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A sedimentary basin is classified as a super basin when its cumulative production exceeds 5 billion barrels of oil equivalent (6.82×108t of oil or 7931.66×108m3 of gas) and its remaining recoverable resources are at least 5 billion barrels of oil equivalent. By the end of 2019, the total output of oil and gas in Sichuan Basin had been 6569×108m3 with the gas-oil ratio of 80:1, and the total remaining recoverable resources reached 136 404×108 m3, which makes it as a second-tier super basin. Because the output is mainly gas, it is a super gas basin. The reason why the Sichuan Basin is a super gas basin is that it has four advantages: (1) The advantage of gas source rocks: it has the most gas source rocks (9 sets) among all the basins in China. (2) The advantage of resource quantity: it has the most total remaining recoverable resources among all the basins in China (136404×108 m3). (3) The advantage of large gas fields: it has the most large gas fields (27) among all the basins in China. (4) The advantage of total production: by the end of 2019, the total gas production had been 6 487.8×108m3, which ranked the first among all the basins in China. There are four major breakthroughs in natural gas exploration in Sichuan Basin: (1) A breakthrough in shale gas: shale gas was firstly found in the Ordovician Wufeng-Silurian Longmaxi formations in China. (2) A breakthrough in tight sandstone gas: the Triassic Xu2 Member gas reservoir in Zhongba gas field is the first high recovery tight sandstone gas reservoir in China. (3) A breakthrough in giant carbonate gas fields. (4) A breakthrough in ultra-deep gas reservoir. These breakthroughs have led to important progress in different basins across the country. Super basins are classified according to three criteria: cumulative production and remaining recoverable resources of oil and gas, tectonic attributes of the basin, and the proportions of oil and gas in cumulative oil and gas production.
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Based on correlation between geochemical characteristics of Sinian and Cambrian source rocks and discovered gas reservoirs, paleoand the analysis on geological conditions of reservoir formation, the sources of natural gas in the Sinian of Sichuan Basin have been discussed to sort out the contribution of Sinian source rocks to the gas reservoirs and effectiveness of Sinian primary gas-bearing system. Through the analysis of natural gas composition, carbon and hydrogen isotopes and effectiveness of Sinian accumulation assemblages, it is concluded that: (1) The natural gas derived from the Sinian source rock is characterized by low ethane content, heavy ethane carbon isotope and light methane hydrogen isotope, and obviously different from the gas generated by the Cambrian source rock. (2) The gas reservoirs discovered in Sinian Dengying Formation are sourced by Sinian and Cambrian source rocks, and the Sinian source rock contributes different proportions to the gas in the 4th member and the 2nd member of the Dengying Formation, specifically, 39% and 55% to the 4th member in marginal zone and intra-platform, 54% and 68% to the 2th member in the marginal zone and intra-platform respectively. (3) The effectiveness of the Sinian primary gas-bearing system depends on the gas generating effectiveness of the source kitchen, reservoir and combination of gas accumulation elements. For high-over mature marine source rocks at the Ro of less than 3.5%, besides gas generated from the thermal cracking of liquid hydrocarbon, the kerogen still has some gas generation potential by thermal degradation. In addition, the Sinian microbial dolomite still preserves relatively good-quality reservoirs despite large burial depths, which match well with other basic conditions for gas accumulation in central Sichuan paleo-uplift, increasing the possibility of Sinian primary gas-bearing system. The research results confirm that the Sinian primary gas-bearing system is likely to form large-scale accumulation.
