Fracability evaluation is critical for efficiently extracting deep shale gas using hydraulic fracturing to avoid blind drilling and fracking.However,existing fracability indices often fail to systematically consider t...Fracability evaluation is critical for efficiently extracting deep shale gas using hydraulic fracturing to avoid blind drilling and fracking.However,existing fracability indices often fail to systematically consider the mechanical behavior of rocks at high temperatures and high pressures(HTHP),coupled with geostress distributions and heterogeneous reservoir characteristics.This critical omission limits their effectiveness in accurately identifying the optimal fracability sweet spots within deep reservoirs.In this work,a fracability evaluation model was proposed based on the combined weighting method,integrating the improved brittleness index,rock strength,geostresses and natural weakness characteristics.A fracability grading evaluation was carried out to determine the potential fracture characteristics corresponding to shales with different fracability levels.Additionally,the fracability index was used for field validation and applications.Results show that rock brittleness and fracability are not equivalent for deep reservoirs.The fracability index is closely related to the pay zones and actual gas production,with a correlation as high as 84%,implying that the proposed method has practical significance in both experimental and field applications.The above findings can provide theoretical guidance for the selection of fracturing candidates and the optimal design of fracturing in deep resource development.展开更多
The fracturing technology for shale gas reservoir is the key to the development of shale gas industrialization.It makes much sense to study the mechanical properties and deformation characteristics of shale,due to its...The fracturing technology for shale gas reservoir is the key to the development of shale gas industrialization.It makes much sense to study the mechanical properties and deformation characteristics of shale,due to its close relationship with the fracability of shale gas reservoir.This paper took marine shale in the Changning area,southern Sichuan Basin of China as the research object.Based on field profile and hand specimen observation,we analyzed the development of natural fractures and collected samples from Wufeng Formation and Longmaxi Formation.Combining with the indoor experiment,we investigated the macroscopic and microscopic structural features and the remarkable heterogeneity of shale samples.Then we illustrated the mechanics and deformation characteristics of shale,through uniaxial compression test and direct shear test.The shale has two types of fracture modes,which depend on the angular relation between loading direction and the bedding plane.Besides,the Wufeng shale has a higher value of brittleness index than the Longmaxi shale,which was calculated using two methods,mechanical parameters and mineral composition.Given the above results,we proposed a fracability evaluation model for shale gas reservoir using the analytic hierarchy process.Four influence factors,brittleness index,fracture toughness,natural fractures and cohesive force,are considered.Finally,under the control of normalized value and weight coefficient of each influence factor,the calculations results indicate that the fracability index of the Wufeng Formation is higher than that of the Longmaxi Formation in Changning area,southern Sichuan Basin.展开更多
Fracability is a property that indicates how easy reservoir rocks can be fractured in hydraulic fracturing operations.It is a key parameter for fracturing design and evaluation.In order to utilize continuous logging d...Fracability is a property that indicates how easy reservoir rocks can be fractured in hydraulic fracturing operations.It is a key parameter for fracturing design and evaluation.In order to utilize continuous logging data to predict fracability,synchronous tests of dynamic and static mechanical parameters of rocks under different confining pressures were conducted on13 tight sandstone samples derived from the central Junggar Basin,China.A modified formula between dynamic and static mechanical parameters was established.Fracability of the tight reservoir in the Junggar Basin was then evaluated based on brittleness index,fracture toughness,and fracability index.The effectiveness of fracturing was analyzed combined with the oil testing curve after hydraulic fracturing.The results show that:(1)The distribution of oil-bearing formations in the studied area coincides well with stratum of higher fracability index.(2)The critical fracability index is determined to be 0.3,three formations are selected as fracturing candidates,and a thin mudstone interbed is identified in the oil-bearing formation.(3)Well testing curve verifies the reliability of the fracability evaluation method and the accuracy of the modified formula between dynamic and static mechanical parameters.This study provides useful information for improving fracturing operations of tight oil and gas reservoirs.展开更多
Fracability characterizes the effectiveness of hydraulic fracturing.The existing assessment methods cannot reflect the actual value of the effectiveness due to a lack of comprehensive consideration and neglect of the ...Fracability characterizes the effectiveness of hydraulic fracturing.The existing assessment methods cannot reflect the actual value of the effectiveness due to a lack of comprehensive consideration and neglect of the influences of engineering factors.This study aims to solve this problem by implementing geological static data and production dynamic data in multivariate analysis in Zhaotong shale gas demonstration zone.First,the reservoir quality index(RQI)was introduced to evaluate the exploration potential by integrating the geological parameters with gray relational analysis.Moreover,the differences in fracturing fluid types and proppant sizes were considered,and the operating parameters were normalized on the basis of the equivalence principle.Finally,the general reservoir fracability index(GRFI)was proposed based on a dimensioned processing of the various parameters.A case study was conducted to verify the accuracy and feasibility of this new approach.The results demonstrate that(1)the organic carbon and gas content are adjusted to contribute the most to the calculation of the RQI,while the effective porosity contributes the least;(2)the fracturing scale is the main operating parameter determining the fracability,which has the strongest correlation with the effectiveness of fracking;and(3)the GRFI has a positive correlation with shale gas production,and the lower limit of the GRFI of 2,000 corresponds to a daily production of 50,000 m3/d;this value is defined as the threshold value of a stripper well.The GRFI is consistent with the productivity trend of shale gas wells in the research block,which suggests that the new model is accurate and practical for well candidate selection.展开更多
