properties of the environment such as capillary pressure at water-oil interface, interfacial tension magnitude, pore channel radius and wetting behavior of solid phase. Due to capillary forces, sharp fluctuations at water-oil interfaces and significant shifts of oil and gas reservoirs relative to vaults of anticlinal structures are observed Introduction in the world so far. Most of the major fields that form the basis of the global oil industry were When dealing with oil and gas geology problems, discovered in the mid-20th century geologists often encounter phenomena that con- (Tsoskounoglou et al., 2008). To this day, the tradict the generally accepted understanding of the anticline concept is the basis for exploration conditions under which oil and gas deposits are plans, oil and gas reservoir modelling, reserves formed. Very often these contradictions are asso- estimation and development design. However, the ciated with deviations in the structure of hydro- number of large reservoirs discovered in recent carbon reservoirs from the principles of anticlinal decades has been declining rapidly. It is clear theory of oil and gas accumulation. This theory that new methods for prospecting and exploring has been effectively applied for many years to the for hydrocarbons are needed to maintain world design of oil and gas exploration for geological production levels. targets of simple structure characterised by high In addition, using the conventional approach reservoir properties. The anticlinal-gravity concept when dealing with complex geological targets char- of oil and gas accumulation dates back to the mid- acterised by low reservoir properties, geologists 19th century, when the American oilman known often have to assign oil and gas reservoirs to var- as Colonel Drake suggested a link between oil ious impermeable screens, of tectonic or sedimen- deposits and anticlinal uplifts. Later on, on the tary origin, the presence of which is not always basis of this method, any objects confined to ele- confirmed by seismic and drilling data. vated reservoir areas in structural plan began to be The reason for these difficulties lies in the fact considered promising. The essence of the anticli- that anticlinal theory does not consider as key, nal concept is that the formation of hydrocarbon forces resisting the migration of hydrocarbons deposits is due to their buoyancy forces. both during formation of the deposit and during According to modern petroleum geology text- its development. The main forces resisting oil books for two centuries, the first and most impor- migration are various capillary effects. Any oil and tant component necessary for oil accumulation gas reservoir, irrespective of the conditions of its and reservoir formation is structural uplift (Ali, formation, facies component and tectonic processes 2017). Based on the approaches used by the anti- occurring both during its formation and operation, clinal concept of oil and gas accumulation, more is a multiphase pore system with an infinite than 50,000 oil and gas fields have been discovered CONTACT A.A. Ponomarev ponomarevaa@tyuiu.ru Department of Oil and Gas Deposits Geology, Tyumen Industrial University, Tyumen, Russia © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 2 E. Y. NEYOLOVA AND A. A. PONOMAREV number of contacts both between different fluids and gas accumulation, this discrepancy in the distri- bution of fluids saturating the reservoir is due to the and in the surrounding pore space. According to presence of various screens, of tectonic or sedimento- the laws of molecular physics, the main role in oil logical origin, which, however, are often not con- and gas water distribution in the pore multiphase firmed by either drilling or seismic data. Figure 1 system is controlled mainly by capillary properties shows a schematic of the reservoir structure in terms of the medium, such as capillary pressure value at of the anticline-gravity concept of oil and gas accumu- the water-oil contact, interfacial tension value, pore lation. The shape of the deposit corresponds to the channel radius and nature of solid phase wetting, structural structure of the reservoir roof, but the oil- i.e., hydrophilicity or hydrophobicity of reservoir saturated part of the deposit near well 706 is separated rock. The combination of these conditions and from the gas-saturated part located at the same hypso- characteristics results in a certain unique oil and metric level by a zone of claying, which has the shape gas trap. As the famous American geologist of some “ancient channel” whose presence has not (A. I. Levorsen, 1948): each oil field is unique in been confirmed by drilling data. This “laced” form of that it has its own development and its formation occurrence of geological bodies according to the con- can be considered the end result of the interaction cepts of classical palaeogeography is characteristic of of many variables. And to such causes, first of all, sand bodies separating impermeable rocks rather than the differently directed action of capillary and grav- of clay impermeable rocks. itational forces, especially in reservoirs with low If we consider the formation patterns of this deposit filtration properties, must be attributed. As from the perspective of the capillary-gravity concept of a result of capillary forces, sharp fluctuations in oil and gas accumulation, the inconsistency in the the position of water-oil contacts and significant shape of the deposit with the modern structural plan shifts of oil and gas reservoirs relative to the vaults can be explained by the action of capillary forces in the of anticlinal structures are observed. Prediction of pore environment of the reservoir. oil and gas contours from the position of capillary- The most significant values determining the influ - gravity concept of oil and gas accumulation allows ence of capillary forces on the distribution of hydro- to explain the reasons of complex distribution of carbons and produced water in the pore space of the oil and gas water in natural hydrocarbon traps and reservoir rock are such surface-molecular properties to outline ways of searching for new oil and gas of the reservoir as the nature of its wettability and the deposits, not subject to structural factor. value of capillary pressure. The influence of reservoir rock wettability on oil recovery and its connection with capillary pressure has been reviewed by many Materials and methods authors (Anderson, 1987; Morrow 1990; Masalmeh 2003). However, these studies practically do not con- As an example of predicting the oil and gas accumula- sider the issues concerning study of capillary forces tion contour from the perspective of the capillary- and their influence on reservoir formation processes. gravity theory of oil and gas accumulation, this article Meanwhile, during both oil recovery and the for- examines the structure of the reservoir of formation mation of an oil or gas reservoir, capillary pressure is BT17 in the R field. the main force resisting migration and filtration of The P field is located in the north of the West hydrocarbons. The nature