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Classification, superimposed evolution and sedimentary filling of prototype basins are analyzed based on the Wilson cycle principle of plate theory, by dissecting the evolution history of 483 sedimentary basins around the world since the Precambrian, combined with the three stress environments of tension, compression and shear. It is found that plate tectonic evolution controls the superimposed development process and petroleum-bearing conditions of the prototype basins in three aspects: first, more than 85% of the sedimentary basins in the world are developed from the superimposed development of two or more prototype basins; second, the superposition evolution process of the prototype basin takes Wilson cycle as the cycle and cycles in a fixed trajectory repeatedly. In each stage of a cycle, a specific type of prototype basin can be formed; third, each prototype basin can form a unique tectonic-sedimentary system, which determines its unique source, reservoir, cap conditions etc. For hydrocarbon accumulation, the later superimposed prototype basin can change the oil and gas accumulation conditions of the earlier prototype basin, and may form new petroleum systems. Based on this, by defining the type of a current basin as its prototype basin formed by the latest plate tectonic movement, 14 types of prototype basins can be classified in the world, namely, intracontinental growth rift, intracontinental aborted rift, intercontinental rift, passive continental margin, interior craton, trench, fore-arc rift, back-arc rift, back-arc depression, back-arc small ocean, peripheral foreland, back-arc foreland, strike-slip pull-apart, and strike-slip flexural basins. The classification scheme can ensure the uniqueness of the types of individual sedimentary basin, and make it possible to predict their oil and gas potential scientifically through analogy.
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To predict the large-scale source rock and reservoir distribution in Cambrian subsalt formations (Upper Sinian to Mid-Cambrian) in the Tarim Basin, more than 50 2D seismic lines and 3D data volume acquired latest, 22 wells and some outcrops around this area were used to study the tectonic-lithofacies paleogeography, define the distribution of large scale source rocks and reservoirs, and sort out favorable zones. (1) The basin experienced evolution from ramp to deep-water mud-rich ramp, low-gradient slope, weak rimmed to strong rimmed platform, and the differentiation in the platform was controlled by pre-Cambrian palaeo-rifts. (2) The Luonan-Yubei ancient rift controlled the distribution of source rocks in the Lower Cambrian Yuertusi Formation, and this rift together with the northern depression are the main source rock area. (3) There are three sets of large-scale reservoirs, the Upper Sinian Qigebulake Formation, the Lower Cambrian Xiaorbulake Formation and the Wusongger Formation, and shoal-mounds, early dolomitization and multi-levels of unconformities controlled reservoir development. (4) Four favorable zones, the north slope of Tazhong, upper slope of Maigaiti area, Keping-Wensu periphery, and south slope of Tabei, were evaluated. Of them, the north slope of Tazhong is most likely to have exploration breakthrough; the south slope of Tabei is the best area for exploring the Upper Sinian large scale weathering crust dolomite; Maigaiti slope and Keping-Wensu periphery area improved in exploration potentials significantly, and are worth prospecting faster.
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Based on geochemical analysis results of core samples from the Triassic Chang 7 Member of Well Feng 75 drilled in the northwest margin of Ordos Basin, combined with geological characteristics of this region, the formation environment of the black shale and its control on shale oil enrichment are comprehensively studied. From the Chang 73 to Chang 71 Sub-members, the black shale have organic carbon contents decreasing from 5.70% to 3.55%, hydrogen indexes decreasing from 345 mg/g to 269 mg/g, while the oxygen indexes increasing gradually from 6 mg/g to 29 mg/g, indicating that the sedimentary environment during the depositional period of Chang 72 and Chang 73 Sub-members was anoxic. Biomarkers in the black shale change regularly, and have an obvious “inflection point” at the depth of 2753-2777 m in the Chang 73 Sub-member, indicating that the input of terrigenous organic matter increased. However, there is a negative drift about 2% of organic carbon isotopic composition near the “inflection point”, which is in conflict with the results of biomarker compounds. This is because the extreme thermal and anoxic events caused by continental volcanic activity in the ancient Qinling region caused negative drift of carbon isotopic composition of the black shale in the Ordos Basin. The volcanic activity caused rise of temperature, polluted air, extreme weathers, more floods and thus more input of terrigenous organic matter, and gave rise to extremely anoxic environment conducive to the preservation of organic matter. Terrigenous organic matter is more conducive to the formation of light oil than aquatic organic matter, so these sections in Yanchang Formation are major “sweet spots” for shale oil enrichment.
