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Aiming at the actual demands of petroleum exploration and development, this paper describes the research progress and application of artificial intelligence (AI) in petroleum exploration and development, and discusses the applications and development directions of AI in the future. Machine learning has been preliminarily applied in lithology identification, logging curve reconstruction, reservoir parameter estimation, and other logging processing and interpretation, exhibiting great potential. Computer vision is effective in picking of seismic first breaks, fault identification, and other seismic processing and interpretation. Deep learning and optimization technology have been applied to reservoir engineering, and realized the real-time optimization of waterflooding development and prediction of oil and gas production. The application of data mining in drilling, completion, and surface facility engineering etc. has resulted in intelligent equipment and integrated software. The potential development directions of artificial intelligence in petroleum exploration and development are intelligent production equipment, automatic processing and interpretation, and professional software platform. The highlights of development will be digital basins, fast intelligent imaging logging tools, intelligent seismic nodal acquisition systems, intelligent rotary-steering drilling, intelligent fracturing technology and equipment, real-time monitoring and control of zonal injection and production.
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By using drilling, high-precision 3D seismic data, data of geochemistry, logging and testing, the reservoir characteristics and accumulation conditions of the KL6-1 lithologic oilfield in the Laibei Low Uplift in the Bohai Sea are examined comprehensively. The study shows that: KL6-1 oilfield is a monolithic, high-quality, large-scale Neogene lithologic oilfield featuring shallow reservoir depth, high productivity, concentrated oil-bearing intervals, large oil-bearing area, and high reserve abundance; hydrocarbon source supply from two directions provides a sufficient material basis for the formation of large oil field; two types of “inherited structural ridge” developed under the effect of block rotation, late active faults formed by Neotectonic movement, and widely distributed contiguous sand bodies provide an efficient oil and gas transportation system for the large-scale accumulation of oil and gas; contiguous channel and lacustrine lowstand system sand bodies developed in low accommodation condition provide the basic condition for the formation of large-scale lithologic traps; deep formations structural ridge, faults (dominant migration pathways) and large-scale superimposed contiguous sand bodies constitute a “vine type” oil and gas migration and accumulation system in the Laibei Low Uplift, which is conducive to the formation of large-scale and high-abundance lithologic reservoir in this area. The successful discovery of KL6-1, 100 million ton reserve order lithologic oil field, has revealed the exploration potential of Neogene large lithologic reservoirs in Bohai Sea, expanded the exploration field, and also has certain reference significance for the exploration of large lithologic reservoirs in similar areas.
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This paper systematically discusses the multiple source characteristics and formation mechanisms of carbonate-rich fine-grained sedimentary rocks through the analysis of material source and rock formation. The hydrocarbon accumulation characteristics of carbonate-rich fine-grained sedimentary rocks are also summarized. The results show that the main reason for the enrichment of fine-grained carbonate materials in rift lake basins was the supply of multiple material sources, including terrestrial material input, formation of intrabasinal authigenic carbonate, volcanic-hydrothermal material feeding and mixed source. The development of carbonate bedrock in the provenance area controlled the filling scale of carbonate materials in rift lake basins. The volcanic-hydrothermal activity might provide an alkaline fluid to the lake basins to strengthen the material supply for the formation of carbonate crystals. Authigenic carbonate crystals induced by biological processes were the main source of long-term accumulation of fine-grained carbonate materials in the lake basins. Carbonate-rich fine-grained sedimentary rocks with multiple features were formed through the interaction of physical, biochemical and chemical processes during the deposition and post-deposition stages. The source and sedimentary origin of the fine-grained carbonate rock controlled the hydrocarbon accumulation in it. In the multi-source system, the types of “sweet spots” of continental shale oil and gas include endogenous type, terrigenous type, volcanic-hydrothermal type and mixed source type.
