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Table of Content

    28 June 2019, Volume 40 Issue 3
    Previous Issue    Next Issue
    Petroleum Geology
    Has China ushered in the shale oil and gas revolution?
    Jin Zhijun, Bai Zhenrui, Gao Bo, Li Maowen
    2019, 40(3):  451-458.  doi:10.11743/ogg20190301
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    With the development of economy,the world energy consumption continues to grow,and the dominant position of oil and gas in the world primary energy consumption will remain stable for a long period.The proportion of shale oil and gas in the global oil and gas supply is increasing continuously,which is mainly due to the success of the American shale revolution.The successful development of shale gas has turned the United States into a net exporter of natural gas,while shale oil has greatly reduced its dependence on foreign oil to a new low of nearly 10%.The shale revolution in the United States not only has been boosting the economy development of the United States,but also has a far-reaching impact on the world's energy pattern.Under the vigorous promotion of the Chinese government and through the active practice of the NOCs,China's shale gas has achieved a major breakthrough in a short period of time,becoming the third country across the world with commercial development capability after the United States and Canada.Although China has made great progress in shale gas exploration and development,it still faces many challenges,the solution to which requires innovation in theories,technologies and business models.China has huge potential of shale oil resources,but the exploration and development of the shale oil are even more challenging.It is necessary to carry out in-depth research on the enrichment mechanism,distribution pattern,sweet-spot prediction and low-cost development technologies of continental shale oil.Overall,China is embracing the shale gas revolution,while the shale oil revolution has not yet arrived.
    Shale oil resource potential and its mobility assessment: A case study of Upper Devonian Duvernay shale in Western Canada Sedimentary Basin
    Chen Zhuoheng, Li Maowen, Jiang Chunqing, Qian Menhui
    2019, 40(3):  459-468.  doi:10.11743/ogg20190302
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    Shale oil reservoir is a source-reservoir-caprock integration characterized by dual-porosity with nano-pores being dominant.Oil mobility in the reservoir is one of the key geological factors for the commercial resource development with success.Shale oil resource evaluation method based on conventional reservoir volumetric approach is subject to the pore structure of simple matrix and not applicable to assess the resources according to their states of occurrence in reservoir.As such it cannot provide adequate information with respect to oil and gas mobility for making resource development decisions.The state of occurrence of shale oil and gas is primarily controlled by the maturity and types of source rocks,and Rock-Eval pyrolysis data contain critical information related to source rock types and maturity.According to the states of oil occurrence in shale reservoir,the oil and gas resources are divided into three categories:non-movable (adsorbed),restricted,and movable resources,to quantitatively describe the mobility of oil and gas in shale reservoir.Combined with the production and reservoir data from the Upper Devonian Duvernay shale oil field in the Western Canada Sedimentary Basin,empirical models are established for assessing resources in terms of shale oil reservoir parameters and pyrolysis data,and then oil formation volume factor is used to estimate the evaporative loss of light hydrocarbons for pyrolysis parameter (S1)during coring and sampling.A single well evaluation of Duvernay shale oil and gas resource potential in Well 8-32-46-9W5 is taken as an example to introduce the principles and demonstrate the application of the proposed method in shale oil resource assessment.The resource potential of the block with Well 8-32-46-9W5 as its core calculated by this method and with 10% recovery discounted,is consistent with the result derived from production decline model based on actual production data in adjacent areas,which demonstrates that the shale oil resource assessment method proposed here based on pyrolysis data and hydrocarbon generation kinetics model is feasible.
    Shale oil exploration and production in the U.S.: Status and outlook
    Zhou Qingfan, Jin Zhijun, Yang Guofeng, Dong Ning, Shang Zhucheng
    2019, 40(3):  469-477.  doi:10.11743/ogg20190303
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    Shale oil(tight oil) plays are widely distributed in Permian,Williston,GOM,Anadarko and many other sedimentary basins in the U.S.The proved reserves of shale plays keep increasing year by year and account for about half of U.S.proved crude oil reserves.Approximately 80% of shale oil production comes from Permian Basin,Bakken and Eagle Fords plays.Favorable geological conditions are the fundamental factor for the shale oil development in the U.S.Affected by the slump in oil prices in 2014,the count of active rigs in the U.S.shale plays dropped dramatically,leading to the decrease of shale oil production.But the growth of shale oil production has recovered and made new highs since mid-2016,indicating a great potential for shale oil development in the U.S.The U.S. shale oil production is expected to keep increasing in the next decade,and to be the primary source of oil production growth in the U.S.
