During gas extraction from deep coal,the rock endures high effective stress,with both the time-dependent deformation and anisotropic structure of the rock controlling the permeability evolution.To reveal this phenomen...During gas extraction from deep coal,the rock endures high effective stress,with both the time-dependent deformation and anisotropic structure of the rock controlling the permeability evolution.To reveal this phenomenon,a numerical simulation framework of the finite volume method and transient embedded discrete fracture model is proposed to establish a new constitutive model that links poroelastoplastic deformation,adsorption-induced swelling,and aperture compression.From this model,anisotropic permeability tensors were derived to further achieve the simulation of coevolution.Meanwhile,our permeability model was verified against the measured permeability data,and the history match of the numerical model showed better results where the mismatch was less than 5%.The results indicate that(1)the long-term permeability evolution clearly showed the competitive effects of multiple deformation mechanisms,which involves three stages:compaction-dominated decline,adsorption-dominated rebound,and creep-controlled loss.(2)The increased number of compressible cleats/fractures accelerated the initial permeability decline,while the increased desorption-induced strain promoted faster rebound and enhancement and higher viscosity coefficients enhanced the creep effect,which led to significant long-term permeability loss.(3)Massive hydraulic fracturing created a larger drainage area,accelerating methane desorption and causing sharp permeability rebound with reduced residual gas,which shows that the permeability remained higher than the initial values even after the extensive extraction via the fractured horizontal wells.The permeability evolution mechanisms displayed varying properties,such as coal rank and burial depth,and distinct characteristics.A precise understanding of multiple competitive stress effects is crucial for optimizing coalbed methane extraction techniques and improving recovery efficiency.展开更多
China’s deep coalbed methane(CBM)resources demonstrate immense potential with extensive developmental prospects.However,the coupling relationship between the negative adsorption effect and the positive desorption-pro...China’s deep coalbed methane(CBM)resources demonstrate immense potential with extensive developmental prospects.However,the coupling relationship between the negative adsorption effect and the positive desorption-promotion effect under high-temperature conditions remains unclear.In this study,a self-built high-temperature adsorption-desorption system was used to investigate the coupled effects of temperature and coal rank on methane adsorption-desorption behavior in deep CBM.The results show that elevated temperatures significantly reduce methane adsorption capacity,with high-rank coals exhibiting greater sensitivity.Conversely,high-temperature conditions significantly enhance methane desorption and diffusion behavior,accelerating initial desorption rates,enabling rapid gas release in a short period,and thus improving desorption efficiency.The desorption volume and desorption-diffusion rate exhibited an asymmetric U-shaped variation with coal rank.By coupling the positive and negative effects of temperature and defining the desorption ratio,it was found that a 10 K increase in temperature raised the desorption ratio by 3.78%-8.05%.Finally,an effective gas content prediction model is proposed,and the key regulatory role of temperature in the resource potential and gas production characteristics of deep CBM is clarified.These findings can provide theoretical guidance for the subsequent optimization of deep CBM exploration and development strategies.展开更多
Deep coalbed methane(DCBM),an unconventional gas reservoir,has undergone significant advancements in recent years,sparking a growing interest in assessing pore pressure dynamics within these reservoirs.While some prod...Deep coalbed methane(DCBM),an unconventional gas reservoir,has undergone significant advancements in recent years,sparking a growing interest in assessing pore pressure dynamics within these reservoirs.While some production data analysis techniques have been adapted from conventional oil and gas wells,there remains a gap in the understanding of pore pressure generation and evolution,particularly in wells subjected to large-scale hydraulic fracturing.To address this gap,a novel technique called excess pore pressure analysis(EPPA)has been introduced to the coal seam gas industry for the first time to our knowledge,which employs dual-phase flow principles based on consolidation theory.This technique focuses on the generation and dissipation for excess pore-water