of pore medium wettability Siberian oil and gas province within the Yamal- determines the direction of capillary forces, while Nenets Autonomous District. The stratigraphic sec- interfacial tension and curvature of interfacial surface tion of the P field is represented by sandy-clay deposits determine the value of capillary pressure. of Mesozoic-Cenozoic sedimentary cover, underlain It is known that when oil or gas comes into by pre-Jurassic basement rocks. The oil and gas bear- contact with a water-saturated porous medium, an ing formation BT17 considered in this article is con- interfacial tension creates a pressure difference at the fined to the Lower Kheta Formation, composed of water-oil (gas) interface, which represents capillary sandstone and siltstone layers. The age of the forma- pressure. tion is Berriasian-Early Valanginian. The concept of surface tension in immiscible The structure of the Bt17 reservoir in question liquids first appeared more than two centuries ago shows a mismatch in the distribution of water, oil (Chen et al., 2006) and is described by the famous and gas to the modern structural plan of the reservoir. Young-Laplace equation (Young, 1805; Laplace, Tests carried out in the north-eastern part of the 1805). This equation states that the magnitude of structure yielded oil flows in the same absolute eleva- capillary pressure Pc when two immiscible phases (in tion interval as gas flows in wells located in the south- our case, water and oil/gas) are introduced into western part of the structure. According to geologists, a porous medium is proportional to the product of who consider oil and gas reservoirs with such interfacial surface curvature 1/r and surface tension γ. a structure solely from the anticlinal concept of oil GEOLOGY, ECOLOGY, AND LANDSCAPES 3 Figure 1. Schematic of the structure of the BT17 reservoir from the perspective of anticlinal-gravity theory of oil and gas accumulation. value. Accordingly, in the case of a hydrophilic reservoir, Pc � � γ�ð1=rÞ (1) oil or gas will tend to occupy relatively large pores, while By convention (Anderson, 1987), the capillary pres- water will fill the shallowest pores. The opposite pattern sure (Pc) in an oil-water system is defined as the in the distribution of liquids and gases would be observed difference between the pressures in the oil (Po) and in a hydrophobic reservoir. water phase P(w): Pc ¼ Po Pw (2) Types of capillary barriers Based on this definition, the radius of curvature of the According to the capillarity theory of oil and gas interfacial surface pointing towards the oil phase is accumulation proposed by Bolshakov (1995), there positive and pointing towards water is negative. are three classes of unconventional capillary-screened Accordingly, depending on the curvature of the inter- deposits (hydrophilic, hydrophobic and mixed, hydro- face, the capillary pressure can be positive or negative. philic-hydrophobic) that may be present in almost When the interface is flat, the capillary pressure is every oil and gas bearing area and are controlled by so- zero. In the case of hydrophilic rock, where the solid called capillary barriers of the first and second kind. phase is predominantly wetted by water, Pc will be The hydrocarbon resources of such reservoirs vary positive, i.e., the pressure inside the oil exceeds the from insignificant to gigantic. pressure inside the water by the value of Pc, and the Two groups have been identified among the hydro- contact surface is concave towards the water phase. If philic class deposits, one of which includes deposits the reservoir is hydrophobic, Pc will be negative, i.e., shielded by capillary barriers of the first kind and the the pressure inside the oil is less than that inside the other by the second kind. Barriers of the first water by the value of Pc, and the contact surface is and second kind are genetically and functionally dis- concave towards the hydrocarbon phase. tinct. The first of them is caused by filtration-lithologic In both cases, the oil or gas element seeks a position variation of the reservoir laterally and acts as an accu- and shape in which its surface and capillary energy, mulating factor, preventing secondary migration of which is the ratio of capillary pressure to fluid density, hydrocarbons. Its origin is due to capillary pressure reach a minimum. Since the density of the fluid is con- surges at the junctions of different porous facies. stant in each case, in order to achieve a minimum capil- The second kind of barrier arises at the water- lary energy, the oil or gas element must be shaped in such hydrocarbon contact of deposits of any type when a way that the capillary pressure also has a minimum 4 E. Y. NEYOLOVA AND A. A. PONOMAREV the oil and gas reservoir cools. It does not play an and gas provinces have been significantly affected by accumulating role, but it prevents oil and gas overflow changes in solar radiation intensity and related climate during deformations and trap opening during tectonic changes. Cooling of sedimentary rocks reached the rearrangements. Deposits shielded by capillary bar- greatest extent, particularly in the northern oil and riers of the first kind may be present in any oil and gas provinces, particularly in the north of Western gas bearing area, whereas capillary-screened hydrocar- Siberia. This change in sediment temperature regime bon accumulations controlled by second kind barriers was facilitated by a decrease in heat flux density simul- are predominantly found in oil and gas bearing areas taneously with climatic cooling. A decrease in reservoir overlain by cryolithozone. However, their presence temperatures at the same time as neotectonic deforma- should not be ruled out even outside the permafrost tion in the Pleistocene has been reported by many area, for example, in areas characterized by low mod- authors. Reduced reservoir temperatures in some ern reservoir temperatures. northern provinces of China reached 60°C (Duan & The genetic difference between the hydrophilic and Wu, 2020). In northern Canada, declines in reservoir hydrophobic classes predetermines a different set of temperatures during this period exceeded 90° prospecting, exploration and development techniques C (Magara, 1976). In areas of permafrost spreading, for each. cooling of oil and gas reservoirs during the neotectonic This article examines the formation of a hydrophilic stage of geological development reached its maximum. reservoir controlled by a second-generation capillary Most authors attribute formation of permafrost to the Late Cenozoic cooling (TODD et al., 2007). barrier. The origin of such reservoir-stabilising screens The Late Cenozoic ice age in the northern hemi- is usually confined to areas where oil-saturated reser- sphere began at the beginning of the Pleistocene voirs have undergone significant reductions in reservoir about 2.7 million years ago. Up to 10 episodes of temperatures since the formation of oil and gas significant ice sheet activity and Northern reservoirs. Hemisphere ice sheet formation were observed in According to known studies the surface tension is