    Geological characteristics and latest progress in exploration and development of Russian shale oil
    Liang Xinping, Jin Zhijun, Alexander Shpilman, Yin Jinyin, Liu Quanyou, Boris Uspensky
    2019, 40(3):  478-490,503.  doi:10.11743/ogg20190304
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    Russia is rich in unconventional shale oil resources,covering almost all sedimentary basins ranging from Cambrian to Neogene in age.The major areas of prospecting potential in Russia are the Bazhenov Formation of the Western Siberian platform and the Domanik formations in the Eastern European platform.Bazhenov Formation is the Upper Jurassic-Lower Cretaceous in age.Its main lithology is siliceous shale,with a thickness of 30 m and TOC of 7% on average.Its TOC is generally above 4%,mainly falling on kerogens TypeⅠ-Ⅱ domain,with the maturity ranging from 0.5% to 1.1%.While Domanik formations,also called "Domanik facies" or "Domanik deposits" from the Paleozoic Upper Devonian (Russian stage-D3 fr2) to Lower Carboniferous (Tournaisian-C1t) in age,in the Volga-Urals and Timan-Pecher oil and gas provinces,are dark siliceous-clayey-bituminous limestones falling on kerogens TypeⅠ-Ⅱ domain,with an overall thickness of 100-600 m,TOC of 0.5-24%,and maturity ranging from 0.5% to 1.5% in the north-central part of Volga-Ural Basin.However,their gas production is mainly concentrated on the southern part of Caspian sea.Despite of the unfavorable impact due to the low international oil prices,the exploration and development of shale oil resources in Russia have never been stopped:146 vertical wells in the Bazhenov Formation were tested in 2016,with an average daily production rate of 10.8 t during the active days; meanwhile 36 horizontal wells were tested with an average daily production rate of 7.5 t during the active days.The Bazhenov technology center,set up by Gazprom in 2018,has cut down the time of hydraulic fracturing to 50%,and unit production cost to 40% in the development of Bazhenov oil.Moreover,the deve-lopment of shale oil resources in Russia can enjoy preferential policies like zero tax rate for mineral exploitations.Together with its existing favorable conditions,including abundant geological data for most oil fields,relatively short distances among these oil fields,and perfect infrastructures,the expectation is very positive for the future commercial development of shale oil in Russia.
    “Chemo-sedimentary facies” analysis: An effective method to study fine-grained sedimentary rocks
    Liu Keyu, Liu Chang
    2019, 40(3):  491-503.  doi:10.11743/ogg20190305
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    Fine-grained sedimentary rocks,a key rock type bearing shale oil and gas,are characterized by fine grain size,colour uniformity and strong heterogeneity.Traditional sedimentological study based on visual observation and optical images alone becomes inadequate to precisely characterize such fine-grained rocks.Therefore,a "chemo-sedimentary facies" model for analyzing fine core-scale samples is established based on the high-resolution(down to 30 μm measurement intervals) 2D inorganic geochemical element mapping of rock slabs obtained by in-situ XRF scanning and optical images.Through the model,it is possible to unambiguously recognize the rock compositions and sedimentary fabrics.And then certain environment-sensitive elemental parameters(ratios) can be calculated in situ from individual chemo-sedimentary facies at millimeter-centimeter scales for the effective interpretation of depositional environment and provenance of fine-grained sediments,as well as their origin.Meanwhile,the method also avoids information interference in the conventional bulk compositional analyses whereby homogenized sample powders are often used.In conclusion,the method can help to establish more fine-scale sedimentary facies models and to provide reliable data for the exploration and development of tight and shale oil & gas.
    Progresses in geophysical characterization of continental shale oil sweet spots
    Liu Xiwu, Liu Yuwei, Liu Zhiyuan, Song Liang, Liu Jiong, Huo Zhizhou, Zhang Jinqiang, Qian Keran, Zhang Yingyan
    2019, 40(3):  504-511.  doi:10.11743/ogg20190306
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    There are still many challenges on geophysical characterization of continental shale oil sweet spot in Eastern China in terms of technique applicability.Petrophysical models for continental shale oil layers are established to analyze geophysical responses based on the study on the fundamentals of continental shale oil.It's believed that the controlling factors for shale oil accumulation are organic-rick laminated lithofacies and fractures,following the integrated investigation of geology,well logging and geophysics.Besides,based on the existing methods for seismic prediction including lithofacies,TOC,rock physical property,oil bearing properties,brittleness,pore pressure and in-situ stress,new geophysical prediction methods are built for laminated lithofacies,fracture and in-situ stress.Based on petrophysical models and anisotropy pre-stack inversion,new methods are developed to predict the density of bedding-parallel fractures in shale,which is in turn regarded as a constraint for sedimentary analysis,so as to effectively predict the distribution of the laminated lithofacies.Orthorhombic anisotropy AVAZ algorithm is given to improve the seismic characterization of fractures;while the prediction of in-situ stress is achieved based onYoung's modulus,bulk modulus,and modulus of rigidity,and the direction of in-situ stress can be computed from interval velocity variation.The study clarifies the main controlling factors for continental shale oil sweet spots,establishes an index system for sweet spots,reveals the characteristics of petrophysical and geophysical responses of sweet spots,and optimizes the geophysical characterization method for sweet spots.These methods are successfully used to identify the favorable target areas for continental shale oil exploration in Jiyang Depression.