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Based on core description, thin section identification, X-ray diffraction analysis, scanning electron microscopy, low-temperature gas adsorption and high-pressure mercury intrusion porosimetry, the shale lithofacies of Shan23 sub-member of Permian Shanxi Formation in the east margin of Ordos Basin was systematically analyzed in this study. The Shan23sub-member has six lithofacies, namely, low TOC clay shale (C-L), low TOC siliceous shale (S-L), medium TOC siliceous shale (S-M), medium TOC hybrid shale (M-M), high TOC siliceous shale (S-H), and high TOC clay shale (C-H). Among them, S-H is the best lithofacies, S-M and M-M are the second best. The C-L and C-H lithofacies, mainly found in the upper part of Shan23 sub-member, generally developed in tide-dominated delta facies; the S-L, S-M, S-H and M-M shales occurring in the lower part of Shan23 sub-member developed in tide-dominated estuarine bay facies. The S-H, S-M and M-M shales have good pore structure and largely organic matter pores and mineral interparticle pores, including interlayer pore in clay minerals, pyrite intercrystalline pore, and mineral dissolution pore. C-L and S-L shales have mainly mineral interparticle pores and clay mineral interlayer pores, and a small amount of organic matter pores, showing poorer pore structure. The C-H shale has organic micro-pores and a small number of interlayer fissures of clay minerals, showing good micro-pore structure, and poor meso-pore and macro-pore structure. The formation of favorable lithofacies is jointly controlled by depositional environment and diagenesis. Shallow bay-lagoon depositional environment is conducive to the formation of type II2 kerogen which can produce a large number of organic cellular pores. Besides, the rich biogenic silica is conducive to the preservation of primary pores and enhances the fracability of the shale reservoir.
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Based on a large number of field outcrops and cores taken systematically from boreholes, by microscopic observation, physical property analysis, mineralogy analysis, geochemical analysis etc., reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin ("Mao 1 Member" for short) are analyzed. (1) Rhythmic limestone-marl reservoirs of this member mostly exist in marl layers are a set of tight carbonate fracture-pore type reservoir with low porosity and low permeability, with multiple types of storage space, mainly secondary dissolution pores and fissures of clay minerals. (2) The clay minerals are mainly diagenetic clay minerals, such as sepiolite, talc and their intermediate products, aliettite, with hardly terrigenous clay minerals, and the reservoir in different regions have significant differences in the types of clay minerals. (3) The formation of high quality tight carbonate reservoir with limestone-marl interbeds is related to the differential diagenesis in the early seawater burial stage and the exposure karstification in the early diagenetic stage. It is inferred through the study that the inner ramp of southwestern Sichuan Basin is more likely to have sweet spots with high production, while the outer ramp in eastern Sichuan Basin is more likely to have large scale contiguous reservoir with low production.
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Based on outcrop profiles, drilling cores, cast thin sections etc., the types, microfacies combinations and distribution pattern of microbial carbonates in the Ordovician middle assemblage of the mid-eastern Ordos Basin have been systematically analyzed. The middle assemblage of Ordovician in the mid-eastern Ordos Basin has microbial carbonates formed by the calcification of cyanobacteria, including microbial biostromes and microbial mounds made of stromatolites, thrombolites, and oncolites. The distribution of the carbonates shows obvious “stratum-control” and “regional” characteristics. The microbial biostromes 2-3 m thick each are controlled by sequence cycles and sedimentary facies changes, and were mainly formed in the tidal flat environment during the depositional stages of the Ma56 and Ma55 sub-members. The microbial biostrome in the Ma55 sub-member occurring near the carbonate-evaporite transition interface in the early stage of the transgression is distributed mainly in the Mizhi subsag in the eastern part of the basin; the microbial biostrome in the Ma56 sub-member turns up near the carbonate-evoporite transition zone in ring shape in the east of the central uplift. The ancient landform had noticeable control on the distribution of microbial mounds. The microbial mounds or mound-shoal complexes developing mainly during the depositional stages of Ma57_Ma510 sub-members are about 15-25 m thick in single layer and distributed largely in the Wushenqi-Jingbian paleouplift. The development model of the microbial carbonate rocks shows that the carbonate-evaporite lithologic transition zone and the Wushenqi-Jingbian paleouplift are favorable exploration zones of microbial carbonates in the Ordovician middle assemblages.