Natural gas hydrates(hereinafter referred to as hydrates)are a promising clean energy source.However,their current development is far from reaching commercial exploitation.Reservoir stimulation tech-nology provides ne...Natural gas hydrates(hereinafter referred to as hydrates)are a promising clean energy source.However,their current development is far from reaching commercial exploitation.Reservoir stimulation tech-nology provides new approaches to enhance hydrate development effectiveness.Addressing the current lack of quantitative and objective methods for evaluating the fracability of hydrate reservoirs,this study clarifies the relationship between geological and engineering fracability and proposes a comprehensive evaluation model for hydrate reservoir fracability based on grey relational analysis and the criteria importance through intercriteria correlation method.By integrating results from hydraulic fracturing experiments on hydrate sediments,the fracability of hydrate reservoirs is assessed.The concept of critical construction parameter curves for hydrate reservoirs is introduced for the first time.Additionally,two-dimensional fracability index evaluation charts and three-dimensional fracability construction condition discrimination charts are established.The results indicate that as the comprehensive fracability index increases,the feasibility of forming fractures in hydrate reservoirs improves,and the required normalized fracturing construction parameters gradually decrease.The accuracy rate of the charts in judging experimental results reached 89.74%,enabling quick evaluations of whether hydrate reservoirs are worth fracturing,easy to fracture,and capable of being fractured.This has significant engineering implications forthehydraulicfracturingof hydratereservoirs.展开更多
Clarify the mechanical properties of different laminations and the fracture mechanism of continental shale under in-situ stress can provide theoretical basis for more comprehensive evaluation of the fracability of con...Clarify the mechanical properties of different laminations and the fracture mechanism of continental shale under in-situ stress can provide theoretical basis for more comprehensive evaluation of the fracability of continental shale oil reservoir.The Chang 72continental shale was used to investigate the mechanical properties of laminations and the effect of natural structure on the crack propagation of the shale.The XRD and thin section tests show that the laminations contain two types:bright sandy lamination with void structure and dark muddy lamination with layer structure.The real-time CT uniaxial compression tests were conducted to investigate the differences of mechanical properties between the muddy lamination and sandy lamination.It found that the uniaxial compression strength and elastic modulus of the sandy lamination are higher,forming a simple crack with large opening,and the Poisson's ratio of the muddy lamination is large,forming obvious lateral deformation and more secondary cracks.On this basis,the cuboid-shaped continental shale specimens were tested under true triaxial compression conditions to study the effect of laminations and interface cracks on crack propagation combining AE and CT techniques.It found that nascent cracks connected laminations and interface cracks to form fracture network under appropriate loading condition,tensile cracks developed in sandy lamination and shear cracks occurred in muddy lamination because of deformation dissonance and brittleness index differences,and more secondary cracks formed in muddy lamination with smaller fracture toughness.Moreover,the combination relationships between nascent and natural cracks mainly conclude direct penetration and deflection,which is affected by the filling degree and morphology of interface cracks and the relationship of lamination types.These conclusions show that laminar continental shale is conducive to forming complex fracture network,which can provide a theoretical basis for the proposal of indicators and methods for fracability evaluation.展开更多
With respect to brittleness,it is about the type of material and its related strength.In comparison with ductile material under load,brittle material has a relatively shorter plastic deformation and responds dominantl...With respect to brittleness,it is about the type of material and its related strength.In comparison with ductile material under load,brittle material has a relatively shorter plastic deformation and responds dominantly by the elastic deformation.With respect to fracability,it is about the rock failure under the ultimate rock strength in either brittle or ductile formation.In comparison,the higher fracable formation should have smaller formation strength than that of the lower fracable formation.In consequence,it is not certain that the brittle formation is easy to fracture than the ductile formation since brittle formation may have greater strength than ductile formation even though the exceptions may exist.More complications arise when evaluating the responses of subsurface formation in great depth to the formation types(e.g.brittle formation or ductile formation).Under this condition,the impact of confinement on the fracability cannot be ignored.In general,the formation subject to higher confinement pressure is more difficult to fracture as the formation strength is greater.Conversely,the formation subject to lower confinement pressure is easy to fracture since the formation strength is smaller.In view of efficient stimulation of tight shale gas reservoirs,it is unclear whether we would choose the brittle interval or the ductile interval to fracture as the strength of either interval is unknown.However,it is apparent that we should choose the formation interval with a higher fracability which is equivalent to the lower formation strength.Under the similar confinements,the lower formation strength may be indicated by the smaller unconfined compressive strength(UCS).As a result,it is advisable that the most fracable interval is the one with lowest UCS.When evaluating the present technology,the formation brittleness should no longer be the associated subject matter as we are unclear about its role to improve the fracability of the tight formation.Disassociating the brittleness with the fracability enables us to focus on identifying the true mechanisms of efficient fracturing of tight shale gas reservoirs.With the objective review and sensible definition of brittleness used in the present petrophysical field to identify the desirable fracturing intervals,the paper presents the ambiguities of using the brittleness to define the formation fracability and points out that the formation brittleness can be unrelated to the formation fracability.As an alternative approach,the paper provides an effective method to define the most fracable formation intervals in designing the hydraulic fracturing in tight shale gas formations.展开更多