northeastern Eurasia over a period of 2.5–0 Ma significantly dependent on the temperature of the (Cavanagh et al., 2006). The cooling effect of the medium (Michaels & Hauser, 1951; Ye et al. 2008; cryolithic zone affected the temperature regime of Hough et al. 1951). oil and gas reservoirs throughout this period. The According to Gimatudinov Sh and Shirkovsky average decline in reservoir temperatures through- (1982) the interfacial tension in the “gas-water” system out the sedimentary cover in the West Siberian oil practically doubles when the temperature drops from and gas province, according to Nesterov et al. 120°C to 70°C, which leads to a corresponding (1982) amounted to 30°C. In the southern part increase in capillary pressure in the pore medium. of the basin, in the roof of the Cenomanian sedi- This phenomenon causes a capillary barrier to arise ments, sedimentary temperatures decreased by 25– at the contact between water and hydrocarbons in the 30°C. In the circumpolar areas of northern West pore environment of the oil and gas reservoir. Once Siberia, sedimentary cover sediments cooled by the reservoir temperature decreases, the deposit 30–50°C. As a result of the Late Cenozoic glacial “freezes” at the site of its initial accumulation due to period, the rocks of the sedimentary basin of the increased capillary pressures. In the absence of tec- West Siberian oil and gas province cooled down to tonic movement and relative homogeneity of reservoir a depth of 4 km. filtration properties, the standard position of the Lower reservoir temperatures caused a significant water-oil contact coincides with a capillary barrier of increase in interfacial tension at the water- the second kind. The stabilizing effect of a capillary hydrocarbon contact and a corresponding increase barrier of the second kind can be demonstrated only in capillary pressure at gas-water and water-oil con- after complete or partial removal of the structural tacts. Thus, the reservoirs were stabilized in areas of factors that led to the accumulation of the reservoir initial hydrocarbon accumulation. Due to increased before reservoir temperatures decrease. capillary pressure, further structural alterations in the neotectonic stage of geological development could no longer lead to reformation of oil and gas Influence of reservoir temperatures on deposits. As a result, the combined effect of active hydrocarbon reservoir stabilisation neotectonic processes and low reservoir tempera- The reservoir temperatures of natural oil and gas tures in the Late Cenozoic led to the emergence of reservoirs and the patterns of their change depend oil and gas accumulations in northern oil and gas mainly on heat flux density. Over the course of geolo- bearing provinces that are not subject to the struc- gical history, heat flux density has changed signifi - tural factor, making it impossible to search for and cantly over time under the influence of various explore them from the position of conventional con- factors. The reservoir temperatures in different oil ventional solutions. GEOLOGY, ECOLOGY, AND LANDSCAPES 5 tension with decrease of reservoir temperature. The Neotectonic deformation and its relation to oil and gas reservoir must cool to such an extent capillary barriers of the second kind that the increase in interfacial tension and, conse- Many authors attest that the northern areas experi- quently, capillary pressure at the water- enced substantial post-glacial deformation (Dehls hydrocarbon contact is sufficient to prevent move- et al., 2000). Deformation in northern Canada was ment of hydrocarbons through the water-saturated greater than one hundred metres (Dyke et al., 1991; formation by gravitational forces during its tectonic Adams & Clague, 1993). Even greater spread of neo- deformations, up to complete disintegration of nat- tectonic movements during this period had been ural traps contained in it. observed in northern territories of Western Siberia Such capillary-screened oil and gas reservoirs (Varlamov, 1985). Neotectonic deformations here may be present in any oil and gas-bearing province reached several hundred meters. Slightly less run-up whose subsoil experienced significant cooling dur- in tectonic movements was observed in the Shirotny ing the neotectonic stage of geological development Priob’ye region – on the order of tens of meters. As or is characterised by low current reservoir a result, some of the local uplifts were dissolved and temperatures. other uplifts were formed. However, they are most widespread in oil and gas The high velocity and broad scope of neotectonic bearing areas overlain by permafrost strata because the transformations have led to a radical structural reor- cryolithozone, along with the weakening in time of ganization of many oil and gas reservoirs. For exam- heat flow, uplift, etc., can be considered as an addi- ple, A. Levorsen (1958) reports that the Capro oil tional significant factor in cooling sedimentary cover reservoir in Arkansas (USA) tilted over a period of rocks to a considerable depth. Of greatest practical 10–12 years. interest are post-stratigraphic hydrocarbon accumula- Active neotectonic processes in the area of cryo- tions, which is due to the comparative ease of their lithozone development, where oil and gas reservoirs prediction. The main elements of local forecasting and have experienced the greatest cooling, suggest that refinement of the contours of such deposits can be a wide variety of oil and gas reservoirs shielded by based on paleostructural and paleogeographical stu- capillary barriers of the second kind are widely dis- dies with the compilation of appropriate models of tributed within these areas. their structure. The connection between the present- However, only two varieties are known to be reli- day structure of hydrocarbon deposits at fields in the able amongst the proposed multitude – post-anticlinal West Siberian oil and gas province and neotectonic oil and gas reservoirs. One type is confined to struc- processes was suggested earlier (Rybak, 1987), but the tures with inherited development (near-anticlinal) and influence of capillary pressures on the stabilization of the other to completely disintegrated anticlines that the deposit at its initial accumulation site was not are not reflected in the present structure. considered at that time. It should be noted that during the formation of paleoswales in rocks characterized by supercapillary pore sizes, such as those of Cenomanian age, due to Principles for predicting the contour of deposits low capillary pressures, fluids could move in accor- shielded by capillary barriers of the second kind dance with new structural forms of newly formed In most cases, the current structure of oil and gas traps. Therefore, the stabilizing role of capillary bar- reservoirs is the main criterion for determining the riers of the second kind is most pronounced for more spatial position of water-oil and gas-water contacts. submerged shallow porous reservoirs of Neocomian This approach often leads to significant errors in and Jurassic ages characterized by low permeability. determining the