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In view of the problem of fine characterization of narrow and thin channels, the maximum entropy criterion is used to enhance the focusing characteristics of Wigner-Ville Distribution. On the basis of effectively improving the time-frequency resolution of seismic signal, a new method of microscopic ancient river channel identification is established. Based on the principle of the equivalence between the maximum entropy power spectrum and the AR model power spectrum, the prediction error and the autoregression coefficient of AR model are obtained using the Burg algorithm and Levinson-Durbin recurrence rule. Under the condition of the first derivative of autocorrelation function being 0, the Wigner-Ville Distribution of seismic signal is calculated, and the Wigner-Ville Distribution time-frequency power spectrum (MEWVD) is obtained under the maximum entropy criterion of the microscopic ancient river channel. Through analysis of emulational seismic signal and forward numerical simulation signal of narrow thin model, it is found that MEWVD can effectively avoid the interference of cross term of Wigner-Ville Distribution, and obtain more accurate spectral characteristics than STFT and CWT signal analysis methods. It is also proved that the narrow and thin river channels of different scales can be identified effectively by MEWVD of different frequencies. The method is applied to the third member of Jurassic Shaximiao Formation (J2s33-2) gas reservoir of the Zhongjiang gas field in Sichuan Basin. The spatial information of width and direction of narrow and thin river channels with width less than 500 m and sandstone thickness less than 35 m is accurately identified, providing bases for well deployment and horizontal well fracturing section selection.
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The transformation mechanism of muddy carbonate rock by the coupling of bioturbation and diagenesis was studied based on core, cast thin section and physical property data of Cretaceous strata in the Mesopotamia Basin, the Middle East. There are 3 ways of biological transformation of rocks: (1) The living creatures transformed formations mechanically to make the rocks looser and intergranular pores increase. (2) After formation, burrows were backfilled with coarse-grained debris, and then unsaturated fluid infiltrated into the burrows during the penecontemporaneous period, resulting in dissolution. (3) Chemical alteration occurred in abandoned burrows and dolomitization produced a large number of intercrystalline pores. The coupling of bioturbation and dissolution occurred mainly in the penecontemporaneous phase, and was controlled by rock type, sea level decline, burrow abundance, infillings, and water environment etc. As the burrows had better physical properties than the matrix, unsaturated fluid preferentially migrated along the burrows, leading to dissolution and expansion of the burrows first and then dissolution of the matrix. The coupling of bioturbation and dolomitization occurred mainly in the burial phase. The rich organic matter and reducing bacteria in the burrow provided material basis, reducing conditions and alkaline environment for dolomitization. The metasomatism in the penecontemporaneous period had little effect on the physical properties of the burrows. When the burrows were separated from the deposition interface, equimolar metasomatism occurred in the burrows in a closed environment, forming euhedral fine-crystalline dolomite with intercrystalline pores. The transformation degree of bioturbation to muddy carbonate reservoir depends on rock type, density, connectivity, infillings and structure of the burrows. With the increase of the carbonate mud content, the improvement to rock physical properties by bioturbation becomes more prominent. When the burrows are filled with coarse-grained debris or fine-crystalline dolomite, the greater the density, the higher the connectivity, and the lower the tortuosity of burrows, the better the physical properties of the muddy carbonate rocks are.
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True tri-axial sanding fracturing experiments are carried out on conglomerate samples from the Permian Wuerhe Formation of Mahu sag, Junggar Basin, to study hydraulic fracture propagation geometry and quartz sand transport in matrix-supported fine conglomerate and grain-supported medium conglomerate. The effect of rough fracture surface on conductivity is analyzed using the 3D-printing technology to reconstruct the rough surface formed in the fractured conglomerate. The hydraulic fractures formed in the matrix-supported fine conglomerate are fairly straight, and only more tortuous when encountering large gravels at local parts; thus, proppants can get into the fractures easily with transport distance about 70%-90% of the fracture length. By contrast, in the grain-supported medium conglomerate, hydraulic fractures tend to bypass the gravels to propagate in tortuous paths and frequently change in width; therefore, proppants are difficult to transport in these fractures and only move less than 30% of the fracture length. As the matrix-supported fine conglomerate has high matrix content and low hardness, proppants embed in the fracture surface severely. In contrast, the grain-supported medium conglomerate has higher gravel content and hardness, so the quartz sand is crushed more severely. Under the high proppant concentration of 5 kg/m2, when the closure stress is increased (above 60 MPa), fractures formed in both matrix-supported fine conglomerate and grain-supported medium conglomerate decrease in width significantly, and drop 88% and 92% in conductivity respectively compared with the case under the low closure stress of 20 MPa. The field tests prove that under high closure stress above 60 MPa, using a high proportion of fine proppants with high concentration allow the proppant to move further in the fracture; meanwhile proppant places more uniformly in the rough fracture, resulting in a higher fracture conductivity and an improved well performance.