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By using the latest geological, seismic, drilling and logging data, this article studies the basic conditions for the formation of the total petroleum system and the orderly coexisting characteristics and accumulation models of conventional & unconventional reservoirs in the Lower Permian Fengcheng Formation in the Junggar Basin. Controlled by thermal evolution, hydrocarbon generation and expulsion process of the high-quality source rocks in alkaline lake as well as the characteristics of multi-type reservoirs (conglomerate, sandstone, dolomite and shale), conventional structure-lithologic reservoirs and tight oil and shale oil reservoirs controlled by source-reservoir structure have been formed. On the plane, mature conventional reservoirs, medium-high mature tight oil, and medium-high mature shale oil reservoirs coexist orderly from the slope area around Mahu sag to the sag. Based on the orderly coexisting characteristics of conventional and unconventional reservoirs in the Fengcheng Formation, it is clear that oil and gas in the Fengcheng Formation accumulate continuously over a large area in three accumulation models: integrated source-reservoir, source-reservoir in close contact, and separated source-reservoir model. The three accumulation models differ in relationship between source-reservoir structure, reservoir lithology and spatial distribution, hydrocarbon migration, oil and gas type. It is pointed out that the conventional & unconventional oil and gas should be explored and developed as a whole to achieve an overall breakthrough of the total petroleum system. This study is expected to enrich the geological theory of oil and gas enrichment in continental basins and to provide an analogy for exploration and research in other hydrocarbon-rich sags.
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By analyzing the structural background, petroleum geological conditions, and typical regional (paleo) oil and gas reservoirs in marine ultra-deep oil and gas regions in China, this paper reveals the evolution processes of the marine ultra-deep oil and gas reservoirs and the key controlling factors of accumulation. The marine ultra-deep oil and gas resources in China are buried at depth of greater than 6000 m, and are mainly distributed in the Precambrian and Lower Paleozoic strata in the Sichuan, Tarim and Ordos cratonic basins. The development of marine ultra-deep source rocks in China is controlled by cratonic rifts and cratonic depressions with the background of global supercontinent breakup-convergence cycles. The source rocks in Sichuan Basin have the most developed strata, followed by Tarim Basin, and the development strata and scale of Ordos Basin needs to be further confirmed. The marine ultra-deep reservoir in China is dominated by carbonate rocks, and the reservoir performance is controlled by high-energy sedimentary environment in the early stage, superimposed corrosion and fracture in the later stage. The regional caprocks are dominated by gypsum salt rocks, shale, and tight carbonate rock. The ultra-deep oil and gas fields in China have generally experienced two stages of oil-reservoir forming, cracking (or partial cracking) of paleo-oil reservoirs, and late finalization of cracked gas (or highly mature to over mature oil and gas). The oil and gas accumulation is controlled by static and dynamic geological elements jointly. Major hydrocarbon generation center, high quality and large-scale reservoir resulted from karstification of high energy facies belt, thick gypsum rock or shale caprock, and stable trapping and preservation conditions are the key factors for accumulation of ultra-deep oil and gas. We propose three favorable exploration directions, i.e. the areas around intracratonic rift and intracratonic depression, and craton margin.
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Taking the mixed pre-salt carbonate rocks in the upper member of Eocene Xiaganchaigou Formation (E32) of Yingxi area in the Qaidam Basin as an example, the lithofacies and controlling mechanisms of reservoir formation are analyzed based on a large dataset of cores, thin sections and geochemical analysis. The reservoirs in E32 pre-salt layers have five types of lithofacies, of them, mixed granular calcareous dolostone, massive calcareous dolostone, plaque calcareous dolostone, and laminated dolomtic limestone are of sedimentary origin, and breccia calcareous dolostone is of tectonic origin. The four types of sedimentary lithofacies are divided into two types of saline sedimentary sequence lithofacies combinations, low-energy type in the sag area and low to high-energy type in the slope and paleo-uplift zone in the depression. Affected by high-frequency supply of continental clastic material, the two types of salty sedimentary sequences are mostly incomplete subtypes of lithofacies. Lithofacies have strong impacts on pre-salt reservoirs in E32: (1) Lithofacies type and sedimentary sequence controlled the formation and distribution of dolomite intercrystalline pores and dissolved pores during the pene-sedimentary period. (2) The structure of laminated dolomitic limestone controlled the formation of large-scale laminated fractures and high permeability channels during the diagenetic period. (3) Granular, massive, plaque calcareous dolostones have low mud content and strong brittleness, in the late tectonic reactivation period, the distribution of the three types of lithofacies, together with their distance from the top large slip faults and secondary faults, controlled the formation and distribution of high-efficiency fracture-cave brecciaed calcareous dolostone reservoirs. The above research led to the composite lithofacies-tectonic formation model of pre-salt reservoir in E32 of Yingxi area. The tempo-spatial distribution of tectonic breccia calcareous dolostone reservoirs, laminated dolomitic limestone shale oil reservoirs and granular, massive calcareous dolostone dissolved-intercrystalline pore tight reservoirs in various structural belts of the studied area have been figured out. These findings gave new insights into tight-shale oil accumulation theory in mixed carbonate successions from saline lacustrine basins, aiding in high efficient exploration and development of petroleum in the studied area.