The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery,which is the key stratum for marine shale gas exploration and development(E&D)in China.Based o...The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery,which is the key stratum for marine shale gas exploration and development(E&D)in China.Based on sedimentary environment,material basis,storage space,fracability and reservoir evolution data,the reservoir characteristics of the Wufeng-Longmaxi shale and their significance for shale gas E&D are systematically compared and analyzed in this paper.The results show that(1)the depocenter of the Wufeng(WF)-Longmaxi(LM)shale gradually migrates from east to west.The high-quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales,which are primarily distributed in the graptolite shale interval of WF2-LM5.The high-quality reservoirs in the southern Sichuan Basin are mainly calcareous-siliceous and organic-rich argillaceous shales,which are distributed in the graptolite shale interval of WF2-LM7.(2)Deep shale gas(the burial depth>3500 m)in the Sichuan Basin has high-ultrahigh pressure and superior physical properties.The organic-rich siliceous,calcareous-siliceous and organic-rich argillaceous shales have suitable reservoir properties.The marginal area of the Sichuan Basin has a higher degree of pressure relief,which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity.(3)Combining shale gas exploration practices and impacts of lithofacies,depth,pressure coefficient and brittle-ductile transition on the reservoir properties,it is concluded that the favorable depth interval of the Wufeng-Longmaxi shale gas is 2200~4000 m under current technical conditions.(4)Aiming at the differential reservoir properties of the Wufeng-Longmaxi shale in the Sichuan Basin and its periphery,several suggestions for future research directions and E&D of shale gas are formulated.展开更多
This paper analyzes the differences in geological and development characteristics between continental shale oil in China and marine shale oil in North America, reviews the evaluation methods and technological progress...This paper analyzes the differences in geological and development characteristics between continental shale oil in China and marine shale oil in North America, reviews the evaluation methods and technological progress of the continental shale oil development in China, and points out the existing problems and development directions of the continental shale oil development. The research progress of development evaluation technologies such as favorable lithofacies identification, reservoir characterization, mobility evaluation, fracability evaluation, productivity evaluation and geological-mathematical modeling integration are introduced. The efficient exploration and development of continental shale oil in China are faced with many problems, such as weak basic theoretical research, imperfect exploration and development technology system, big gap in engineering technology between China and other countries, and high development cost. Three key research issues must be studied in the future:(1) forming differentiated development technologies of continental shale oil through geological and engineering integrated research;(2) strengthening the application of big data and artificial intelligence to improve the accuracy of development evaluation;(3) tackling enhanced shale oil recovery technology and research effective development method, so as to improve the development effect and benefit.展开更多
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 ...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.展开更多
In view of the bottleneck of shale gas development technologies,the latest progresses in such theories on the fields like fracture network fracability evaluation,fracture network propagation mechanism,stimulated reser...In view of the bottleneck of shale gas development technologies,the latest progresses in such theories on the fields like fracture network fracability evaluation,fracture network propagation mechanism,stimulated reservoir volume(SRV)evaluation and fracturing fluid research&development were analyzed comprehensively in reference to the existing achievements and experiences in North American shale gas reservoir development and based on the probes,practices and cognitions of shale gas development technologies in China since 2005.First,the study on the shale brittleness develops from the definition of mineral and mechanical semi-quantitative threshold measurement to a comprehensive evaluation which integrates rock constituents,elastic mechanics and development characteristics of natural fractures together.Second,the study on the fracture network development develops from the directional extension theory to the simulation of fracture network formation in the stochastic distribution pattern of natural fractures.Third,the study on SRV develops from the micro-seismic monitoring based instrument and technology to the mathematical theory evaluation method dependent on the discrete fracture network and extended finite element.And fourth,the study on fracturing fluids develops from the wide application of slick water(drag reducing water)and a linear gel fracturing fluid system to the development and application of novel fracturing fluids with little or no water.Then,the theoretical and technological challenges were pointed out,including the comprehensive fracability evaluation of reservoir fracture networks,the deep seated shale gas fracturing,the operation curve diagnosis of shale gas fracturing,the shale gas refracturing theory,and the research&development and flowback control of novel fracturing fluids.Finally,the development trend of related technologies was predicted so as to provide a theoretical and technical guidance for the efficient shale gas development in China in the future.展开更多