position of oil and gas-bearing con- Partial or complete divergence in reservoir shape from tours and, consequently, to errors in determining the the modern structural plan of oil and gas-bearing deposit area and oil and gas saturated volume. reservoirs located in northern areas is quite common. According to Bolshakov (1995), the following prin- Accordingly, exploration and supplementary explora- ciples should be followed when defining oil-bearing tion of such reservoirs should not be approached contours of capillary-screened deposits: solely from the traditional view of oil and gas reservoir In the regional prediction stage, thermometric stu- formation. dies of oil and gas reservoirs are the main ones, as well as assessing the extent of their neotectonic alteration. The purpose of thermometric studies is to assess the Results and discussion extent to which reservoir temperatures have decreased Determinant for the second kind of barriers is since hydrocarbon accumulation began in its traps. If cooling of oil-and-gas reservoirs, which causes cooling of the oil and gas reservoir was sufficient to increase of capillary pressures at water- create stabilizing capillary barriers, the intensity of hydrocarbon contacts due to increase of interfacial subsequent neotectonic movements is studied. The 6 E. Y. NEYOLOVA AND A. A. PONOMAREV intensity of tectonic movements is assessed to deter- In order to carry out paleostructural analysis to mine whether anticlinal paleovolcanic reservoirs can clarify the reservoir contour from the perspective be fully or partially disintegrated. of capillary-gravity theory of oil and gas accumu- Next, structural and palaeostructural plots are lation, the Turonian basement, which satisfactorily carried out. Paleostructural maps should be correlates with the seismic reflection horizon D, plotted at the time of maximum cooling of the was used as the horizon closest to the time of oil and gas reservoir in question. If it is impossible maximum cooling of the sedimentary cover due to obtain a well traceable horizon in the Upper to the absence of reliable benchmark horizons of Cenozoic interval, a horizon chronologically clo- the upper part of the section. According to the sest to the time of the lowest reservoir tempera- paleostructural plan, oil and gas accumulations are tures can be taken as a reference for isopachite confined to local anticlinal uplifts separated by map construction. For the West Siberian section, a structural sag. There is also a local structure in this could be the basement of the Turonian Stage. the southwestern part of the area in question, Obviously, this may lead to certain errors in iden- within which a post-aniclinal hydrocarbon deposit tifying and delineating deposits. However, the may be present. Thus, prior to declining reservoir errors mostly introduce quantitative rather than temperatures and subsequent neotectonic transfor- qualitative distortions in the true structure of pro- mations, oil and gas accumulations were fully ductive horizons under the generally inherited consistent with the principles of anticlinal theory tectonic development of the region. Using the of oil and gas accumulation and gas-water con- resulting paleostructural maps, anticlinal traps tacts had a near-horizontal position (Figure 2). are identified according to the anticlinal concept Thus, substantiating the position of oil-water of oil and gas accumulation, and with the available and gas-water contacts on the modern structural data on the oil and gas content of the reservoir plan of the reservoir in question does not require based on well test results, the position of the the construction of any unconfirmed zones of inferred horizontal water-oil contacts is deter- claying or faulting. When projecting the contours mined, which are then projected onto the present- of oil and gas bearing capacity, identified on the day structure. basis of paleostructural analysis, onto the modern When predicting the oil-bearing contours of structure of the reservoir (Figure 3), a rather sharp productive formations in West Siberia, it should discrepancy between the shape of the deposit and be kept in mind that maximum reservoir tempera- the modern structure of the reservoir is detected. tures up to 120°C in the sedimentary cover were Nevertheless, this position of contact boundaries is observed in the early Oligocene (Kurchikov & fully explained from the perspective of the capil- Stavitsky, 1987). Accordingly, conditions for lary-gravity concept of oil and gas accumulation. hydrocarbon migration were optimal in the early Oligocene because interfacial tensions were almost completely compensated. The mobility of fluids Conclusions due to this fact contributed to the most complete The use of capillary reservoir characterization data subordination of reservoir contours to the form of allows for more accurate prediction of the position anticlinal traps. The onset of formation of the of oil-water contacts that do not correspond to the permafrost strata is attributed to the cooling of current reservoir structure. This eliminates the the early Pleistocene. At that time, hydrocarbon need to assign non-existent screens such as reservoirs were stabilized by second-order capil- impermeable faults or unconfirmed zones of reser- lary barriers due to increased interfacial tension, voir claying to oil and gas reservoirs. and further tectonic deformation could no longer Based on the Young-Laplace equation, capillary result in fluid overflows due to increased capillary screens arising at the water-hydrocarbon interface pressures at water-hydrocarbon contacts. that control reservoir shape can genetically be of As an example of predicting the contour of oil and two types. The first occurs at junctions of different gas accumulation from the position of capillary- porous facies, i.e., increase in capillary pressure in gravity theory of oil and gas accumulation, this article this case occurs due to variation in pore channel examines the structure of the reservoir of formation radius of curvature. BT17 of the R field. The structure of the reservoir of The second type of capillary screen, discussed in formation BT17, made on the basis of the traditional this article, owes its origin to a decrease in reservoir view of the processes of formation of hydrocarbon temperatures, which led to an increase in interfacial deposits is shown in Figure 1. GEOLOGY, ECOLOGY, AND LANDSCAPES 7 Figure 2. Schematic of the hydrocarbon reservoir structure of the BT17 formation at Early Turonian time. Figure 3. Schematic of hydrocarbon reservoir structure from the capillary-gravity theory of oil and gas accumulation. 8 E. Y. NEYOLOVA AND A. A. PONOMAREV tension and consequently a capillary pressure surge Hough, E. W., Rzasa, M. J., & Wood, B. B. (1951). Interfacial tensions at reservoir pressures and temperatures; and subsequent neotectonic deformations. The distri- Apparatus and the water-methane system. 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The application of basin modeling of oil and gas systems based on the capillary-gravity concept

Neyolova, E.Y.; Ponomarev, A.A.