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Based on analysis of outcrop, drilling, logging and seismic data, and geotectonic background, the lithofacies paleogeography and paleokarst geomorphology of the Middle Permian Maokou Formation in the northwestern Sichuan Basin were reconstructed, and the petroleum geological significance of the lithofacies paleogeography and paleokarst geomorphology were discussed. The Maokou Formation is divided into 3 long-term cycles, namely LSC1, LSC2 and LSC3, which correspond to the Member 1, Member 2 and Member 3 of the Maokou Formation, respectively. Controlled by the extensional structure caused by opening of the Mianlue Ocean in the north margin of the upper Yangtze blocks and basement faults produced by mantle plume uplifting, the area had tectonic differentiation in NWW and NE, and sedimentary basement took on episodic settlement from north to south, as a result, the sedimentary systems of Member 1 to Member 3 gradually evolved from carbonate platform to platform-slope-continental shelf. According to the residual thickness, paleokarst geomorphologic units such as karst highland, karst slope and karst depression at different stages were reconstructed. The karst geomorphological units were developed successively on the basis of sedimentary geomorphology. Sedimentary facies and paleokarst geomorphology are of great significance for oil and gas accumulation. The Maokou Formation in northwestern Sichuan has two kinds of most favorable reservoir zone combinations: high energy grain shoal and karst monadnock, platform margin slope and karst slope. Based on this understanding, the planar distribution of the two kinds of reservoir zones were predicted by overlapping the favorable reservoir facies belt with paleokarst geomorphology. The study results provide a new idea and reference for the exploration deployment of the Middle Permian Maokou Formation in the Sichuan Basin.
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Based on the observation of outcrops and cores, thin section identification, restoration of paleogeomorphology by residual thickness method, fine description of seismic facies, etc., the coupling relationships between the development patterns of various types of Cambrian platform margin mound-shoal complexes and paleogeomorphology in the Gucheng area of Tarim Basin have been examined. The Cambrian platform margin mound-shoal complex is divided into mound base, mound core, mound front, mound back and mound flat microfacies, which are composed of dolomites of seven textures with facies indication. The different paleogeomorphology before the deposition of mound-shoal complex in each period was reconstructed, and three types of mound-shoal complex sedimentary models corresponding to the paleogeomorphologies of four stages were established: namely, the first stage of gentle slope symmetric accretion type, the second stage of steep slope asymmetric accretion type and the third and fourth stages of steep slope asymmetric progradation type. Their microfacies are respectively characterized by “mound base - mound back + (small) mound core + mound front - mound flat” symmetric vertical accretion structure, “mound base - (large) mound core + mound front - mound flat” asymmetric vertical accretion structure, “mound base - (small) mound core + mound front - mound flat” asymmetric lateral progradation structure. With most developed favorable reservoir facies belt, the steep slope asymmetric accretion type mound-shoal complex with the characteristics of “large mound and large shoal” is the exploration target for oil and gas reservoir.