Brownshale is a lithology unit in the middle of the Pematang Formation consisting of brown to black shale that is deposited in the lacustrine environment. Brownshale from the results of previous studies stated as the ...Brownshale is a lithology unit in the middle of the Pematang Formation consisting of brown to black shale that is deposited in the lacustrine environment. Brownshale from the results of previous studies stated as the main source rock in the Central Sumatra Basin, which is spread over several troughs, namely Balam, Aman, Rangau, Kiri, and Bengkalis Troughs, where Bengkalis Trough is the most extensive Trough. In the shale hydrocarbon prospecting analysis, Brownshale from previous researchers concluded that it had good prospects, based on several parameters including: TOC values with poor to very good quality. Brownshale formation is a type of kerogene as kerogen type of II/III, brittleness index greater than 0.48, and rock compressive strength below 10,000 Psi. One method in the development phase of shale hydrocarbon is to determine the fracable sweetspot window using drill cuttings and TOC, because there is no core data available. Based on the results of the well log analysis of well BS-03, it is obtained information that the Brownshale formation has a thickness of 1028 feet with intercalation laminated shale/sand section, so the mineral content varies greatly. From the ternary diagram of XRD (bulk analysis) results of drill cuttings of Brownshale formation of well BS-03, it can be seen that mineral distribution of Quartz-Clay-Calcite (Q-C-C) is spread between zone 1 to zone 3, namely: Dominant Quartz - Minor Clay & Carbonate (Zone 1: Brittle Quartz Rich), Dominant Carbonate - Quartz & Minor Clay (Zone 2: Brittle Carbonate Rich), and Quartz & Carbonate Balance - Clay minor (Zone 3: Ductile, hard to frac). This shows that not all Brownshale formation intervals are easy to frac (high fracability). From the XRD result, percentage of mineral content (bulk analysis) of Brownshale drill cuttings, there is an interesting phenomenon, i.e. the presence of sillimanite and kaliophilite minerals significantly starting at a depth of 10,780 ft and below, where both minerals have tenacity: brittle, and also from the results of the MBT analysis seen an interesting phenomenon, i.e. at a depth interval of about 10,780 ft the value of CEC drops below 3 meq/100 grams, and can be categorized as the brittle shale. Referring to the presence of sillimanite and kaliophilite minerals, as well as low MBT values, then at intervals of 10,780 ft below, it can be seen that at the bottom of the depth interval as a fracable sweetspot window, and at the upper depth interval of the Brownshale formation, it is believed to be a fracture barrier.展开更多
Accurate rock elastic property determination is vital for effective hydraulic fracturing,particularly Young’s modulus due to its link to rock brittleness.This study integrates interdisciplinary data for better predic...Accurate rock elastic property determination is vital for effective hydraulic fracturing,particularly Young’s modulus due to its link to rock brittleness.This study integrates interdisciplinary data for better predictions of elastic modulus,combining data mining,experiments,and calibrated synthetics.We used the microstructural insights extracted from rock images for geomechanical facies analysis.Additionally,the petrophysical data and well logs were correlated with shear wave velocity(Vs)and Young’s modulus.We developed a machine-learning workflow to predict Young’s modulus and assess rock fracturability,considering mineral composition,geomechanics,and microstructure.Our findings indicate that artificial neural networks effectively predict Young’s modulus,while K-Means clustering and hierarchical support vector machines excel in identifying rock and geomechanical facies.Utilizing Microscale thin section analysis in conjunction with fracture modeling enhances our understanding of fracture geometries and facilitates fracturability assessment.Notably,fracturability is controlled by specific geomechanical facies during initiation and propagation and influenced by continuity of geomechanical facies in small depth intervals.In conclusion,this study demonstrates data mining and machine learning potential for predicting rock properties and assessing fracturability,aiding hydraulic fracturing design optimization through diverse data and advanced methods.展开更多
Shale oil and gas plays in continental rift basins are complicated and have not been reported elsewhere.In the Luojia area of the Jiyang Depression,an evaluation workflow for shale oil and gas in this continental rift...Shale oil and gas plays in continental rift basins are complicated and have not been reported elsewhere.In the Luojia area of the Jiyang Depression,an evaluation workflow for shale oil and gas in this continental rift basin is proposed.Based on analysis of oil-and gas-related geological conditions,a favorable area of shale oil and gas can be identified,and a high-frequency sequence stratigraphic framework of the target area can be established,therefore,the spatiotemporal distribution of shale has been elucidated in the Luojia area.According to the rock texture,structure,composition and color,petrographic classification criteria for shale are determined,and well log data are used to demarcate,track and predict high-quality lithofacies.Based on geochemical analyses and physical simulations of hydrocarbon generation,abundance,types and maturity of organic matter are analyzed,furthermore,geochemical parameters criteria of hydrocarbon generation and the characteristics of oil and gas occurrence in shales can be determined.Storage space types,assemblages and evolution characteristics of shale reservoirs are studied through core observation,thin-section analysis,electron microscopy examination and fluorescence spectrometry.Combined with analysis of reservoir physical properties,the reservoir performance is evaluated.A saturation model is established based on core analysis,well-log interpretation and well-test production data.The model is further used for evaluation of the movable hydrocarbon contents and integrated assessment of the oil potential.Finally,the shale oil and gas production capacity and exploration prospects in the Luojia area are forecasted based on the analyses of factors controlling production capacity and the rock fracability.Through an integrated analysis of multi-factors(including the lithofacies,source rocks,reservoir properties,oil saturation,and production capacity),the shales in the Luojia area can be divided into three categories,i.e.,Class I(high porosity-high resistivity),Class II(medium porosity-medium resistivity),and Class III(low porosity-medium resistivity).展开更多
基金supported by the National Natural Science Foun-dation of China(Nos.U24B2035 and U22A20166)the Postdoctoral Fellowship Program and China Postdoctoral Science Foundation(Nos.BX20250034 and 2024M763503)the Natural Science Foundation of Hubei Province of China(No.2024AFD374).