pressure(EPWP)and excess pore-gas pressure(EPGP)in stimulated deep coal reservoirs.Equations have been developed respectively and numerical solutions have been provided using the finite element method(FEM).Application of this model to a representative field example reveals that excess pore pressure arises from rapid loading,with overburden weight transferred under undrained condition due to intense hydraulic fracturing,which significantly redistributes the weight-bearing role from the solid coal structure to the injected fluid and liberated gas within artificial pores over a brief timespan.Furthermore,field application indicates that the dissipation of EPWP and EPGP can be actually considered as the process of well production,where methane and water are extracted from deep coalbed methane wells,leading to consolidation for the artificial reservoirs.Moreover,history matching results demonstrate that the excess-pressure model established in this study provides a better explanation for the declining trends observed in both gas and water production curves,compared to conventional practices in coalbed methane reservoir engineering and petroleum engineering.This research not only enhances the understanding of DCBM reservoir behavior but also offers insights applicable to production analysis in other unconventional resources reliant on hydraulic fracturing.展开更多
Deep coal seams show low permeability,low elastic modulus,high Poisson’s ratio,strong plasticity,high fracture initiation pressure,difficulty in fracture extension,and difficulty in proppants addition.We proposed the...Deep coal seams show low permeability,low elastic modulus,high Poisson’s ratio,strong plasticity,high fracture initiation pressure,difficulty in fracture extension,and difficulty in proppants addition.We proposed the concept of large-scale stimulation by fracture network,balanced propagation and effective support of fracture network in fracturing design and developed the extreme massive hydraulic fracturing technique for deep coalbed methane(CBM)horizontal wells.This technique involves massive injection with high pumping rate+high-intensity proppant injection+perforation with equal apertures and limited flow+temporary plugging and diverting fractures+slick water with integrated variable viscosity+graded proppants with multiple sizes.The technique was applied in the pioneering test of a multi-stage fracturing horizontal well in deep CBM of Linxing Block,eastern margin of the Ordos Basin.The injection flow rate is 18 m^(3)/min,proppant intensity is 2.1 m^(3)/m,and fracturing fluid intensity is 16.5 m^(3)/m.After fracturing,a complex fracture network was formed,with an average fracture length of 205 m.The stimulated reservoir volume was 1987×10^(4)m^(3),and the peak gas production rate reached 6.0×10^(4)m^(3)/d,which achieved efficient development of deep CBM.展开更多
Deep coalbed methane(CBM)resources are enormous and have become a hot topic in the unconventional exploration and development of natural gas.The fractures in CBM reservoirs are important channels for the storage and m...Deep coalbed methane(CBM)resources are enormous and have become a hot topic in the unconventional exploration and development of natural gas.The fractures in CBM reservoirs are important channels for the storage and migration of CBM and control the high production and enrichment of CBM.Therefore,fracture prediction in deep CBM reservoirs is of great significance for the exploration and development of CBM.First,the basic principles of calculating texture attributes by gray-level cooccurrence matrix(GLCM)and gray-level run-length matrix(GLRLM)were introduced.A geological model of the deep CBM reservoirs with fractures was then constructed and subjected to seismic forward simulation.The seismic texture attributes were extracted using the GLCM and GLRLM.The research results indicate that the texture attributes calculated by both methods are responsive to fractures,with the 45°and 135°gray level inhomogeneity texture attributes based on the GLRLM showing better identification effects for fractures.Fracture prediction of a deep CBM reservoir in the Ordos Basin was carried out based on the GLRLM texture attributes,providing an important basis for the efficient development and utilization of deep CBM.展开更多