    Geological characteristics and effectiveness of the shale oil reservoir in Dongying sag
    Liu Huimin, Zhang Shun, Bao Youshu, Fang Zhengwei, Yao Suping, Wang Yong
    2019, 40(3):  512-523.  doi:10.11743/ogg20190307
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    The reservoir space types of various sizes and their connection patterns of the Paleogene shale in Jiyang Depression were characterized through core observation,polarizing microscope and argon ion milling-scanning electron microscopy (SEM) analyses,and high pressure mercury injection,as well as small angle X ray scattering experiment; in addition,the lower limits of effective pore diameter and porosity were clarified according to data such as porosity and oil saturation.A spatial development and distribution model of shale reservoir in Dongying sag was established to predict the favorable reservoir facies belt through integrating thermal maturity (Ro) with the characteristics of lithofacies,formation pressure,and reservoir space (porosity and pore diameter).The results show that:(1) the pores of the shale reservoir in the study area are mainly intergranular pores,shrinkage pores in organic matter and clay minerals,and intercrystalline pores in carbonate,all of which are connected through a three-level pore-fracture network; (2) the intraparticle pores in clay minerals and intercrystalline pores in carbonate are the most effective pores with the highest contribution to the total porosity,averaging at 50% to 70%,followed by clay shrinkage cracks and tension fissure; (3) the lower limit of pore diameter for shale oil occurrence,the minimum pore diameter for free oil occurrence,and the threshold pore diameter of free oil enrichment are 5nm,10nm and 30mm,respectively; (4) the laminated shale rich in organic matter and carbonate minerals is the highest in total porosity,pore connectivity,and contribution of pores favorable for free oil storage to porosity,so it is a preferential lithofacies; (5) the development and distribution characteristics of shale reservoir space are controlled by the lithofacies type,diagenetic stage,formation pressure and fracture development degree,all of which also serve as important factors for the evaluation of shale oil sweet spots; the prediction of favorable reservoir facies belt is to identify laminated organic-rich shale with relatively higher thermal maturity,highly-developed fracture and high formation pressure under the guidance of graded evaluation scheme.
    Geological characteristics and accumulation mechanism of continental shale oil in Jimusaer sag,Junggar Basin
    Zhi Dongming, Tang Yong, Yang Zhifeng, Guo Xuguang, Zheng Menglin, Wan Min, Huang Liliang
    2019, 40(3):  524-534.  doi:10.11743/ogg20190308
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    We studied the geological characteristics and accumulation mechanism of continental shale oil in Jimusaer sag,based on the data collected from core and casting thin section observation,well log,Rock-Eval pyrolysis,field emission scanning electron microscopy (FE-SEM),laser-scanning confocal microscopy and high pressure mercury injection.The results show that the Lucaogou Formation in Jimusaer sag is originated from the organic-rich sediments in saline lacustrine basins,which serve as both source rocks and reservoirs,and the shale oil reservoirs herein are geologically characterized by "indigenous accumulation".There are two types of lithologic associations:one is siltstone-mudstone,and the other is dolomite-mudstone.Oil is pervasive in the Lucaogou Formation,enriches in "sweet spots",and has lower maturity.The shale oil reservoir is oil-wet,and pressurization caused by hydrocarbon generation is the main driving force for its shale oil migration and accumulation.The micro-and nano-pore throats,bedding-parallel fractures and tectonic fractures are favorable reservoir space and vadose channels for the Lucaogou shale oil in Jimusaer sag.The shale oil accumulates within or close to the source rocks as a whole,and occurs in the "sweet spots" and shale intervals in free or absorbed phases.
    Characteristics and oil-bearing capability of shale oil reservoir in the Permian Lucaogou Formation,Jimusaer sag
    Xu Lin, Chang Qiusheng, Yang Chengke, Tao Qin'e, Wang Shili, Fei Liying, Xu Shilu
    2019, 40(3):  535-549.  doi:10.11743/ogg20190309
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    The characteristics,controlling factors and oil-bearing capability of the Lucaogou reservoir in Jimusaer sag were studied based on core observation,thin section features,as well as data from scanning electron microscopy (SEM),physical and geochemical analysis.The results indicate that the sedimentary environment of the Lucaogou Formation is of saline lacustrine,in which a set of fine-grained hybrid sedimentation of lacustrine dark mudstone,silty-fine sand and carbonate was developed.Its lithology is mainly diamictite characterized by alternating thin beds of silty-fine sand,clay and carbonate.Reservoir space is dominated by intergranular dissolved pores and intercrystalline micro-pores,with residual intergranular pores ranked second and followed by a few cracks.The controlling factors for the reservoir properties of Lucaogou shale include sedimentary facies,lithology,organic carbon content (TOC) and diagenesis.The reservoir properties of the Lucaogou shale in the study area show a remarkable control on oil-bearing capability,and the oil film was well developed in dolomitic rocks.Reservoir wettability is primarily neutral,followed by localized lipophobicity.In addition,the reservoir micro-wettability change from oil-wet and oil-bearing to hydrophilic and moisture-containing with smaller pore sizes,as showcased by the field emission scanning electron microscopy.It demonstrates that the shale oil reservoir in the Lucaogou Formation,Jimusaer sag,is capable of high saturation oil-bearing,which has laid a sound foundation for the shale oil exploration and development in the region.