文摘Fracability evaluation is critical for efficiently extracting deep shale gas using hydraulic fracturing to avoid blind drilling and fracking.However,existing fracability indices often fail to systematically consider the mechanical behavior of rocks at high temperatures and high pressures(HTHP),coupled with geostress distributions and heterogeneous reservoir characteristics.This critical omission limits their effectiveness in accurately identifying the optimal fracability sweet spots within deep reservoirs.In this work,a fracability evaluation model was proposed based on the combined weighting method,integrating the improved brittleness index,rock strength,geostresses and natural weakness characteristics.A fracability grading evaluation was carried out to determine the potential fracture characteristics corresponding to shales with different fracability levels.Additionally,the fracability index was used for field validation and applications.Results show that rock brittleness and fracability are not equivalent for deep reservoirs.The fracability index is closely related to the pay zones and actual gas production,with a correlation as high as 84%,implying that the proposed method has practical significance in both experimental and field applications.The above findings can provide theoretical guidance for the selection of fracturing candidates and the optimal design of fracturing in deep resource development.
基金financially supported by the National Natural Science Foundation of China(Grant No.41530315,41372213)the National Science and Technology Major Project of China(Grant No.2016ZX05066003,2016ZX05066006)the“Climate Change:Carbon Budget and Related Issues”Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDA05030100)
文摘The fracturing technology for shale gas reservoir is the key to the development of shale gas industrialization.It makes much sense to study the mechanical properties and deformation characteristics of shale,due to its close relationship with the fracability of shale gas reservoir.This paper took marine shale in the Changning area,southern Sichuan Basin of China as the research object.Based on field profile and hand specimen observation,we analyzed the development of natural fractures and collected samples from Wufeng Formation and Longmaxi Formation.Combining with the indoor experiment,we investigated the macroscopic and microscopic structural features and the remarkable heterogeneity of shale samples.Then we illustrated the mechanics and deformation characteristics of shale,through uniaxial compression test and direct shear test.The shale has two types of fracture modes,which depend on the angular relation between loading direction and the bedding plane.Besides,the Wufeng shale has a higher value of brittleness index than the Longmaxi shale,which was calculated using two methods,mechanical parameters and mineral composition.Given the above results,we proposed a fracability evaluation model for shale gas reservoir using the analytic hierarchy process.Four influence factors,brittleness index,fracture toughness,natural fractures and cohesive force,are considered.Finally,under the control of normalized value and weight coefficient of each influence factor,the calculations results indicate that the fracability index of the Wufeng Formation is higher than that of the Longmaxi Formation in Changning area,southern Sichuan Basin.
基金supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China(2016ZX05002-002)the National Natural Science Foundation of China(41972138).
文摘Fracability is a property that indicates how easy reservoir rocks can be fractured in hydraulic fracturing operations.It is a key parameter for fracturing design and evaluation.In order to utilize continuous logging data to predict fracability,synchronous tests of dynamic and static mechanical parameters of rocks under different confining pressures were conducted on13 tight sandstone samples derived from the central Junggar Basin,China.A modified formula between dynamic and static mechanical parameters was established.Fracability of the tight reservoir in the Junggar Basin was then evaluated based on brittleness index,fracture toughness,and fracability index.The effectiveness of fracturing was analyzed combined with the oil testing curve after hydraulic fracturing.The results show that:(1)The distribution of oil-bearing formations in the studied area coincides well with stratum of higher fracability index.(2)The critical fracability index is determined to be 0.3,three formations are selected as fracturing candidates,and a thin mudstone interbed is identified in the oil-bearing formation.(3)Well testing curve verifies the reliability of the fracability evaluation method and the accuracy of the modified formula between dynamic and static mechanical parameters.This study provides useful information for improving fracturing operations of tight oil and gas reservoirs.
基金funded by the Research Project of the Hubei Provincial Department of Education under grant no.Q20181307the Project of Shandong Provincial Key Laboratory of Depositional Mineralization&Sedimentary Mineral under grant no.DMSM2019001+2 种基金the Project of the Key Laboratory of Well Stability and Fluid&Rock Mechanics in Oil and Gas Reservoir of Shaanxi Province,Xi’an Shiyou University under grant no.WSFRM20190302001the National Natural Science Foundation of China under grant no.51574039the National Science and Technology Major Project under grant no.2016ZX05061-009
文摘Fracability characterizes the effectiveness of hydraulic fracturing.The existing assessment methods cannot reflect the actual value of the effectiveness due to a lack of comprehensive consideration and neglect of the influences of engineering factors.This study aims to solve this problem by implementing geological static data and production dynamic data in multivariate analysis in Zhaotong shale gas demonstration zone.First,the reservoir quality index(RQI)was introduced to evaluate the exploration potential by integrating the geological parameters with gray relational analysis.Moreover,the differences in fracturing fluid types and proppant sizes were considered,and the operating parameters were normalized on the basis of the equivalence principle.Finally,the general reservoir fracability index(GRFI)was proposed based on a dimensioned processing of the various parameters.A case study was conducted to verify the accuracy and feasibility of this new approach.The results demonstrate that(1)the organic carbon and gas content are adjusted to contribute the most to the calculation of the RQI,while the effective porosity contributes the least;(2)the fracturing scale is the main operating parameter determining the fracability,which has the strongest correlation with the effectiveness of fracking;and(3)the GRFI has a positive correlation with shale gas production,and the lower limit of the GRFI of 2,000 corresponds to a daily production of 50,000 m3/d;this value is defined as the threshold value of a stripper well.The GRFI is consistent with the productivity trend of shale gas wells in the research block,which suggests that the new model is accurate and practical for well candidate selection.