Geology Ecology and Landscapes , Volume OnlineFirst: 8 – Jun 10, 2022
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GEOLOGY, ECOLOGY, AND LANDSCAPES INWASCON https://doi.org/10.1080/24749508.2022.2085852 RESEARCH ARTICLE The application of basin modeling of oil and gas systems based on the capillary-gravity concept E.Y. Neyolova and A.A. Ponomarev Department of Oil and Gas Deposits Geology, Tyumen Industrial University, Tyumen, Russia ANNOTATION ARTICLE HISTORY Received 1 April 2022 Oil and gas reservoirs exhibiting low reservoir properties often have significant deviations in Accepted 31 May 2022 structure of hydrocarbon reservoirs they contain, based on anticlinal theory of oil and gas accu- mulation. First, these deviations are related to sharp fluctuations found in water-oil and water- KEYWORDS gas interfaces and failure to subordinate the shape of deposits to structural factor. When dealing Oil; gas; reservoir; capillary with such objects, geologists often have to assign unreasonable structural deflections, various pressure; interfacial tension; impermeable seals of tectonic or sedimentary origin whose presence is not always confirmed by capillary barriers seismic and drilling data. The reason for such difficulties is that anticlinal theory does not consider capillary forces counteracting hydrocarbon migration to be key to reservoir formation. Any oil and gas reservoir is a multiphase pore system with countless contacts, both between different fluids and in host pore space. As per molecular physics laws, the major role in oil and gas water distribution in such systems is controlled primarily by capillary properties of the environment such as capillary pressure at water-oil interface, interfacial tension magnitude, pore channel radius and wetting behavior of solid phase. Due to capillary forces, sharp fluctuations at water-oil interfaces and significant shifts of oil and gas reservoirs relative to vaults of anticlinal structures are observed Introduction in the world so far. Most of the major fields that form the basis of the global oil industry were When dealing with oil and gas geology problems, discovered in the mid-20th century geologists often encounter phenomena that con- (Tsoskounoglou et al., 2008). To this day, the tradict the generally accepted understanding of the anticline concept is the basis for exploration conditions under which oil and gas deposits are plans, oil and gas reservoir modelling, reserves formed. Very often these contradictions are asso- estimation and development design. However, the ciated with deviations in the structure of hydro- number of large reservoirs discovered in recent carbon reservoirs from the principles of anticlinal decades has been declining rapidly. It is clear theory of oil and gas accumulation. This theory that new methods for prospecting and exploring has been effectively applied for many years to the for hydrocarbons are needed to maintain world design of oil and gas exploration for geological production levels. targets of simple structure characterised by high In addition, using the conventional approach reservoir properties. The anticlinal-gravity concept when dealing with complex geological targets char- of oil and gas accumulation dates back to the mid- acterised by low reservoir properties, geologists 19th century, when the American oilman known often have to assign oil and gas reservoirs to var- as Colonel Drake suggested a link between oil ious impermeable screens, of tectonic or sedimen- deposits and anticlinal uplifts. Later on, on the tary origin, the presence of which is not always basis of this method, any objects confined to ele- confirmed by seismic and drilling data. vated reservoir areas in structural plan began to be The reason for these difficulties lies in the fact considered promising. The essence of the anticli- that anticlinal theory does not consider as key, nal concept is that the formation of hydrocarbon forces resisting the migration of hydrocarbons deposits is due to their buoyancy forces. both during formation of the deposit and during According to modern petroleum geology text- its development. The main forces resisting oil books for two centuries, the first and most impor- migration are various capillary effects. Any oil and tant component necessary for oil accumulation gas reservoir, irrespective of the conditions of its and reservoir formation is structural uplift (Ali, formation, facies component and tectonic processes 2017). Based on the approaches used by the anti- occurring both during its formation and operation, clinal concept of oil and gas accumulation, more is a multiphase pore system with an infinite than 50,000 oil and gas fields have been discovered CONTACT A.A. Ponomarev ponomarevaa@tyuiu.ru Department of Oil and Gas Deposits Geology, Tyumen Industrial University, Tyumen, Russia © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 2 E. Y. NEYOLOVA AND A. A. PONOMAREV number of contacts both between different fluids and gas accumulation, this discrepancy in the distri- bution of fluids saturating the reservoir is due to the and in the surrounding pore space. According to presence of various screens, of tectonic or sedimento- the laws of molecular physics, the main role in oil logical origin, which, however, are often not con- and gas water distribution in the pore multiphase firmed by either drilling or seismic data. Figure 1 system is controlled mainly by capillary properties shows a schematic of the reservoir structure in terms of the medium, such as capillary pressure value at of the anticline-gravity concept of oil and gas accumu- the water-oil contact, interfacial tension value, pore lation. The shape of the deposit corresponds to the channel radius and nature of solid phase wetting, structural structure of the reservoir roof, but the oil- i.e., hydrophilicity or hydrophobicity of reservoir saturated part of the deposit near well 706 is separated rock. The combination of these conditions and from the gas-saturated part located at the same hypso- characteristics results in a certain unique oil and metric level by a zone of claying, which has the shape gas trap. As the famous American geologist of some “ancient channel” whose presence has not (A. I. Levorsen, 1948): each oil field is unique in been confirmed by drilling data. This “laced” form of that it has its own development and its formation occurrence of geological bodies according to the con- can be considered the end result of the interaction cepts of classical palaeogeography is characteristic of of many variables. And to such causes, first of all, sand bodies separating impermeable rocks rather than the differently directed action of capillary and grav- of clay impermeable rocks. itational forces, especially in reservoirs with low If we consider the formation patterns of this deposit filtration properties, must be attributed. As from the perspective of the capillary-gravity concept of a result of capillary forces, sharp fluctuations in oil and gas accumulation, the inconsistency in the the position of water-oil contacts and significant shape of the deposit with the modern structural plan shifts of oil and gas reservoirs relative to the vaults can be explained by the action of capillary forces in the of anticlinal structures are observed. Prediction of pore environment of the reservoir. oil and gas contours from the position of capillary- The most significant values determining the influ - gravity concept of oil and gas accumulation allows ence of capillary forces on the distribution of hydro- to explain