    Determination of the starting time for measurement of NMR effective porosity in shale oil reservoir: A case study of the Permian Lucaogou shale oil reservoir,Jimusaer sag
    Wang Wei, Zhao Yanwei, Mao Rui, Sun Zhongchun, Mu Liwei
    2019, 40(3):  550-557.  doi:10.11743/ogg20190310
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    Due to the fine-grained lithology and complex pore throat structure of the Lucaogou shale oil reservoir in Jimusaer sag,Junggar Basin,it is difficult to accurately calculate the effective porosity by conventional logging interpretation methods.To solve this difficulty,30 sandstone samples of five clay types were selected for measurements such as porosity under helium expansion,nuclear magnetic resonance (NMR) T2 spectrum under different conditions,type and content of clay.The mechanism of clay type effect on short lateral relaxation component of reservoir NMR T2 signal was revealed through the analysis of matching experimental data.Subsequently,combined with the experimental results and the NMR relaxation time of bound water in various clay types,the effective porosity of shale oil reservoirs in the study area was calculated accurately by iteration method.The initial NMR time was determined to be 1.7 ms,which have been verified well by core analysis and actual logging.The study serves as a new technical solution for effective porosity evaluation of shale oil reservoir.
    Discussion on prospecting potential of shale oil in the 3rd sub-member of the Triassic Chang 7 member in Binchang block,southwestern Ordos Basin
    Li Zhiming, Tao Guoliang, Li Maowen, Qian Menhui, Xie Xiaomin, Jiang Qigui, Liu Peng, Bao Yunjie, Xia Dongling
    2019, 40(3):  558-570.  doi:10.11743/ogg20190311
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    To discuss the prospecting potential of shale oil in the 3rd sub-member of the Triassic Chang 7 member(Chang 73) in Binchang block,southwestern Ordos Basin,the oil content and oil occurrence of typical rock types in Well B-1 were systematically observed and evaluated based on systematic observation and description of cores and using the results of Rock-Eval pyrolysis,thermovaporization in different temperature ranges and organic petrological analysis in combination with scanned images of cores with white light and fluorescence.The results show that the cored interval in Chang 73 from Well B-1 is quality source rock at early stage of major oil window.The black laminated shale and layered mudstone are characterized by high oil content but dominance of immobile oil,and the average OSI is 45 mg/g(n=116),indicating no prospecting potential for shale oil.Whereas,the gray-brown laminated,thin-bedded and massive tuff and the interbedded tuffaceous(argillaceous) siltstone are characterized by high oil content,predominance of free oil and contributions from migrated oil.The oil mainly occurs in microfractures,intergranular pores and grain boundary fractures of detrital minerals,and its average OSI is 200 mg/g(n=28),indicating good prospecting potential for shale oil.In conclusion,the gray-brown tuff and the interbedded tuffaceous(argillaceous) siltstone on the Yishan slope in Binchang block are favorable intervals for shale oil exploration and have some shale oil prospecting potential.
    Experimental procedures of well-site geological evaluation for shale oil and related technological progress
    Jiang Qigui, Li Maowen, Qian Menhui, Bao Yunjie, Liu Peng, Tao Guoliang, Ma Xiaoxiao, Li Zhiming, Cao Tingting, Wu Shiqiang
    2019, 40(3):  571-582.  doi:10.11743/ogg20190312
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    In the drilling and completion process of a shale oil exploration well,it is necessary to evaluate the oil-bearing property and movability of shale oil in time.However,the existing well-site evaluation techniques are developed primarily for conventional sandstone reservoirs,thus cannot meet the requirements of shale heterogeneity description.Therefore,it is in urgent need to establish a specific experimental procedure for shale oil drilling.In accordance with the requirements of rapid well-site geological evaluation and the technical limitations of available instrumentations,we proposed item selection,sampling,sample preservation and technical workflow for on-site experiment.Furthermore,this workflow was tested and applied in several well-site operations.A magnetohydrodynamic variable density rock bulk volume testing device was developed to solve the problem that it is hard to rapidly measure physical properties due to the difficulty to sample plungers for shales prone to loose and deform.In addition,a technique was invented to analyze the physical properties of shales with no fixed geometry.In order to minimize the evaporative loss of light hydrocarbons and to make Rock-Eval pyrolysis analysis data more reliable,shale samples were ground in liquid nitrogen freezing instruments.Analytical programs for well-site and lab tests were optimized and synergized to ensure more efficient workflow.These new methods were applied successfully to the Qianjiang Formation in Jianghan Basin,Shahejie Formation in Jiyang Depression and Chang 7 in Ordos Basin,laying a sound foundation for fast geological evaluation of continental shale oil exploration.Meanwhile,what should be pointed out is that the current well-site experimental technology is still far from perfect for China's shale oil exploration,for instance,a lack of related specifications and criteria.What we should do further is to improve the existing well-site experimental and applied technologies,develop normative methodology and technology systems,and build a rapid geological evaluation platform for shale oil exploration.