基金support of the National Natural Science Foundation of China(Grant No.52074332).
文摘Natural gas hydrates(hereinafter referred to as hydrates)are a promising clean energy source.However,their current development is far from reaching commercial exploitation.Reservoir stimulation tech-nology provides new approaches to enhance hydrate development effectiveness.Addressing the current lack of quantitative and objective methods for evaluating the fracability of hydrate reservoirs,this study clarifies the relationship between geological and engineering fracability and proposes a comprehensive evaluation model for hydrate reservoir fracability based on grey relational analysis and the criteria importance through intercriteria correlation method.By integrating results from hydraulic fracturing experiments on hydrate sediments,the fracability of hydrate reservoirs is assessed.The concept of critical construction parameter curves for hydrate reservoirs is introduced for the first time.Additionally,two-dimensional fracability index evaluation charts and three-dimensional fracability construction condition discrimination charts are established.The results indicate that as the comprehensive fracability index increases,the feasibility of forming fractures in hydrate reservoirs improves,and the required normalized fracturing construction parameters gradually decrease.The accuracy rate of the charts in judging experimental results reached 89.74%,enabling quick evaluations of whether hydrate reservoirs are worth fracturing,easy to fracture,and capable of being fractured.This has significant engineering implications forthehydraulicfracturingof hydratereservoirs.
基金funded by the National Natural Science Foundation of China(42102309 and 42007243)the Natural Science Foundation of Liaoning Province(2023-MSBA-120)the National Key Research and Development Program(2022YFB3304705)。
文摘Clarify the mechanical properties of different laminations and the fracture mechanism of continental shale under in-situ stress can provide theoretical basis for more comprehensive evaluation of the fracability of continental shale oil reservoir.The Chang 72continental shale was used to investigate the mechanical properties of laminations and the effect of natural structure on the crack propagation of the shale.The XRD and thin section tests show that the laminations contain two types:bright sandy lamination with void structure and dark muddy lamination with layer structure.The real-time CT uniaxial compression tests were conducted to investigate the differences of mechanical properties between the muddy lamination and sandy lamination.It found that the uniaxial compression strength and elastic modulus of the sandy lamination are higher,forming a simple crack with large opening,and the Poisson's ratio of the muddy lamination is large,forming obvious lateral deformation and more secondary cracks.On this basis,the cuboid-shaped continental shale specimens were tested under true triaxial compression conditions to study the effect of laminations and interface cracks on crack propagation combining AE and CT techniques.It found that nascent cracks connected laminations and interface cracks to form fracture network under appropriate loading condition,tensile cracks developed in sandy lamination and shear cracks occurred in muddy lamination because of deformation dissonance and brittleness index differences,and more secondary cracks formed in muddy lamination with smaller fracture toughness.Moreover,the combination relationships between nascent and natural cracks mainly conclude direct penetration and deflection,which is affected by the filling degree and morphology of interface cracks and the relationship of lamination types.These conclusions show that laminar continental shale is conducive to forming complex fracture network,which can provide a theoretical basis for the proposal of indicators and methods for fracability evaluation.
文摘With respect to brittleness,it is about the type of material and its related strength.In comparison with ductile material under load,brittle material has a relatively shorter plastic deformation and responds dominantly by the elastic deformation.With respect to fracability,it is about the rock failure under the ultimate rock strength in either brittle or ductile formation.In comparison,the higher fracable formation should have smaller formation strength than that of the lower fracable formation.In consequence,it is not certain that the brittle formation is easy to fracture than the ductile formation since brittle formation may have greater strength than ductile formation even though the exceptions may exist.More complications arise when evaluating the responses of subsurface formation in great depth to the formation types(e.g.brittle formation or ductile formation).Under this condition,the impact of confinement on the fracability cannot be ignored.In general,the formation subject to higher confinement pressure is more difficult to fracture as the formation strength is greater.Conversely,the formation subject to lower confinement pressure is easy to fracture since the formation strength is smaller.In view of efficient stimulation of tight shale gas reservoirs,it is unclear whether we would choose the brittle interval or the ductile interval to fracture as the strength of either interval is unknown.However,it is apparent that we should choose the formation interval with a higher fracability which is equivalent to the lower formation strength.Under the similar confinements,the lower formation strength may be indicated by the smaller unconfined compressive strength(UCS).As a result,it is advisable that the most fracable interval is the one with lowest UCS.When evaluating the present technology,the formation brittleness should no longer be the associated subject matter as we are unclear about its role to improve the fracability of the tight formation.Disassociating the brittleness with the fracability enables us to focus on identifying the true mechanisms of efficient fracturing of tight shale gas reservoirs.With the objective review and sensible definition of brittleness used in the present petrophysical field to identify the desirable fracturing intervals,the paper presents the ambiguities of using the brittleness to define the formation fracability and points out that the formation brittleness can be unrelated to the formation fracability.As an alternative approach,the paper provides an effective method to define the most fracable formation intervals in designing the hydraulic fracturing in tight shale gas formations.