Deep coalbed methane(CBM)has become one of the most significant potential sources of natural gas in China.However,the exploration and development of deep CBM in China is still in an initial stage,and its accumulation-...Deep coalbed methane(CBM)has become one of the most significant potential sources of natural gas in China.However,the exploration and development of deep CBM in China is still in an initial stage,and its accumulation-forming characteristics require further study.Therefore,taking the No.8 deep coal seam in the central-eastern region of Ordos Basin as an example,this study investigated the geologic characteristics of CBM accumulations to establish a numerical model.The evolution of the burial and accumulation of CBM in the area was reconstructed.The modeling results suggest that the No.8 coal seam experienced continuous subsidence from the Late Cretaceous to the Triassic,alternating subsidence and uplift during the Jurassic,rapid burial throughout the Early Cretaceous,and continuous uplift since the Late Cretaceous.The coal reached its maximum maturity at the end of the Early Cretaceous.Furthermore,CBM generation in the region was divided into four stages of thermal eventsdbiogenic and early thermogenic gas,cracking of light oil into gas,cracking of the remaining kerogen into gas,and hydrocarbon generation ceasingdwhich accelerated coal maturity and generation.The adsorption capacity presented an overall declining trend prior to the end of the Cretaceous,followed by a rapid increase since the Late Cretaceous.As for adsorption mass evolution,the CBM successively un-derwent unsaturated minor adsorption,unsaturated rapid-rising adsorption,saturated decreasing adsorption,and saturated rising adsorption.The in-situ gas mass was found to be controlled by a combination of generation,adsorption,and expulsion of hydrocarbons,with its present-day value being 9-29×10^(4)t/km^(2)and the corresponding gas volume per ton of coal being 12-28 m^(3)/t.Moreover,free gas evolution initially showed an increasing trend,followed by a decline,ultimately accounting for 11%-28%of the total gas content.展开更多
Developing deep fragmented soft coalbed methane(CBM)can significantly enhance domestic natural gas supplies,reduce reliance on imported energy,and bolster national energy security.This manuscript provides a comprehens...Developing deep fragmented soft coalbed methane(CBM)can significantly enhance domestic natural gas supplies,reduce reliance on imported energy,and bolster national energy security.This manuscript provides a comprehensive review of commonly employed coalbed methane extraction technologies.It then delves into several critical issues in the current stage of CBM exploration and development in China,including the compatibility of existing technologies with CBM reservoirs,the characteristics and occurrence states of CBM reservoirs,critical desorption pressure,and gas generation mechanisms.Our research indicates that current CBM exploration and development technologies in China have reached an internationally advanced level,yet the industry is facing unprecedented challenges.Despite progress in low-permeability,high-value coal seams,significant breakthroughs have not been achieved in exploring other types of coal seams.For different coal reservoirs,integrated extraction technologies have been developed,such as surface pre-depressurisation and segmented hydraulic fracturing of coal seam roof strata.Additionally,techniques like large-scale volume fracturing in horizontal wells have been established,significantly enhancing reservoir stimulation effects and coalbed methane recovery rates.However,all of these technologies are fundamentally based on permeation.These technologies lack direct methods aimed at enhancing the diffusion rate of CBM,thereby failing to fully reflect the unique characteristics of CBM.Current CBM exploration and development theories and technologies are not universally applicable to all coal seams.They do not adequately account for the predominantly adsorbed state of CBM,and the complex and variable gas generation mechanisms further constrain CBM development in China.Finally,continuous exploration of new deep CBM exploration technologies is necessary.Integrating more effective reservoir stimulation technologies is essential to enhance technical adaptability concerning CBM reservoir characteristics,gas occurrence states,and gas generation mechanisms,ultimately achieving efficient CBM development.We conclude that while China possesses a substantial foundation of deep fractured CBM resources,industry development is constrained and requires continuous exploration of new CBM exploration and development technologies to utilize these resources effectively.展开更多
The publisher regrets that the article type for this publication was incorrectly labeled as a Research Article.The correct designation should be Review Article.This correction does not affect the content or conclusion...The publisher regrets that the article type for this publication was incorrectly labeled as a Research Article.The correct designation should be Review Article.This correction does not affect the content or conclusions of the article.The publisher apologizes for any inconvenience caused.展开更多