    Quantitative evaluation models of adsorbed and free shale oil and its microscopic occurrence mechanism
    Li Junqian, Lu Shuangfang, Zhang Jie, Zhang Pengfei, Xue Haitao
    2019, 40(3):  583-592.  doi:10.11743/ogg20190313
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    The occurrence state of shale oil affects both its mobility and recoverability.In order to quantitatively evaluate the shale oil contents in different occurrence states (adsorbed and free) and their ratios,a series of mathematical models and a theoretical framework for quantitative evaluation of adsorbed and free shale oil were established.They can be used to evaluate:①the contents of adsorbed and free single-component hydrocarbons under unsaturated adsorption conditions (0<p/p0<1);②the contents of adsorbed and free single-component hydrocarbons and mixed hydrocarbons (remaining oil) and their ratios under saturated adsorption conditions (p/p0 ≥ 1).The models have comprehensively taken into account the microscopic occurrence characteristics of shale oil (density of adsorbed and free phases,thickness of adsorbed phase),microscopic structure of shale pores (morphology,size,pore volume,specific surface area) and physical properties of shale reservoir (porosity,oil saturation,apparent density).According to the models and the target area of lacustrine shale oil in eastern China (a case study of Dongying sag),the microscopic occurrence mechanisms of shale oil are preliminarily revealed and the following understandings are obtained:① the contents of adsorbed and free oil and their ratios are jointly controlled by multiple influential parameters;② the impact of pore size is significant when the pore size is less than 50 nm or so,while when the pore diameter is larger than 50 nm or so,the impact is relatively weak,and the content of free oil depends on the volume of oil-bearing pores.
    Evaluation of movable shale oil reserves in the Es1L of the Raoyang sag, Jizhong Depression
    Chen Fangwen, Zhao Hongqin, Wang Shuping, Lu Shuangfang, Wang Min, Ding Xue
    2019, 40(3):  593-601.  doi:10.11743/ogg20190314
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    The Es1L shale oil reservoir in the Raoyang sag,Jizhong Depression,was studied to measure the reservoir rock,crude oil and formation water compressibilities as well as original oil saturation variation with depth,which enable the determination of the variation of the maximum movable oil reserves in massive and laminated shale core samples from diffe-rent burial depth.These shale core samples were analyzed in terms of organic carbon content (TOC),Rock-Eval and porosity according to the well logging plot for the studied horizons.The results show that the movable oil reserves of natural depletion and dissolution gas drive increase with increasing burial depth both in massive and laminated shale reservoirs in Es1L,except for a sweat spot area located at a burial depth of 2 500 m at the immature production stage.Under natural depletion conditions,the movable oil ratio in massive reservoirs is markedly larger than that in laminated reservoirs,while slightly smaller under solution gas drive conditions.The movable reserves in per unit volume of massive shale oil reservoirs are slightly and obviously smaller than those in laminated shale oil reservoirs under natural depletion and dissolution gas drive respectively.The average movable reserves under natural depletion are 0.13×10-3 t/m3 and 0.14×10-3 t/m3,and those under dissolution gas drive are 0.56×10-3 t/m3 and 1.27×10-3 t/m3 in massive and laminated shale reservoirs respectively.
    Petroleum Development and Engineering
    Progresses in shale gas well integrity research
    Gao Deli, Liu Kui
    2019, 40(3):  602-615.  doi:10.11743/ogg20190315
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    The sustainable annular pressure (SCP) caused by sealing failure of cement sheath and the casing deformation during hydraulic fracturing,are the major well problems integrity encountered in shale gas development.Based on the existing research results,we focus on summarizing and analyzing the progresses in research of factors contributing to failure of shale gas well integrity.With the progress in researches about casing deformation during hydraulic fracturing in shale gas wells worldwide,we know that the major influential factors include temperature,stress,asymmetric fracturing in the reservoir,poor cementing and the slip of faults or natural fractures.Among them,the shear deformation mechanism of the casing in shale gas well caused by the slip of faults or natural fractures during fracturing has attracted more and more attention among the researchers.It is proposed that the casing deformation can be effectively reduced by increasing the casing strength and the cementing performance,or avoiding the areas with fault or natural fracture slips.Besides,the sealing failure of cement sheath in shale gas wells is mainly caused by yield failure of cement sheath out of pressure variation within the casing or eccentricity of casing and channeling through interface cracks.The sealing failure of cement sheath can be effectively reduced by applying expansive cement,flexible cement or annular pre-stressing techniques,while the optimization of special cement slurry system in shale gas wells is particularly important for improving the sealing integrity of cement sheath.Given the limitation of the cement sheath sealing ability,additional mechanical sealing methods can also be used to meet the requirements of sealing integrity for certain well annulus areas.The research results about the shale gas well integrity are also of referential value to the design and control of other unconventional oil & gas wells to be completed through volume-fracturing.