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Based on the cores from 47 wells and logging data of 130 wells, the main types of sand bodies in the shallow water delta of the Chang 82 sub-member in the Triassic Yanchang Formation, Longdong Area, Ordos Basin were comprehensively analyzed, the distribution characteristics of sand bodies more than 20 m thick were identified, and the genetic types of the thick sand bodies were sorted out. In this region, thick-bed sand bodies can be divided into two types according to the shape and position: type 1 is the sand body in wide strip and is distributed between the average high water mark and the average low water mark; type 2 is distributed on both sides of the average low water mark and is in irregular lump shape. Based on the principle of the volume distribution of sediments and the change rule of accommodation space, the genetic models of two types of thick-bed sand bodies in the Chang 82 sub-member are superimposed distributary channel sand bodies in high accommodation space and superimposed composite sand bodies in low accommodation space.
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With many types of salt structures developed in the Lower Cretaceous Aptian Formation, the passive continental marginal basins in the middle segment of the south Atlantic are hot areas of deep-water petroleum exploration. Based on analysis of differential deformations of salt structures, the influences of the inclination of subsalt slope, subsalt topographic reliefs and basement uplifting on the formation of salt structures were analyzed by physical modeling in this work. The experimental results show that the subsalt slopes in the middle West Africa basins are steeper, so the salt rock is likely to rapidly flow towards the ocean to form larger and fewer salt diapirs. In the Santos and Campos basins, the basement uplifts outside the basins are far from the provenances, which is conducive to the intrusion and accumulation of salt rock on the top of the basement uplifts. In contrast, in the middle West Africa, the basement uplifts are close to the basin margin, the residual salt layers above them are thin, and small triangular salt structures develop on both sides of the uplifts. Moreover, the uplifting of the African plate is also conducive to the full development of salt diapirs in the middle West Africa and results in large-scale thrust faults and folds in the front compressional zone.
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To understand the evolution of the Miocene gravity flow deposits in the Lower Congo-Congo Fan Basin, this paper documents the Miocene sequence stratigraphic framework, the depositional characteristics and the controlling factors of the gravity flow system. Based on the establishment of high-resolution sequence stratigraphic framework, lithofacies characteristics and sedimentary units of the gravity flow deposits in the region are identified by using seismic, well logging and core data comprehensively, and the sedimentary evolution process is revealed and the controlling factors are discussed. The Miocene can be divided into four 3rd-order sequences (SQ1-SQ4). The gravity flow deposits mainly include siliciclastic rock and pelite. The main sedimentary units include slumping deposits, mass transport deposits (MTD), channel fills, levee-overbank deposits, and frontal lobes. In the Early Miocene (SQ1), mainly gull-wing, weakly restricted to unrestricted depositional channel-overbank complexes and lobes were formed. In the early Middle Miocene (SQ2), W-shaped and weakly restricted erosional-depositional channels (multi-phase superposition) were subsequently developed. In the late Middle Miocene (SQ3), primarily U-shaped and restricted erosional channels were developed. In the Late Miocene (SQ4), largely V-shaped and deeply erosional isolated channels were formed in the study area. Climate cooling and continuous fall of the sea level made the study area change from toe of slope-submarine plain to lower continental slope, middle continental slope and finally to upper continental slope, which in turn affected the strength of the gravity flow. The three times of tectonic uplifting and climate cooling in the West African coast provided abundant sediment supply for the development of gravity flow deposits. Multistage activities of salt structures played important roles in redirecting, restricting, blocking and destroying the gravity flow deposits. Clarifying the characteristics, evolution and controlling factors of the Miocene gravity flow deposits in the Lower Congo-Congo Fan Basin can provide reference for deep-water petroleum exploration in this basin.