Based on the coalbed methane(CBM)/coal-rock gas(CRG)geological,geophysical,and experimental testing data from the Daji block in the Ordos Basin,the coal-forming and hydrocarbon generation&accumulation characterist...Based on the coalbed methane(CBM)/coal-rock gas(CRG)geological,geophysical,and experimental testing data from the Daji block in the Ordos Basin,the coal-forming and hydrocarbon generation&accumulation characteristics across different zones were dissected,and the key factors controlling the differential CBM/CRG enrichment were identified.The No.8 coal seam of the Carboniferous Benxi Formation in the Daji block is 8-10 m thick,typically overlain by limestone.The primary hydrocarbon generation phase occurred during the Early Cretaceous.Based on the differences in tectonic evolution and CRG occurrence,and with the maximum vitrinite reflectance of 2.0%and burial depth of 1800 m as boundaries,the study area is divided into deeply buried and deeply preserved,deeply buried and shallowly preserved,and shallowly buried and shallowly preserved zones.The deeply buried and deeply preserved zone contains gas content of 22-35 m^(3)/t,adsorbed gas saturation of 95%-100%,and formation water with total dissolved solid(TDS)higher than 50000 mg/L.This zone features structural stability and strong sealing capacity,with high gas production rates.The deeply buried and shallowly preserved zone contains gas content of 16-20 m^(3)/t,adsorbed gas saturation of 80%-95%,and formation water with TDS of 5000-50000 mg/L.This zone exhibits localized structural modification and hydrodynamic sealing,with moderate gas production rate.The shallowly buried and shallowly preserved zone contains gas content of 8-16 m^(3)/t,adsorbed gas saturation of 50%-70%,and formation water with TDS lower than 5000 mg/L.This zone experienced intense uplift,resulting in poor sealing and secondary alteration of the primary gas reservoir,with partial adsorbed gas loss,and low gas production rate.A depositional unification and structural divergence model is proposed,that is,although coal seams across the basin experienced broadly similar depositional and tectonic histories,differences in tectonic intensity have led to spatial heterogeneity in the maximum burial depth(i.e.,thermal maturity of coal)and current burial depth and occurrence of CRG(i.e.,gas content and occurrence state).The research results provide valuable guidance for advancing the theoretical understanding of CBM/CRG enrichment and for improving exploration and development practices.展开更多
Low-salinity fracturing fluids tend to induce ion migration,alter wettability,and cause fluctuations in gas desorption efficiency when penetrating deep coal seams.Taking the No.8 coal from the Daning-Jixian area in th...Low-salinity fracturing fluids tend to induce ion migration,alter wettability,and cause fluctuations in gas desorption efficiency when penetrating deep coal seams.Taking the No.8 coal from the Daning-Jixian area in the Ordos Basin,NW China,as a representative example,this study employs physical simulation experiments to reveal the coupled control mechanism of salinity gradient on the ion-coal matrix-gas/water interfacial system and its key role in the imbibition-desorption process.The increasing ionic concentration improves the hydrophobicity of coal,with multivalent ions exhibiting particularly significant effects.The imbibition and ion diffusion occur in opposite directions,with imbibition equilibrium being achieved earlier than ionic equilibrium.Water-coal interactions induce both mineral dissolution and secondary precipitation.When a low-salinity fracturing fluid is injected into a high-salinity reservoir,the osmotic-pressure difference drives imbibition,promotes CH4 desorption,but results in higher fluid loss.Conversely,injecting high-salinity fracturing fluid into a low-salinity reservoir creates a reverse osmotic gradient that suppresses leak-off while improving flowback efficiency.Based on these findings,a high-low salinity sequential injection strategy is proposed for deep coal seams:high-salinity fluid is first injected to form stable fracture networks,followed by low-salinity fluid to enlarge the imbibition zone and enhance CH4 desorption and diffusion.Moderate well soaking is recommended to increase the imbibition volume,thereby achieving multiple positive effects such as maintaining reservoir pressure,preserving formation energy,and promoting imbibition-driven displacement.展开更多