基金granted by the National Science and Technology Major Project of the Ministry of Science and Technology of China (Grant No. 2017ZX05036002–001)National Natural Science Foundation of China (No. 41202103, 41872124)SINOPEC Ministry of Science and Technology Project (Grant No. P17027–2)
文摘The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery,which is the key stratum for marine shale gas exploration and development(E&D)in China.Based on sedimentary environment,material basis,storage space,fracability and reservoir evolution data,the reservoir characteristics of the Wufeng-Longmaxi shale and their significance for shale gas E&D are systematically compared and analyzed in this paper.The results show that(1)the depocenter of the Wufeng(WF)-Longmaxi(LM)shale gradually migrates from east to west.The high-quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales,which are primarily distributed in the graptolite shale interval of WF2-LM5.The high-quality reservoirs in the southern Sichuan Basin are mainly calcareous-siliceous and organic-rich argillaceous shales,which are distributed in the graptolite shale interval of WF2-LM7.(2)Deep shale gas(the burial depth>3500 m)in the Sichuan Basin has high-ultrahigh pressure and superior physical properties.The organic-rich siliceous,calcareous-siliceous and organic-rich argillaceous shales have suitable reservoir properties.The marginal area of the Sichuan Basin has a higher degree of pressure relief,which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity.(3)Combining shale gas exploration practices and impacts of lithofacies,depth,pressure coefficient and brittle-ductile transition on the reservoir properties,it is concluded that the favorable depth interval of the Wufeng-Longmaxi shale gas is 2200~4000 m under current technical conditions.(4)Aiming at the differential reservoir properties of the Wufeng-Longmaxi shale in the Sichuan Basin and its periphery,several suggestions for future research directions and E&D of shale gas are formulated.
基金National Science and Technology Major Project(2017ZX05049)。
文摘This paper analyzes the differences in geological and development characteristics between continental shale oil in China and marine shale oil in North America, reviews the evaluation methods and technological progress of the continental shale oil development in China, and points out the existing problems and development directions of the continental shale oil development. The research progress of development evaluation technologies such as favorable lithofacies identification, reservoir characterization, mobility evaluation, fracability evaluation, productivity evaluation and geological-mathematical modeling integration are introduced. The efficient exploration and development of continental shale oil in China are faced with many problems, such as weak basic theoretical research, imperfect exploration and development technology system, big gap in engineering technology between China and other countries, and high development cost. Three key research issues must be studied in the future:(1) forming differentiated development technologies of continental shale oil through geological and engineering integrated research;(2) strengthening the application of big data and artificial intelligence to improve the accuracy of development evaluation;(3) tackling enhanced shale oil recovery technology and research effective development method, so as to improve the development effect and benefit.
基金Supported by the China National Science and Technology Major Project(2016ZX05046)National Key R&D Program(2018YFE0196000)Consulting Research Project of Chinese Academy of Engineering(2019-XZ-61)。
文摘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.
基金supported by National Natural Science Foundation of China“Dynamic fracture control mechanism and water-free fracturing theory for shale formations”(No.51490653).
文摘In view of the bottleneck of shale gas development technologies,the latest progresses in such theories on the fields like fracture network fracability evaluation,fracture network propagation mechanism,stimulated reservoir volume(SRV)evaluation and fracturing fluid research&development were analyzed comprehensively in reference to the existing achievements and experiences in North American shale gas reservoir development and based on the probes,practices and cognitions of shale gas development technologies in China since 2005.First,the study on the shale brittleness develops from the definition of mineral and mechanical semi-quantitative threshold measurement to a comprehensive evaluation which integrates rock constituents,elastic mechanics and development characteristics of natural fractures together.Second,the study on the fracture network development develops from the directional extension theory to the simulation of fracture network formation in the stochastic distribution pattern of natural fractures.Third,the study on SRV develops from the micro-seismic monitoring based instrument and technology to the mathematical theory evaluation method dependent on the discrete fracture network and extended finite element.And fourth,the study on fracturing fluids develops from the wide application of slick water(drag reducing water)and a linear gel fracturing fluid system to the development and application of novel fracturing fluids with little or no water.Then,the theoretical and technological challenges were pointed out,including the comprehensive fracability evaluation of reservoir fracture networks,the deep seated shale gas fracturing,the operation curve diagnosis of shale gas fracturing,the shale gas refracturing theory,and the research&development and flowback control of novel fracturing fluids.Finally,the development trend of related technologies was predicted so as to provide a theoretical and technical guidance for the efficient shale gas development in China in the future.