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Based on the pseudo-analytical equation of electromagnetic log for layered formation, an optimal boundary match method is proposed to adaptively truncate the encountered formation structures. An efficient integral method is put forward to significantly accelerate the convergence of Sommerfeld integral. By asymptotically approximating and subtracting the first reflection/transmission waves from the scattered field, the new Sommerfeld integral method has addressed difficulties encountered by the traditional digital filtering method, such as low computational precision and limited operating range, and realized the acceleration of the computation speed of logging-while-drilling electromagnetic measurements (LWD EM). By making use of the priori information from the offset/pilot wells and interactively adjusting the formation model, the optimum initial guesses of the inversion model is determined in order to predict the nearby formation boundaries. The gradient optimization algorithm is developed and an interactive inversion system for the LWD EM data from the horizontal wells is established. The inverted results of field data demonstrated that the real-time interactive inversion method is capable of providing the accurate boundaries of layers around the wellbore from the LWD EM, and it will benefit the wellbore trajectory optimization and reservoir interpretation.
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Taking reservoir rocks and fluids of the Daqing, Dagang and Changqing oilfields as research objects, the EOR mechanisms and technical approach of polymer flooding were discussed. By comparing the displacement performances of ordinary polymer, glycerol, polymer in “sheet-net” structure and heterogeneous weak gel at the same viscosity and concentration, the relationship between the viscosity of polymer displacement agents and displacement performance was demonstrated, and the method of improving polymer flooding effect was worked out. The main mechanism of polymer flooding to increase oil recovery is the swept volume expansion of water injection due to polymer retention in porous media. The viscosity of polymer agents has no positive correlation with polymer flooding effect. Although polymer of “sheet-net” structure has strong capacity in increasing viscosity, it has poor compatibility with pore throat structure of reservoir rock, low injectivity and low shear resistance. Heterogeneous weak gel system has higher adsorption and capture capacity in porous media, which is easy to retain in porous media, and can effectively establish seepage resistance in high permeability layers (zones). Compared with polymer solutions with the same viscosity or concentration, it has stronger ability to expand swept volume. Long term injection of polymer flooding agents will inevitably lead to fluid entry profile reversal, and thus worsening of polymer flooding effect. Alternate injection of high retention and low or non-retention displacement agents can further improve the displacement effect of polymer flooding agents.
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A new method for selecting dimensionless relaxation time in the lattice Boltzmann model was proposed based on similarity criterion and gas true physical parameters. At the same time, the dimensionless relaxation time was modified by considering the influence of the boundary Knudsen layer. On this basis, the second-order slip boundary condition of the wall was considered, and the key parameters in the corresponding combined bounce-back/specular-reflection boundary condition were deduced to build a new model of unconventional gas microscale flow simulation based on the lattice Boltzmann method suitable for high temperatures and high pressures. The simulation results of methane gas flow driven by body force in infinite micro-channels and flow driven by inlet-outlet pressure differential in long straight channels were compared with the numerical and analytical solutions in the literature to verify the accuracy of the model, and the dimensionless relaxation time modification was formally optimized. The results show that the new model can effectively characterize the slippage effect, compression effect, gas density and the effect of boundary Knudsen layer in the micro-scale flow of unconventional natural gas. The new model can achieve a more comprehensive characterization of the real gas flow conditions and can be used as a basic model for the simulation of unconventional gas flow on the micro-nano scale.
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By conducting relative permeability experiments of multi-cycle gas-water displacement and imbibition on natural cores, we discuss relative permeability hysteresis effect in underground gas storage during multi-cycle injection and production. A correction method for relative permeability hysteresis in numerical simulation of water-invaded gas storage has been worked out using the Carlson and Killough models. A geologic model of water-invaded sandstone gas storage with medium-low permeability is built to investigate the impacts of relative permeability hysteresis on fluid distribution and production performance during multi-cycle injection and production of the gas storage. The study shows that relative permeability hysteresis effect occurs during high-speed injection and production in gas storage converted from water-invaded gas reservoir, and leads to increase of gas-water transition zone width and thickness, shrinkage of the area of high-efficiency gas storage, and decrease of the peak value variation of pore volume containing gas, and then reduces the storage capacity, working gas volume, and high-efficiency operation span of the gas storage. Numerical simulations exhibit large prediction errors of performance indexes if this hysteresis effect is not considered. Killough and Carlson methods can be used to correct the relative permeability hysteresis effect in water-invaded underground gas storage to improve the prediction accuracy. The Killough method has better adaptability to the example model.