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Light crude oil from the lower member of the Paleogene Xiaganchaigou Formation of Gaskule in Qinghai Oilfield was selected to carry out thermal kinetic analysis experiments and calculate the activation energy during the oil oxidation process. The oxidation process of crude oil in porous medium was modeled by crude oil static oxidation experiment, and the component changes of crude oil before and after low-temperature oxidation were compared through Fourier transform ion cyclotron resonance mass spectrometry and gas chromatography; the dynamic displacement experiment of oxygen-reduced air was combined with NMR technology to analyze the oil recovery degree of oxygen-reduced air flooding. The whole process of crude oil oxidation can be divided into four stages: light hydrocarbon volatilization, low-temperature oxidation, fuel deposition, and high temperature oxidation; the high temperature oxidation stage needs the highest activation energy, followed by the fuel deposition stage, and the low-temperature oxidation stage needs the lowest activation energy; the concentration of oxygen in the reaction is negatively correlated with the activation energy required for the reaction; the higher the oxygen concentration, the lower the average activation energy required for oxidation reaction is; the low-temperature oxidation reaction between crude oil and air generates a large amount of heat and CO, CO2 and CH4, forming flue gas drive in the reservoir, which has certain effects of mixing phases, reducing viscosity, lowering interfacial tension and promoting expansion of crude oil, and thus helps enhance the oil recovery rate. Under suitable reservoir temperature condition, the degree of recovery of oxygen-reduced air flooding is higher than that of nitrogen flooding for all scales of pore throat, and the air/oxygen-reduced air flooding development should be preferred.
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Based on the analysis of the production composition of reservoirs developed by the second & tertiary recovery combination (STRC), the relationship between the overall output of the STRC project and the production level during the blank water flooding stage is proposed. According to the basic principle of reservoir engineering that the “recovery factor is equal to sweeping coefficient multiplied by oil displacement efficiency”, the formula for calculating the ultimate oil recovery factor of chemical combination flooding reservoir was established. By dividing the reservoir into a series of grids according to differential calculus thinking, the relationship between the ultimate recovery factor of a certain number of grids and the recovery degree of the reservoir was established, and then the variation law of oil production rate of the STRC reservoir was obtained. The concept of “oil rate enlargement factor of chemical combination flooding” was defined, and a production calculation method of reservoir developed by STRC was put forward based on practical oilfield development experience. The study shows that the oil production enhancing effect of STRC increases evenly with the increase of the ratio of STRC displacement efficiency to water displacement efficiency, and increases rapidly with the increase of the ratio of recovery degree at flooding mode conversion to the water displacement efficiency. STRC is more effective in increasing oil production of reservoir with high recovery degree. Through practical tests of the alkali free binary flooding (polymer/surfactant) projects, the relative error of the oil production calculation method of STRC reservoir is about ±10%, which meets the requirements of reservoir engineering.
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Thermosensitivity experiments and simulation calculations were conducted on typical oil sand core samples from Kinosis, Canada to predict the steam chamber development with time-lapse seismic data during the steam-assisted gravity drainage (SAGD). Using an ultrasonic base made of polyether ether ketone resin instead of titanium alloy can improve the signal energy and signal-to-noise ratio and get clear first arrival; with the rise of temperature, heavy oil changes from glass state (at -34.4 °C), to quasi-solid state, and to liquid state (at 49.0 °C) gradually; the quasi-solid heavy oil has significant frequency dispersion. For the sand sample with high oil saturation, its elastic property depends mainly on the nature of the heavy oil, while for the sand sample with low oil saturation, the elastic property depends on the stiffness of the rock matrix. The elastic property of the oil sand is sensitive to temperature noticeably, when the temperature increases from 10 °C to 175 °C, the oil sand samples decrease in compressional and shear wave velocities significantly. Based on the experimental data, the quantitative relationship between the compressional wave impedance of the oil sand and temperature was worked out, and the temperature variation of the steam chamber in the study area was predicted by time-lapse seismic inversion.