文摘Brownshale is a lithology unit in the middle of the Pematang Formation consisting of brown to black shale that is deposited in the lacustrine environment. Brownshale from the results of previous studies stated as the main source rock in the Central Sumatra Basin, which is spread over several troughs, namely Balam, Aman, Rangau, Kiri, and Bengkalis Troughs, where Bengkalis Trough is the most extensive Trough. In the shale hydrocarbon prospecting analysis, Brownshale from previous researchers concluded that it had good prospects, based on several parameters including: TOC values with poor to very good quality. Brownshale formation is a type of kerogene as kerogen type of II/III, brittleness index greater than 0.48, and rock compressive strength below 10,000 Psi. One method in the development phase of shale hydrocarbon is to determine the fracable sweetspot window using drill cuttings and TOC, because there is no core data available. Based on the results of the well log analysis of well BS-03, it is obtained information that the Brownshale formation has a thickness of 1028 feet with intercalation laminated shale/sand section, so the mineral content varies greatly. From the ternary diagram of XRD (bulk analysis) results of drill cuttings of Brownshale formation of well BS-03, it can be seen that mineral distribution of Quartz-Clay-Calcite (Q-C-C) is spread between zone 1 to zone 3, namely: Dominant Quartz - Minor Clay & Carbonate (Zone 1: Brittle Quartz Rich), Dominant Carbonate - Quartz & Minor Clay (Zone 2: Brittle Carbonate Rich), and Quartz & Carbonate Balance - Clay minor (Zone 3: Ductile, hard to frac). This shows that not all Brownshale formation intervals are easy to frac (high fracability). From the XRD result, percentage of mineral content (bulk analysis) of Brownshale drill cuttings, there is an interesting phenomenon, i.e. the presence of sillimanite and kaliophilite minerals significantly starting at a depth of 10,780 ft and below, where both minerals have tenacity: brittle, and also from the results of the MBT analysis seen an interesting phenomenon, i.e. at a depth interval of about 10,780 ft the value of CEC drops below 3 meq/100 grams, and can be categorized as the brittle shale. Referring to the presence of sillimanite and kaliophilite minerals, as well as low MBT values, then at intervals of 10,780 ft below, it can be seen that at the bottom of the depth interval as a fracable sweetspot window, and at the upper depth interval of the Brownshale formation, it is believed to be a fracture barrier.
文摘Accurate rock elastic property determination is vital for effective hydraulic fracturing,particularly Young’s modulus due to its link to rock brittleness.This study integrates interdisciplinary data for better predictions of elastic modulus,combining data mining,experiments,and calibrated synthetics.We used the microstructural insights extracted from rock images for geomechanical facies analysis.Additionally,the petrophysical data and well logs were correlated with shear wave velocity(Vs)and Young’s modulus.We developed a machine-learning workflow to predict Young’s modulus and assess rock fracturability,considering mineral composition,geomechanics,and microstructure.Our findings indicate that artificial neural networks effectively predict Young’s modulus,while K-Means clustering and hierarchical support vector machines excel in identifying rock and geomechanical facies.Utilizing Microscale thin section analysis in conjunction with fracture modeling enhances our understanding of fracture geometries and facilitates fracturability assessment.Notably,fracturability is controlled by specific geomechanical facies during initiation and propagation and influenced by continuity of geomechanical facies in small depth intervals.In conclusion,this study demonstrates data mining and machine learning potential for predicting rock properties and assessing fracturability,aiding hydraulic fracturing design optimization through diverse data and advanced methods.
基金This work was funded by National Science and Technology Major Project of China(Grant No.2011ZX05006-003).
文摘Shale oil and gas plays in continental rift basins are complicated and have not been reported elsewhere.In the Luojia area of the Jiyang Depression,an evaluation workflow for shale oil and gas in this continental rift basin is proposed.Based on analysis of oil-and gas-related geological conditions,a favorable area of shale oil and gas can be identified,and a high-frequency sequence stratigraphic framework of the target area can be established,therefore,the spatiotemporal distribution of shale has been elucidated in the Luojia area.According to the rock texture,structure,composition and color,petrographic classification criteria for shale are determined,and well log data are used to demarcate,track and predict high-quality lithofacies.Based on geochemical analyses and physical simulations of hydrocarbon generation,abundance,types and maturity of organic matter are analyzed,furthermore,geochemical parameters criteria of hydrocarbon generation and the characteristics of oil and gas occurrence in shales can be determined.Storage space types,assemblages and evolution characteristics of shale reservoirs are studied through core observation,thin-section analysis,electron microscopy examination and fluorescence spectrometry.Combined with analysis of reservoir physical properties,the reservoir performance is evaluated.A saturation model is established based on core analysis,well-log interpretation and well-test production data.The model is further used for evaluation of the movable hydrocarbon contents and integrated assessment of the oil potential.Finally,the shale oil and gas production capacity and exploration prospects in the Luojia area are forecasted based on the analyses of factors controlling production capacity and the rock fracability.Through an integrated analysis of multi-factors(including the lithofacies,source rocks,reservoir properties,oil saturation,and production capacity),the shales in the Luojia area can be divided into three categories,i.e.,Class I(high porosity-high resistivity),Class II(medium porosity-medium resistivity),and Class III(low porosity-medium resistivity).