    Hydraulic fracture propagation behavious and geometry under supercritical CO2 fracturing in shale reservoirs
    Su Jianzheng, Li Fengxia, Zhou Tong
    2019, 40(3):  616-625.  doi:10.11743/ogg20190316
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    The supercritical CO2 fracturing technology is yet to be improved,and the mechanisms of fracture generation and propagation are still not clear at present.In order to get an in-depth understanding of hydraulic fracture propagation behaviors and geometry under supercritical CO2 fracturing,we introduced the Pen-Robinson equation to simulate the process of supercritical CO2 fracturing,based on the displacement discontinuity boundary element method.Combined with lab physical simulation experiments,the differences of hydraulic fracture propagation behaviors and geometry between conventional fracturing with water-based fluid and fracturing with supercritical CO2 in the shale reservoir were discussed.The results show that the pressurization in the pores of surrounding rocks will function to reduce the constraint of in-situ stress on fracture propagation,thanks to the diffusivity and good permeability of supercritical CO2,and in turn the initiation pressure for fractures is lower than that of conventional fracturing.The incremental volumetric strain generated during and the failure of fractures after supercritical CO2 fracturing are higher than those under conventional fracturing,thus the fracture geometry under supercritical CO2 fracturing is more complex than that under fracturing with water-based fluid; meanwhile,the fracture plane under supercritical CO2 fracturing is more complex and uneven,and has higher tortuosity than that under conventional fracturing with water-based fluid.
    Fracturing technology of stimulated reservoir volume with subdivision cutting for shale oil horizontal wells in Ordos Basin
    Mu Lijun, Zhao Zhenfeng, Li Xianwen, Zhang Kuangsheng, Tang Meirong, Du Xianfei, Bai Xiaohu
    2019, 40(3):  626-635.  doi:10.11743/ogg20190317
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    The Ordos Basin is rich in shale oil resources.But due to factors such as tight reservoir,low primary reservoir pressure coefficient and high heterogeneity of lacustrine sedimentation,the production of early conventional vertical single well is extremely low.Since 2011,the multistage SRV(stimulated reservoir volume) fracturing technology for horizontal wells has been successfully applied to shale oil reservoirs of Type I and Ⅱ,and the daily production per well has increased to more than 10t/d.However,we still have to face the problems including high decline rate of production,poor benefit under low oil price in large-scale development,and a fairly low transmission efficiency from resource to reserve,then to production,and finally to economical development.With a view to further improving the individual well production and recovery degree,an integrated set of evaluation criteria for horizontal reservoir quality and engineering quality has been established based on the composite evaluation of hydraulic fracture propagation patterns and fracture geometry,and combined with the evaluation of lithology,physical property,oil-bearing property and compressibility,targeting at maximizing the fracture-controlled reserves.The nonhomogeneous multi-cluster fracturing design with subdivision cutting based on the classification of horizontal laterals and the method to optimize the fracture-controlled volume fracturing parameters have been improved.In view of the theory of limited entry and bridging,the fracture-controlled technologies such as limit multi-cluster perforation and dynamic temporary plugging diversion have been integrated to improve the multi-cluster effectiveness.The cost of SRV fracturing with subdivision cutting can be controlled and reduced by applying dissolvable bridge plug fracturing tools,low-cost slick water fracturing fluid and combined particle-size quartz sand proppants.Following multiple researches and practices,the fracturing technology of SRV with subdivision cutting for long horizontal wells has been created and applied to develop shale oil reservoirs of Type Ⅰ and Ⅱ economically.From 2017 to 2018,this technology were put into effect on 85 horizontal wells with the initial oil production per well reaching above 16 t.And their accumulative production was 4 850 t for the first year,which was 1 120 t more year-on-year.While the decline rate fell by 15% in the first year and the single-well production was predicted to be improved by 10,000 t or so.The Break Even Point (BEP)has declined from $60 per barrel to around $40.Field practice has proved that the technique of fracturing with subdivision cutting and intensive reformation for long horizontal wells is effective to efficiently develop shale oil,and it is of reference value for improving the efficient mobilization and development of other shale oil resources.
    Reservoir conditions and mechanism of shale oil flow
    Dong Mingzhe, Li Yajun, Sang Qian, Li Sheng, Li Meng, Su Yuliang
    2019, 40(3):  636-644.  doi:10.11743/ogg20190318
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    With the development of horizontal wells through multi-stage hydraulic fracturing techniques,the shale oil production rate and its decline rate are closely related to the flow ability of shale oil in the matrixes.Although the shale re-servoir rich in crude oil is characterized by fine pores and extremely low permeability,it is possible to accomplish commercial production of shale oil if the laminated texture (featuring sandstone alternating with shale) is well developed.The mechanism of shale oil flow in laminated formations is not clear yet.The commonly applied dual-media or equivalent permeability methods in field-scale reservoir simulations cannot be used to describe the microscopic phenomena of shale oil flow in laminated reservoirs,and consequently the simulated shale oil production is not reliable.Therefore,we established a macroscopic conceptual model for shale oil reservoirs based on the features of continental shale reservoirs,and analyzed the flow mechanisms of shale oil in laminated reservoirs.Besides,the effects of laminated texture properties and fracture distribution on shale oil flow in shale reservoirs during primary production were investigated through simulation.The simulated results verify the presence of transverse flow and the significant control of laminated sandstone intervals' permeability on the overall reservoir permeability,and demonstrate the important role of both natural and artificial fractures in enhancing fluid communication within the laminated reservoirs.