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A method is proposed to characterize the fast neutron scattering cross-section (σf) quantitatively by the combination of inelastic gamma rays and captured gamma rays, so as to realize the gas saturation evaluation of CO2-injected heavy oil reservoirs based on the three-detector pulsed neutron logging technology. Factors influencing of the evaluation effect of this method are analyzed and the effectiveness of this method is verified by a simulation example. By using the Monte Carlo simulation method and the physical model of bulk-volume rock, the relationship between σf and CO2 saturation is studied, and the saturation interpretation model is established. The influences of formation temperature and pressure, heavy oil density, borehole fluid and reservoir methane content on the evaluation results of CO2 saturation are analyzed. The results show that the characterization of σf by the combination of secondary gamma information can eliminate the influence of formation lithology, borehole fluid and methane content are the main factors affecting the quantitative monitoring of CO2 saturation, and the effects of formation temperature and pressure and heavy oil density are negligible. The simulation example verified the feasibility of the method for evaluating the CO2 saturation of CO2-injected heavy oil reservoirs.
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Microscopic computed tomography (Micro-CT) is used to visualize microscopic flow in sandstone core samples during emulsion flooding. The images obtained during the experiment are processed quantitatively with a series of methods to evaluate the occurrence characteristics and oil recovery enhancement mechanisms of emulsion. (1) The emulsion is distributed in the cores in spherical shape, and its sphericity is significantly different from that of the remaining oil. Sphericity can be taken as a characteristic parameter to identify emulsion. (2) The emulsion with specific size prefers to stay in pores with matching sizes; when the emulsion volume is smaller than the product of the lower limit of pore occupancy and the corresponding pore volume, it will not be able to effectively trap in the pore to achieve conformance control. In the process of emulsion displacement designing, we need to design emulsion with suitable particle size according to the pore distribution of the reservoir. (3) Mobilization ratio of the pores can be increased from 23.1% to 59.3% by emulsion flooding after subsequent water flooding, and the average oil displacement efficiency at the pore-scale can be increased from 22.9% to 75.8% under the test conditions; (4) After emulsion flooding, the clustered remaining oil and the oil phase in the trapped emulsion are the main targets for further EOR.
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Since the production regime of shut-in after fracturing is generally adopted for wells in shale oil reservoir, a shut-in pressure drop model coupling wellbore-fracture network-reservoir oil-water two-phase flow has been proposed. The model takes into account the effects of wellbore afterflow, fracture network channeling, and matrix imbibition and oil exchange after stop of pumping. The simulated log-log curve of pressure-drop derivative by the model presents W-shape, reflecting the oil-water displacement law between wellbore, fracture network and matrix, and is divided into eight main control flow stages according to the soaking time. In the initial stage of pressure drop, the afterflow dominates; in the early stage, the pressure drop is controlled by the cross-flow and leakoff of the fracture system, and the fractures close gradually; in the middle stage of pressure drop, matrix imbibition and oil exchange take dominance, and the fracturing fluid loss basically balances with oil replaced from matrix; the late stage of pressure drop is the reservoir boundary control stage, and the leakoff rate of fracturing-fluid and oil exchange rate decrease synchronously till zero. Finally, the fracture network parameters such as half-length of main fracture, main fracture conductivity and secondary fracture density were inversed by fitting the pressure drop data of five wells in Jimsar shale oil reservoir, and the water imbibition volume of matrix and the oil replacement volume in fracture were calculated by this model. The study results provide a theoretical basis for comprehensively evaluating the fracturing effect of shale oil horizontal wells and understanding the oil-water exchange law of shale reservoir after fracturing.