the reasons of complex distribution of carbons and produced water in the pore space of the oil and gas water in natural hydrocarbon traps and reservoir rock are such surface-molecular properties to outline ways of searching for new oil and gas of the reservoir as the nature of its wettability and the deposits, not subject to structural factor. value of capillary pressure. The influence of reservoir rock wettability on oil recovery and its connection with capillary pressure has been reviewed by many Materials and methods authors (Anderson, 1987; Morrow 1990; Masalmeh 2003). However, these studies practically do not con- As an example of predicting the oil and gas accumula- sider the issues concerning study of capillary forces tion contour from the perspective of the capillary- and their influence on reservoir formation processes. gravity theory of oil and gas accumulation, this article Meanwhile, during both oil recovery and the for- examines the structure of the reservoir of formation mation of an oil or gas reservoir, capillary pressure is BT17 in the R field. the main force resisting migration and filtration of The P field is located in the north of the West hydrocarbons. The nature of pore medium wettability Siberian oil and gas province within the Yamal- determines the direction of capillary forces, while Nenets Autonomous District. The stratigraphic sec- interfacial tension and curvature of interfacial surface tion of the P field is represented by sandy-clay deposits determine the value of capillary pressure. of Mesozoic-Cenozoic sedimentary cover, underlain It is known that when oil or gas comes into by pre-Jurassic basement rocks. The oil and gas bear- contact with a water-saturated porous medium, an ing formation BT17 considered in this article is con- interfacial tension creates a pressure difference at the fined to the Lower Kheta Formation, composed of water-oil (gas) interface, which represents capillary sandstone and siltstone layers. The age of the forma- pressure. tion is Berriasian-Early Valanginian. The concept of surface tension in immiscible The structure of the Bt17 reservoir in question liquids first appeared more than two centuries ago shows a mismatch in the distribution of water, oil (Chen et al., 2006) and is described by the famous and gas to the modern structural plan of the reservoir. Young-Laplace equation (Young, 1805; Laplace, Tests carried out in the north-eastern part of the 1805). This equation states that the magnitude of structure yielded oil flows in the same absolute eleva- capillary pressure Pc when two immiscible phases (in tion interval as gas flows in wells located in the south- our case, water and oil/gas) are introduced into western part of the structure. According to geologists, a porous medium is proportional to the product of who consider oil and gas reservoirs with such interfacial surface curvature 1/r and surface tension γ. a structure solely from the anticlinal concept of oil GEOLOGY, ECOLOGY, AND LANDSCAPES 3 Figure 1. Schematic of the structure of the BT17 reservoir from the perspective of anticlinal-gravity theory of oil and gas accumulation. value. Accordingly, in the case of a hydrophilic reservoir, Pc � � γ�ð1=rÞ (1) oil or gas will tend to occupy relatively large pores, while By convention (Anderson, 1987), the capillary pres- water will fill the shallowest pores. The opposite pattern sure (Pc) in an oil-water system is defined as the in the distribution of liquids and gases would be observed difference between the pressures in the oil (Po) and in a hydrophobic reservoir. water phase P(w): Pc ¼ Po Pw (2) Types of capillary barriers Based on this definition, the radius of curvature of the According to the capillarity theory of oil and gas interfacial surface pointing towards the oil phase is accumulation proposed by Bolshakov (1995), there positive and pointing towards water is negative. are three classes of unconventional capillary-screened Accordingly, depending on the curvature of the inter- deposits (hydrophilic, hydrophobic and mixed, hydro- face, the capillary pressure can be positive or negative. philic-hydrophobic) that may be present in almost When the interface is flat, the capillary pressure is every oil and gas bearing area and are controlled by so- zero. In the case of hydrophilic rock, where the solid called capillary barriers of the first and second kind. phase is predominantly wetted by water, Pc will be The hydrocarbon resources of such reservoirs vary positive, i.e., the pressure inside the oil exceeds the from insignificant to gigantic. pressure inside the water by the value of Pc, and the Two groups have been identified among the hydro- contact surface is concave towards the water phase. If philic class deposits, one of which includes deposits the reservoir is hydrophobic, Pc will be negative, i.e., shielded by capillary barriers of the first kind and the the pressure inside the oil is less than that inside the other by the second kind. Barriers of the first water by the value of Pc, and the contact surface is and second kind are genetically and functionally dis- concave towards the hydrocarbon phase. tinct. The first of them is caused by filtration-lithologic In both cases, the oil or gas element seeks a position variation of the reservoir laterally and acts as an accu- and shape in which its surface and capillary energy, mulating factor, preventing secondary migration of which is the ratio of capillary pressure to fluid density, hydrocarbons. Its origin is due to capillary pressure reach a minimum. Since the density of the fluid is con- surges at the junctions of different porous facies. stant in each case, in order to achieve a minimum capil- The second kind of barrier arises at the water- lary energy, the oil or gas element must be shaped in such hydrocarbon contact of deposits of any type when a way that the capillary pressure also has a minimum 4 E. Y. NEYOLOVA AND A. A. PONOMAREV the oil and gas reservoir cools. It does not play an and gas provinces have been significantly affected by accumulating role, but it prevents oil and gas overflow changes in solar radiation intensity and related climate during deformations and trap opening during tectonic changes. Cooling of sedimentary rocks reached the rearrangements. Deposits shielded by capillary bar- greatest extent, particularly in the northern oil and riers of the first kind may be present in any oil and gas provinces, particularly in the north of Western gas bearing area, whereas capillary-screened hydrocar- Siberia. This change in sediment temperature regime bon accumulations controlled by second kind barriers was facilitated by a decrease in heat flux density simul- are predominantly found in oil and gas bearing areas taneously with climatic cooling. A decrease in reservoir overlain by cryolithozone. However, their presence temperatures at the same time as neotectonic deforma- should not be ruled out even outside the permafrost tion in the Pleistocene has been reported by many area, for example, in areas characterized by low mod- authors. Reduced reservoir temperatures in some ern reservoir temperatures. northern provinces of China reached 60°C (Duan & The genetic difference between the hydrophilic and Wu, 2020). In northern Canada, declines in reservoir hydrophobic classes predetermines a different set of temperatures during this period exceeded 90° prospecting, exploration and development techniques C (Magara, 1976). In areas of permafrost spreading, for each. cooling of oil and gas reservoirs during the neotectonic This article