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A forecasting method of oil well production based on multivariate time series (MTS) and vector autoregressive (VAR) machine learning model for waterflooding reservoir is proposed, and an example application is carried out. This method first uses MTS analysis to optimize injection and production data on the basis of well pattern analysis. The oil production of different production wells and water injection of injection wells in the well group are regarded as mutually related time series. Then a VAR model is established to mine the linear relationship from MTS data and forecast the oil well production by model fitting. The analysis of history production data of waterflooding reservoirs shows that, compared with history matching results of numerical reservoir simulation, the production forecasting results from the machine learning model are more accurate, and uncertainty analysis can improve the safety of forecasting results. Furthermore, impulse response analysis can evaluate the oil production contribution of the injection well, which can provide theoretical guidance for adjustment of waterflooding development plan.
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To get a deeper understanding on the synergistic enhancement effect of low frequency artificial seismic wave on foam stability, a micro-kinetic model of enhanced foam stability under low frequency artificial seismic wave is established based on a vertical liquid film drainage model and elastic wave theory. The model is solved by non-dimensional transformation of the high order partial differential equations and a compound solution of implicit and explicit differences and is verified to be accurate. The foam film thickness, surfactant concentration distribution and drainage velocity under the action of low frequency artificial seismic wave are quantitatively analyzed. The research shows that low-frequency vibration can reduce the difference between the maximum and minimum concentrations of surfactant in the foam liquid film at the later stage of drainage, enhance the effect of Marangoni effect, and improve the stability of the foam liquid film. When the vibration frequency is close to the natural frequency of the foam liquid film, the vibration effect is the best, and the best vibration frequency is about 50 Hz. The higher the vibration acceleration, the faster the recovery rate of surfactant concentration in the foam liquid film is. The higher the vibration acceleration, the stronger the ability of Marangoni effect to delay the drainage of foam liquid film and the better the foam stability is. It is not the higher the vibration acceleration, the better. The best vibration acceleration is about 0.5 times of gravity acceleration. Reasonable vibration parameters would greatly enhance the effect of Marangoni effect. The smaller the initial concentration of surfactant, the better the vibration works in enhancing Marangoni effect.
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By reviewing the development history of stimulation techniques for deep/ultra-deep oil and gas reservoirs, the new progress in this field in China and abroad has been summed up, including deeper understanding on formation mechanisms of fracture network in deep/ultra-deep oil and gas reservoir, performance improvement of fracturing fluid materials, fine stratification of ultra-deep vertical wells, and mature staged multi-cluster fracturing technique for ultra-deep and highly deviated wells/horizontal wells. In light of the exploration and development trend of ultra-deep oil and gas reservoirs in China, the requirements and technical difficulties in ultra-deep oil and gas reservoir stimulation are discussed: (1) The research and application of integrated geological engineering technology is difficult. (2) The requirements on fracturing materials for stimulation are high. (3) It is difficult to further improve the production in vertical profile of the ultra-deep and hugely thick reservoirs. (4) The requirements on tools and supporting high-pressure equipment on the ground for stimulation are high. (5) It is difficult to achieve efficient stimulation of ultra-deep, high-temperature and high-pressure wells. (6) It is difficult to monitor directly the reservoir stimulation and evaluate the stimulation effect accurately after stimulation. In line with the complex geological characteristics of ultra-deep oil and gas reservoirs in China, seven technical development directions are proposed: (1) To establish systematic new techniques for basic research and evaluation experiments; (2) to strengthen geological research and improve the operational mechanism of integrating geological research and engineering operation; (3) to develop high-efficiency fracturing materials for ultra-deep reservoirs; (4) to research separated layer fracturing technology for ultra-deep and hugely thick reservoirs; (5) to explore fracture-control stimulation technology for ultra-deep horizontal well; (6) to develop direct monitoring technology for hydraulic fractures in ultra-deep oil and gas reservoirs; (7) to develop downhole fracturing tools with high temperature and high pressure tolerance and supporting wellhead equipment able to withstand high pressure.