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Aiming at the synergistic rock-breaking mechanism of supercritical carbon dioxide (SC-CO2) jet pressure and temperature difference, a heat-fluid-solid calculation model of rock-breaking stress was established and verified to be effective, and the variations of jet flow field and rock stress with jet standoff distance of SC-CO2, water and nitrogen were studied. With the increase of jet standoff distance, the jet pressure of SC-CO2 decreases and the jet temperature difference increases. The SC-CO2 jet is higher in pressure than the nitrogen jet and differs little from the water jet. Temperature difference of SC-CO2 jet is 5 times that of water jet and more than 2.5 times that of nitrogen jet when the jet standoff distance is larger than 10. The temperature stress is the main reason why SC-CO2 jet is superior to water and nitrogen jets in rock-breaking. The rock under the SC-CO2 jet has greater rock stress, effective rock-breaking jet standoff distance and rock-breaking area. The jet pressure plays a major role in rock-breaking when the jet standoff distance is small, while the jet temperature difference plays a major role in rock-breaking when the jet standoff distance is large. The SC-CO2 jet is an efficient volume rock-breaking method, which results in tensile and shear failure on the rock surface under short time jet and large area tensile failure inside the rock simultaneously under long time jet.
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Gas hydrates and wax are the major flow assurance problems for the transportation of produced hydrocarbons through pipelines. However, in most research works both these two problems are studied separately. Although simultaneous precipitation or deposition of these compounds in pipelines can lead to different mitigation/prevention strategies, the investigations in which both these problems are considered simultaneously appeared only recently. There is no information in the literature on the texture/composition and features of decomposition process of mixed wax/hydrate plugs. At the same time, this information could be useful to understand how to treat the problem of formation of these plugs. In this work, three wax/gas hydrate plugs were collected at quasi-static conditions from a water-in-oil emulsion to study their texture, composition and the features of decomposition process. Powder X-ray diffraction and IR (infrared spectroscopy) analyses showed that the plugs consisted of wax and gas hydrate. Thermovolumetric and DSC (Differential Scanning Calorimetry) experiments showed that the main part of gas hydrate in the plugs at the ambient pressure started to decompose at about 268 K. This temperature was higher than the equilibrium temperature of carbon dioxide hydrate at this pressure, indicating that the gas hydrate in the plugs could be effectively preserved at temperatures below the ice melting point (273.2 K). It was found through observation of the hydrate decomposition process in the plugs under the microscope that the gas in the samples released in small bubbles, while the hydrate particles were not visible at this magnification, indicating that the hydrate was indeed highly dispersed in the samples. A residual wax was jelly-like after decomposition of hydrate in all the cases. Rheological experiments showed that the plugs residues after decomposition of the hydrates had higher yield points and viscosities than the initial waxy crude oil originally used for the experiments.
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Based on the current research status of shale oil exploration and development at home and abroad, combing the field observations, dissection of typical shale oil regions, analysis and testing of organic-rich shale samples, etc., we compare the differences in geological and engineering characteristics of shale oil reservoirs in marine and continental basins between China and the United States. We put forward 8 issues worthy of attention in the exploration and development of lacustrine shale oil in typical basins of China, including the concept of tight oil and shale oil, differences between continental and marine shale oil reservoirs, medium-low maturity and medium-high maturity, vertical permeability and horizontal permeability, source-reservoir and source-caprock, geology and engineering, selection criteria of favorable areas and “sweet spots”, and basic scientific research and application research. By comparing and analyzing organic-rich shales in the Triassic Yanchang Formation of the Ordos Basin, the Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin, the Permian Fengcheng Formation in the Mahu Sag, the Cretaceous Qingshankou & Nenjiang Formation in the Songliao Basin and the Paleogene Kongdian & Shahejie Formation in the Bohai Bay Basin, from shale oil exploration to development, three key scientific issues must be studied in-depth in the future: (1) the physical, chemical and biological processes during the deposition of terrestrial fine-grained sediments and the formation mechanism of terrestrial organic-rich shale; (2) diagenesis-hydrocarbon-generation and storage dynamics, hydrocarbon occurrence and enrichment mechanism; (3) the fracturing mechanisms of terrestrial shale layers in different diagenetic stages and the multi-phase and multi-scale flow mechanism of shale oil in shale layers of different maturities. Clarifying the main controlling factors of shale oil reservoir characterization, oil-bearing properties, compressibility and fluidity of shale oil with different maturities and establishing a lacustrine shale oil enrichment model and the evaluation methodology can provide effective development methods, and theoretical foundation, and technical support for the large scale economical exploration and development of lacustrine shale oil resources in China.