examines the formation of a hydrophilic stage of geological development reached its maximum. reservoir controlled by a second-generation capillary Most authors attribute formation of permafrost to the Late Cenozoic cooling (TODD et al., 2007). barrier. The origin of such reservoir-stabilising screens The Late Cenozoic ice age in the northern hemi- is usually confined to areas where oil-saturated reser- sphere began at the beginning of the Pleistocene voirs have undergone significant reductions in reservoir about 2.7 million years ago. Up to 10 episodes of temperatures since the formation of oil and gas significant ice sheet activity and Northern reservoirs. Hemisphere ice sheet formation were observed in According to known studies the surface tension is northeastern Eurasia over a period of 2.5–0 Ma significantly dependent on the temperature of the (Cavanagh et al., 2006). The cooling effect of the medium (Michaels & Hauser, 1951; Ye et al. 2008; cryolithic zone affected the temperature regime of Hough et al. 1951). oil and gas reservoirs throughout this period. The According to Gimatudinov Sh and Shirkovsky average decline in reservoir temperatures through- (1982) the interfacial tension in the “gas-water” system out the sedimentary cover in the West Siberian oil practically doubles when the temperature drops from and gas province, according to Nesterov et al. 120°C to 70°C, which leads to a corresponding (1982) amounted to 30°C. In the southern part increase in capillary pressure in the pore medium. of the basin, in the roof of the Cenomanian sedi- This phenomenon causes a capillary barrier to arise ments, sedimentary temperatures decreased by 25– at the contact between water and hydrocarbons in the 30°C. In the circumpolar areas of northern West pore environment of the oil and gas reservoir. Once Siberia, sedimentary cover sediments cooled by the reservoir temperature decreases, the deposit 30–50°C. As a result of the Late Cenozoic glacial “freezes” at the site of its initial accumulation due to period, the rocks of the sedimentary basin of the increased capillary pressures. In the absence of tec- West Siberian oil and gas province cooled down to tonic movement and relative homogeneity of reservoir a depth of 4 km. filtration properties, the standard position of the Lower reservoir temperatures caused a significant water-oil contact coincides with a capillary barrier of increase in interfacial tension at the water- the second kind. The stabilizing effect of a capillary hydrocarbon contact and a corresponding increase barrier of the second kind can be demonstrated only in capillary pressure at gas-water and water-oil con- after complete or partial removal of the structural tacts. Thus, the reservoirs were stabilized in areas of factors that led to the accumulation of the reservoir initial hydrocarbon accumulation. Due to increased before reservoir temperatures decrease. capillary pressure, further structural alterations in the neotectonic stage of geological development could no longer lead to reformation of oil and gas Influence of reservoir temperatures on deposits. As a result, the combined effect of active hydrocarbon reservoir stabilisation neotectonic processes and low reservoir tempera- The reservoir temperatures of natural oil and gas tures in the Late Cenozoic led to the emergence of reservoirs and the patterns of their change depend oil and gas accumulations in northern oil and gas mainly on heat flux density. Over the course of geolo- bearing provinces that are not subject to the struc- gical history, heat flux density has changed signifi - tural factor, making it impossible to search for and cantly over time under the influence of various explore them from the position of conventional con- factors. The reservoir temperatures in different oil ventional solutions. GEOLOGY, ECOLOGY, AND LANDSCAPES 5 tension with decrease of reservoir temperature. The Neotectonic deformation and its relation to oil and gas reservoir must cool to such an extent capillary barriers of the second kind that the increase in interfacial tension and, conse- Many authors attest that the northern areas experi- quently, capillary pressure at the water- enced substantial post-glacial deformation (Dehls hydrocarbon contact is sufficient to prevent move- et al., 2000). Deformation in northern Canada was ment of hydrocarbons through the water-saturated greater than one hundred metres (Dyke et al., 1991; formation by gravitational forces during its tectonic Adams & Clague, 1993). Even greater spread of neo- deformations, up to complete disintegration of nat- tectonic movements during this period had been ural traps contained in it. observed in northern territories of Western Siberia Such capillary-screened oil and gas reservoirs (Varlamov, 1985). Neotectonic deformations here may be present in any oil and gas-bearing province reached several hundred meters. Slightly less run-up whose subsoil experienced significant cooling dur- in tectonic movements was observed in the Shirotny ing the neotectonic stage of geological development Priob’ye region – on the order of tens of meters. As or is characterised by low current reservoir a result, some of the local uplifts were dissolved and temperatures. other uplifts were formed. However, they are most widespread in oil and gas The high velocity and broad scope of neotectonic bearing areas overlain by permafrost strata because the transformations have led to a radical structural reor- cryolithozone, along with the weakening in time of ganization of many oil and gas reservoirs. For exam- heat flow, uplift, etc., can be considered as an addi- ple, A. Levorsen (1958) reports that the Capro oil tional significant factor in cooling sedimentary cover reservoir in Arkansas (USA) tilted over a period of rocks to a considerable depth. Of greatest practical 10–12 years. interest are post-stratigraphic hydrocarbon accumula- Active neotectonic processes in the area of cryo- tions, which is due to the comparative ease of their lithozone development, where oil and gas reservoirs prediction. The main elements of local forecasting and have experienced the greatest cooling, suggest that refinement of the contours of such deposits can be a wide variety of oil and gas reservoirs shielded by based on paleostructural and paleogeographical stu- capillary barriers of the second kind are widely dis- dies with the compilation of appropriate models of tributed within these areas. their structure. The connection between the present- However, only two varieties are known to be reli- day structure of hydrocarbon deposits at fields in the able amongst the proposed multitude – post-anticlinal West Siberian oil and gas province and neotectonic oil and gas reservoirs. One type is confined to struc- processes was suggested earlier (Rybak, 1987), but the tures with inherited development (near-anticlinal) and influence of capillary pressures on the stabilization of the other to completely disintegrated anticlines that the deposit at its initial accumulation site was not are not reflected in the present structure. considered at that time. It should be noted that during the formation of paleoswales in rocks characterized by supercapillary pore sizes, such as those of Cenomanian age, due to Principles for predicting the contour of deposits low capillary pressures, fluids could move in accor- shielded by capillary barriers of the second kind dance with new structural forms of newly formed In most cases, the current structure of oil and gas traps. Therefore, the stabilizing role of capillary