    Numerical simulation of shale oil coupled flow in multi-pore media
    Su Yuliang, Lu Mingjing, Li Meng, Zhang Qi, Wang Wendong, Dong Mingzhe
    2019, 40(3):  645-652,660.  doi:10.11743/ogg20190319
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    Shale reservoirs are characterized by various pores with micro-and nano-pores well developed.Commercial shale oil development can be achieved by means of staged fracturing in horizontal wells.The interaction between oil molecules and pore wall is more complex than that between methane molecules and pore wall,but it is still unclear about the migration mechanism of shale oil in inorganic and organic nano-pores at present.Accurate simulation of the microscopic migration mechanisms of shale oil and the coupled flow in multi-pore media is of great significance to productivity evaluation and production prediction in shale oil reservoirs.Considering wettability and mechanisms of liquid adsorption,velocity slip and physical property change,we established the apparent permeability model for fluids in micro-and nano-scale multi-pore media based on the complex structural parameters (including tortuosity,porosity and organic pore content),to explore the effects of different migration mechanisms on the apparent permeability of micro-and nano-scale multi-pore media.Subsequently,the mathematical model with matrix,natural fracture and artificial fracture coupled was set up for multistage fractured horizontal wells in shale oil reservoirs.Besides,the finite element method was used to solve the mo-del,and to analyze the factors affecting productivity.The results show that when the pore radius is less than 10 nm,the effect of velocity slip in micro-and nano-pores is significant,while when the pore radius is greater than 100 nm,the effect of microscopic migration mechanisms may be neglected.The less the number of organic pores and the more the number of fractures,the greater the productivity of multistage fractured horizontal wells will be.The optimal fracture network is that has neither spacing nor overlapping between the adjacent fractures.The highlight of the study is to enrich the theories on oil and gas migration in micro-and nano-pores,so as to contribute to the development simulation of shale reservoirs with theoretical methods.
    Characteristics of the shale bedding planes and their control on hydraulic fracturing
    Li Xiao, He Jianming, Yin Chao, Huang Beixiu, Li Guanfang, Zhang Zhaobin, Li Lihui
    2019, 40(3):  653-660.  doi:10.11743/ogg20190320
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    Shale,a kind of sedimentary rocks,is characterized by significant bedding.The investigation of the impact of bedding planes of shale on hydraulic fracturing is of great significance for the exploitation of shale gas.The characteristics of sedimentary structures at various scales (ranging from meter to ten micron scales) of shale in the Yanchang Formation in southern Ordos Basin were analyzed,and the hydraulic fracturing tests for multi-sized shale specimens under different dip angles of bedding plane were carried out,both of which function to explain the characteristics of the bedding planes of shale and their controlling effects on hydraulic fracturing process and outcome.The results show that the average thickness of laminae is of fractal features with a fractal dimension of 1.06,and is 2.26 m,2.09 dm,1.70 cm,1.48 mm and 11.7 μm,corresponding to meter,decimeter,centimeter,millimeter and 10-micron scales respectively.The behaviors of hydraulic fracturing are significantly influenced by the bedding planes of shale,clearly shown by the contrast of fracture geometry before and after fracturing and the fracturing pressure.When the dip angles of the bedding planes are less than 30°,the fractured shale specimen is remarkably different from its original one in fracture geometry,featuring much more artificial fractures,and higher fracturing pressure,which tends to slump as the dip angle of bedding plane increases; while when the dip angles of the shale specimens are above 45°,the fracture geometry of the shale specimens after fracturing is almost the same with that of the original specimens,featuring lower fracturing pressure which tends to fluctuate a little with the increase of the dip angle of the bedding planes.Generally,the fracturing pressures of shale under different dip angles of bedding plane fluctuates in the shape of oblique "S".Besides,the controlling effects of structural dip angles of the bedding planes on the fracture propagation,hydraulic pressure curve and fracturing pressure are also reflected in the tests.