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A coupled CFD-DEM method is used to simulate the formation process of fracture plugging zone. A photo-elastic system characterizing mesoscale force chain network developed by our own is used to model the pressure evolution in fracture plugging zone to reveal the evolution mechanism of the structure of fracture plugging zone. A theoretical basis is provided for improving the lost circulation control effect in fractured reservoirs and novel methods are proposed for selecting loss control materials and designing loss control formula. CFD-DEM simulation results show that bridging probability is the key factor determining the formation of fracture plugging zone and fracture plugging efficiency. Critical and absolute bridging concentrations are proposed as the key indexes for loss control formula design. With the increase of absolute bridging concentration, the governing factor of bridging is changed from material grain size to the combination of material grain size and friction force. Results of photo-elastic experiments show that mesoscale force chain network is the intrinsic factor affecting the evolution of pressure exerting on the fracture plugging zone and determines the macroscopic strength of fracture plugging zone. Performance parameters of loss control material affect the force chain network structure and the ratio of stronger force chain, and further impact the stability and strength of fracture plugging zone. Based on the study results, the loss control formula is optimized and new-type loss control material is designed. Laboratory experiments results show that the fracture plugging efficiency and strength is effectively improved.
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The crack volume strain method and acoustic emission (AE) method are used to analyze the anisotropy of the crack initiation strength, damage strength, the failure mode and the AE characteristics of coal reservoir. The results show that coal reservoirs show obvious anisotropic characteristics in compressive strength, cracking initiation strength and damage strength. The compressive strength of coal reservoirs decreases with the increase of bedding angle, but the reservoirs with bedding angles of 45° and 90° differ little in compressive strength. The crack initiation strength and damage strength decrease first and then increase with the increase of bedding angle. The crack initiation strength and damage strength are the highest, at the bedding angle of 0°, moderate at the bedding angle of 90°, and lowest at the bedding angle of 45°. When the bedding angle is 0°, the failure of the coal reservoirs is mainly steady propagation of large-scale fractures. When the bedding angle is 45°, one type of failure is caused by steady propagation of small-scale fractures, and the other type of failure is due to a sudden instability of large-scale fractures. When the bedding angle is 90°, the failure is mainly demonstrated by a sudden-instability of small-scale fractures. Compared with the cumulative count method of the AE, the cumulative energy method is more suitable for determining crack initiation strength and damage strength of coal reservoirs.
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This paper targets the shale oil reservoirs of middle to high maturity in four major basins of China, including the Permian Lucaogou Formation of the Jimsar Sag in the Junggar Basin, the Chang 73 Member of the Triassic Yanchang Formation in the Longdong area of the Ordos Basin, the Kong 2 Member of the Paleogene Kongdian Formation in Cangdong Sag of the Bohai Bay Basin, and the Qing 1 Member of the Cretaceous Qingshankou Formation in Changling Sag of the Songliao Basin. The key parameters of the shale oil reservoirs in the four basins, such as reservoirs effectiveness, oil content, crude oil movability, and fracability, have been revealed under identical experimental conditions using the same evaluation technical system, on the basis of technique development and integrated application of multi-scale spatial distribution depiction, effective connectivity calculation, movable oil assessment based on the charging effect, and simulation of fracture propagation during reservoir stimulation. This research overcomes insufficient resolutions of conventional analysis approaches and difficulties in quantitative evaluation, develops the evaluation method for resource recoverability of different types of shale oil, and gains insights into different types of shale oil via comparison. The results of experiments and comparative analysis show that there are significant differences in the endowment of continental shale oil resources in the four major basins in China. Among them, the Lucaogou Formation in the Junggar Basin has more effective shale reservoirs, the Chang 73 sub-member of the Ordos Basin has a comparatively good proportion of movable oil and the Kong 2 Member of the Bohai Bay Basin has the best fracability. These results can provide references and basis for choosing development plans and engineering techniques.