bar- reservoirs is the main criterion for determining the riers of the second kind is most pronounced for more spatial position of water-oil and gas-water contacts. submerged shallow porous reservoirs of Neocomian This approach often leads to significant errors in and Jurassic ages characterized by low permeability. determining the position of oil and gas-bearing con- Partial or complete divergence in reservoir shape from tours and, consequently, to errors in determining the the modern structural plan of oil and gas-bearing deposit area and oil and gas saturated volume. reservoirs located in northern areas is quite common. According to Bolshakov (1995), the following prin- Accordingly, exploration and supplementary explora- ciples should be followed when defining oil-bearing tion of such reservoirs should not be approached contours of capillary-screened deposits: solely from the traditional view of oil and gas reservoir In the regional prediction stage, thermometric stu- formation. dies of oil and gas reservoirs are the main ones, as well as assessing the extent of their neotectonic alteration. The purpose of thermometric studies is to assess the Results and discussion extent to which reservoir temperatures have decreased Determinant for the second kind of barriers is since hydrocarbon accumulation began in its traps. If cooling of oil-and-gas reservoirs, which causes cooling of the oil and gas reservoir was sufficient to increase of capillary pressures at water- create stabilizing capillary barriers, the intensity of hydrocarbon contacts due to increase of interfacial subsequent neotectonic movements is studied. The 6 E. Y. NEYOLOVA AND A. A. PONOMAREV intensity of tectonic movements is assessed to deter- In order to carry out paleostructural analysis to mine whether anticlinal paleovolcanic reservoirs can clarify the reservoir contour from the perspective be fully or partially disintegrated. of capillary-gravity theory of oil and gas accumu- Next, structural and palaeostructural plots are lation, the Turonian basement, which satisfactorily carried out. Paleostructural maps should be correlates with the seismic reflection horizon D, plotted at the time of maximum cooling of the was used as the horizon closest to the time of oil and gas reservoir in question. If it is impossible maximum cooling of the sedimentary cover due to obtain a well traceable horizon in the Upper to the absence of reliable benchmark horizons of Cenozoic interval, a horizon chronologically clo- the upper part of the section. According to the sest to the time of the lowest reservoir tempera- paleostructural plan, oil and gas accumulations are tures can be taken as a reference for isopachite confined to local anticlinal uplifts separated by map construction. For the West Siberian section, a structural sag. There is also a local structure in this could be the basement of the Turonian Stage. the southwestern part of the area in question, Obviously, this may lead to certain errors in iden- within which a post-aniclinal hydrocarbon deposit tifying and delineating deposits. However, the may be present. Thus, prior to declining reservoir errors mostly introduce quantitative rather than temperatures and subsequent neotectonic transfor- qualitative distortions in the true structure of pro- mations, oil and gas accumulations were fully ductive horizons under the generally inherited consistent with the principles of anticlinal theory tectonic development of the region. Using the of oil and gas accumulation and gas-water con- resulting paleostructural maps, anticlinal traps tacts had a near-horizontal position (Figure 2). are identified according to the anticlinal concept Thus, substantiating the position of oil-water of oil and gas accumulation, and with the available and gas-water contacts on the modern structural data on the oil and gas content of the reservoir plan of the reservoir in question does not require based on well test results, the position of the the construction of any unconfirmed zones of inferred horizontal water-oil contacts is deter- claying or faulting. When projecting the contours mined, which are then projected onto the present- of oil and gas bearing capacity, identified on the day structure. basis of paleostructural analysis, onto the modern When predicting the oil-bearing contours of structure of the reservoir (Figure 3), a rather sharp productive formations in West Siberia, it should discrepancy between the shape of the deposit and be kept in mind that maximum reservoir tempera- the modern structure of the reservoir is detected. tures up to 120°C in the sedimentary cover were Nevertheless, this position of contact boundaries is observed in the early Oligocene (Kurchikov & fully explained from the perspective of the capil- Stavitsky, 1987). Accordingly, conditions for lary-gravity concept of oil and gas accumulation. hydrocarbon migration were optimal in the early Oligocene because interfacial tensions were almost completely compensated. The mobility of fluids Conclusions due to this fact contributed to the most complete The use of capillary reservoir characterization data subordination of reservoir contours to the form of allows for more accurate prediction of the position anticlinal traps. The onset of formation of the of oil-water contacts that do not correspond to the permafrost strata is attributed to the cooling of current reservoir structure. This eliminates the the early Pleistocene. At that time, hydrocarbon need to assign non-existent screens such as reservoirs were stabilized by second-order capil- impermeable faults or unconfirmed zones of reser- lary barriers due to increased interfacial tension, voir claying to oil and gas reservoirs. and further tectonic deformation could no longer Based on the Young-Laplace equation, capillary result in fluid overflows due to increased capillary screens arising at the water-hydrocarbon interface pressures at water-hydrocarbon contacts. that control reservoir shape can genetically be of As an example of predicting the contour of oil and two types. The first occurs at junctions of different gas accumulation from the position of capillary- porous facies, i.e., increase in capillary pressure in gravity theory of oil and gas accumulation, this article this case occurs due to variation in pore channel examines the structure of the reservoir of formation radius of curvature. BT17 of the R field. The structure of the reservoir of The second type of capillary screen, discussed in formation BT17, made on the basis of the traditional this article, owes its origin to a decrease in reservoir view of the processes of formation of hydrocarbon temperatures, which led to an increase in interfacial deposits is shown in Figure 1. GEOLOGY, ECOLOGY, AND LANDSCAPES 7 Figure 2. Schematic of the hydrocarbon reservoir structure of the BT17 formation at Early Turonian time. Figure 3. Schematic of hydrocarbon reservoir structure from the capillary-gravity theory of oil and gas accumulation. 8 E. Y. NEYOLOVA AND A. A. PONOMAREV tension and consequently a capillary pressure surge Hough, E. W., Rzasa, M. J., & Wood, B. B. (1951). Interfacial tensions at reservoir pressures and temperatures; and subsequent neotectonic deformations. The distri- Apparatus and the water-methane system. 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Journal

Geology Ecology and Landscapes – Taylor & Francis

Published: Jun 10, 2022

Keywords: Oil; gas; reservoir; capillary pressure; interfacial tension; capillary barriers

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