    Rock mechanics and fracability evaluation of the Lucaogou Formation oil shales in Jimusaer sag, Junggar Basin
    Wang Xiaojun, Liang Lixi, Zhao Long, Liu Xiangjun, Qin Zhijun, Li Wei
    2019, 40(3):  661-668.  doi:10.11743/ogg20190321
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    The shale oil reservoirs are characterized by low oil and gas abundance,poor permeability,no natural productivity or lower natural production capacity in single wells compared with the lower limit of commercial oil flow,and fast energy decay,etc.The application of efficient volumetric fracturing technique to horizontal or multi-lateral wells is the key to achieving efficient development of shale oils.In order to evaluate the fracability of the shale oil reservoir in the Lucaogou Formation in Jimusaer sag,sufficient experimental and theoretical studies were carried out.The characteristics of deformation and failure,mechanical strength and their lateral variations in the Lucaogou oil-bearing reservoir in Jimusaer sag were recognized.And a new method for evaluating the fracability indexes was established based on the analysis of the factors influencing the shale reservoir fracability in the Lucaogou Formation.The results show that due to the significant brittleness of shale under deformation and the well development of fabric planes such as bedding and micro-fractures,the intrinsic geological and mechanical conditions of the Lucaogou Formation are favorable for the formation of complex fracture networks under fracturing.There are interlayers characterized by high rock mechanical strength and in-situ stress in the Lucaogou shale reservoir.Therefore,to maximize the effective volume of fracturing in such reservoir,the artificial fractures should be designed to increase the connectivity between upper and lower reservoirs,while the fracture networks should be as complicated as possible.Besides,a new fracability evaluation method was established using brittleness index,horizontal stress difference,interlayer stress difference and fracture ductility,which can simultaneously characterize the ability of volumetric fracturing network in horizontal wells and the vertical connectivity of artificial fractures.The analysis based on microseismic fracturing monitoring shows that the fracability index evaluation method is very suitable to evaluate thin alternating shale formations typically shown in the oil shales in Jimusaer sag.
    Fracture extension and distribution pattern of volume fracturing in tight reservoir: An analysis based on actual coring data after fracturing
    Niu Xiaobing, Feng Shengbin, You Yuan, Liang Xiaowei, Xin Honggang, Dan Weidong, Li Tingyan, Ren Jisheng
    2019, 40(3):  669-677.  doi:10.11743/ogg20190322
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    The extension and distribution of hydraulic fractures in stimulated reservoir volume (SRV) after volume fracturing are the key for unconventional oil and gas exploration and development.The artificial fractures created in reservoirs after volume fracturing were identified and characterized based on the actual coring data,logging data and laboratory data of the coring wells completed in the tight oil pilot area of the Ordos Basin.Subsequently,the development characteristics of artificial fractures in the simulated reservoir volume (SRV) were analyzed,and the effective extension and spatial distribution of the hydraulic fractures were discussed.The results show that under volume fracturing,the nearly vertical extensional mirco-fractures of 0.13 m in length were created in the oil-bearing massive sandstones in the Chang 7 of the tight oil pilot zone X233,and many bedding-parallel or -crossing fractures were observed in oil layers about 1.49 m away from the operational area.Hydrocarbon extravasation along these fractures is significant,indicating that they are the artificial fractures generated by fracturing.By comprehensive analysis,we found that a hydraulic fracture network consists of limited number of main fractures,a large number of micro-fractures generated and bedding-parallel fractures extended or expanded by fracturing.Meanwhile,the extension range of macro-fracture network in the SRV is limited,but the micro-fractures are widely developed,and the remaining oil saturation of the reservoir stays high,all of which indicate that there is still room to optimize the volume fracturing technology for tight oil reservoir.
    Organic matter pyrolysis kinetics and formation permeability variation during upgrading process of low-maturity shale oil
    Wang Yiwei, Wang Youping, Meng Xianglong, Su Jianzheng, Long Qiulian
    2019, 40(3):  678-684.  doi:10.11743/ogg20190323
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    Shale oil upgrading entails heating of formation through boreholes to convert heavy hydrocarbons in pore space into light ones,and it also stimulate the generation of oil and gas from kerogen through pyrolysis.Prediction of hydrocarbon yield through pyrolysis and changes of formation porosity and permeability are the challenges for shale oil upgrading and recovery.Organic matter pyrolysis and composition transformation in sample cores from shale wells during heating process using gold tube testing device were observed,and the formative kinetic parameters of hydrocarbon gases,light and heavy oil were calculated.The results show that oil generation increases first and then decreases through the temperature range of 280℃-500℃ while gas generation increases constantly.Under lower heating rate,the conversion curve shifts to the left and the pyrolysis temperature decreases.The kinetic parameters of pyrolysis of heavy oil,light oil and hydrocarbon gases were obtained,and the kinetic model can predict hydrocarbon generation amount at any time.The activation energy of heavy and light oil as well as gaseous hydrocarbons is 39-49 kcal/mol,57-74 kcal/mol and 56-59 kcal/mol respectively.In addition,shale permeability variation during the heating process (from ambient temperature to 550℃) were measured through nitrogen tests under triaxial stresses and high temperature.The results show that the permeability curve during the heating process can be divided into three stages,namely descending,ascending and stabilizing stages.Both the matrix permeability and fracture permeability improve remarkably by 1-2 orders of magnitude when reaching pyrolysis temperature.In short,generation of pyrolysis hydrocarbons and permeability variation may serve as a basis for production prediction of oil upgrading and recovery in low-maturity shales.