Commercial exploration and development of deep buried coalbed methane (CBM) in Daning-Jixian Block, eastern margin of Ordos Basin, have rapidly increased in recent decades. Gas content, saturation and well productivit...Commercial exploration and development of deep buried coalbed methane (CBM) in Daning-Jixian Block, eastern margin of Ordos Basin, have rapidly increased in recent decades. Gas content, saturation and well productivity show significant heterogeneity in this area. To better understand the geological controlling mechanism on gas distribution heterogeneity, the burial history, hydrocarbon generation history and tectonic evolution history were studied by numerical simulation and experimental simulation, which could provide guidance for further development of CBM in this area. The burial history of coal reservoir can be classified into six stages, i.e., shallowly buried stage, deeply burial stage, uplifting stage, short-term tectonic subsidence stage, large-scale uplifting stage, sustaining uplifting and structural inversion stage. The organic matter in coal reservoir experienced twice hydrocarbon generation. Primary and secondary hydrocarbon generation processes were formed by the Early and Middle Triassic plutonic metamorphism and Early Cretaceous regional magmatic thermal metamorphism, respectively. Five critical tectonic events of the Indosinian, Yanshanian and Himalayan orogenies affect different stages of the CBM reservoir accumulation process. The Indosinian orogeny mainly controls the primary CBM generation. The Yanshanian Orogeny dominates the second gas generation and migration processes. The Himalayan orogeny mainly affects the gas dissipation process and current CBM distribution heterogeneity.展开更多
Deep coal reservoirs(buried depth>2000 m)represent a significant yet underexploited resource for coalbed methane(CBM)production.In these reservoirs,CBM primarily exists in adsorbed and free phase,with the pore stru...Deep coal reservoirs(buried depth>2000 m)represent a significant yet underexploited resource for coalbed methane(CBM)production.In these reservoirs,CBM primarily exists in adsorbed and free phase,with the pore structure playing a critical role in gas storage and migration.The Jiaxian block in the northeastern Ordos Basin,has emerged as a key area for deep CBM exploration due to its promising resource potential.However,the pore structure characteristics of the No.8 coal seam in Jiaxian block and their implications for gas storage and production remain poorly understood.A comprehensive characterization of the No.8 coal seam's pore structure is conducted in the study using multiple methods including high-pressure mercury injection,N2/CO_(2)adsorption experiments,and integration of measured core gas content data and production history.The study results reveal that the pores can be mainly classified as vesicles and cellular pores,and the fractures are mainly static pressure fractures.Micropores(pore diameter<10 nm)dominate the pore system(accounting for more than 99%of the total specific surface area),providing important adsorption sites for gas storage.Although mesopores(pore diameter of 100-1000 nm)and macropores(pore diameter>1000 nm)account for a small proportion,they feature effective storage spaces and interconnectivity,resulting in a high proportion of free gas.Therefore,the reservoirs shows great development potential after stimulation(such as hydraulic fracturing).These findings emphasize the feasibility of large-scale and long-term development of CBM in the Jiaxian block in terms of reservoir space,gas content and production characteristics.This study serves to lay a scientific basis for its efficient exploitation.展开更多
An improved evaluation method for estimating gas content during the inversion process of deep-burial coal was established based on the on-site natural desorption curves.The accuracy of the US Bureau of Mines(USBM),Pol...An improved evaluation method for estimating gas content during the inversion process of deep-burial coal was established based on the on-site natural desorption curves.The accuracy of the US Bureau of Mines(USBM),Polynomial fitting,Amoco,and the improved evaluation methods in the predicting of lost gas volume in deep seams in the Mabidong Block of the Qinshui Basin were then compared.Furthermore,the calculation errors of these different methods in simulating lost gas content based on coring time were compared.A newly established nonlinear equation was developed to estimate the minimum error value,by controlling the lost time within 16 min,the related errors can be reduced.The improved evaluation was shown to accurately and rapidly predict the gas content in deep seams.The results show that the deep coal bed methane accumulation is influenced by various factors,including geological structure,hydrodynamic conditions,roof lithology,and coalification.Reverse faults and weak groundwater runoff can hinder the escape of methane,and these factors should be considered in the future exploration and development of coalbed methane.展开更多
基金support of the National Natural Science Foundation of China(U23B6004 and 52404045)the CAST Young Talent Support Program,Doctoral Student Special Project.
文摘During gas extraction from deep coal,the rock endures high effective stress,with both the time-dependent deformation and anisotropic structure of the rock controlling the permeability evolution.To reveal this phenomenon,a numerical simulation framework of the finite volume method and transient embedded discrete fracture model is proposed to establish a new constitutive model that links poroelastoplastic deformation,adsorption-induced swelling,and aperture compression.From this model,anisotropic permeability tensors were derived to further achieve the simulation of coevolution.Meanwhile,our permeability model was verified against the measured permeability data,and the history match of the numerical model showed better results where the mismatch was less than 5%.The results indicate that(1)the long-term permeability evolution clearly showed the competitive effects of multiple deformation mechanisms,which involves three stages:compaction-dominated decline,adsorption-dominated rebound,and creep-controlled loss.(2)The increased number of compressible cleats/fractures accelerated the initial permeability decline,while the increased desorption-induced strain promoted faster rebound and enhancement and higher viscosity coefficients enhanced the creep effect,which led to significant long-term permeability loss.(3)Massive hydraulic fracturing created a larger drainage area,accelerating methane desorption and causing sharp permeability rebound with reduced residual gas,which shows that the permeability remained higher than the initial values even after the extensive extraction via the fractured horizontal wells.The permeability evolution mechanisms displayed varying properties,such as coal rank and burial depth,and distinct characteristics.A precise understanding of multiple competitive stress effects is crucial for optimizing coalbed methane extraction techniques and improving recovery efficiency.
基金supported by the National Natural Science Fund of China(No.42272195)the National Natural Science Fund of China(No.42130802)+2 种基金the Fundamental Research Funds for the Central Universities(No.2025ZDPY10)the China National Petroleum Co.,Ltd..Research applied science and technology special(No.2023ZZ18)the PetroChina Changqing oilfield science and technology major project(No.2023DZZ01).
文摘China’s deep coalbed methane(CBM)resources demonstrate immense potential with extensive developmental prospects.However,the coupling relationship between the negative adsorption effect and the positive desorption-promotion effect under high-temperature conditions remains unclear.In this study,a self-built high-temperature adsorption-desorption system was used to investigate the coupled effects of temperature and coal rank on methane adsorption-desorption behavior in deep CBM.The results show that elevated temperatures significantly reduce methane adsorption capacity,with high-rank coals exhibiting greater sensitivity.Conversely,high-temperature conditions significantly enhance methane desorption and diffusion behavior,accelerating initial desorption rates,enabling rapid gas release in a short period,and thus improving desorption efficiency.The desorption volume and desorption-diffusion rate exhibited an asymmetric U-shaped variation with coal rank.By coupling the positive and negative effects of temperature and defining the desorption ratio,it was found that a 10 K increase in temperature raised the desorption ratio by 3.78%-8.05%.Finally,an effective gas content prediction model is proposed,and the key regulatory role of temperature in the resource potential and gas production characteristics of deep CBM is clarified.These findings can provide theoretical guidance for the subsequent optimization of deep CBM exploration and development strategies.
基金supported by the National Natural Science Foundation of China(Nos.42272195 and 42130802)supported by the Key Applied Science and Technology Project of PetroChina(No.2023ZZ18)the Major Science and Technology Project of Changqing Oilfield(No.2023DZZ01).
文摘Deep coalbed methane(DCBM),an unconventional gas reservoir,has undergone significant advancements in recent years,sparking a growing interest in assessing pore pressure dynamics within these reservoirs.While some production data analysis techniques have been adapted from conventional oil and gas wells,there remains a gap in the understanding of pore pressure generation and evolution,particularly in wells subjected to large-scale hydraulic fracturing.To address this gap,a novel technique called excess pore pressure analysis(EPPA)has been introduced to the coal seam gas industry for the first time to our knowledge,which employs dual-phase flow principles based on consolidation theory.This technique focuses on the generation and dissipation for excess pore-water pressure(EPWP)and excess pore-gas pressure(EPGP)in stimulated deep coal reservoirs.Equations have been developed respectively and numerical solutions have been provided using the finite element method(FEM).Application of this model to a representative field example reveals that excess pore pressure arises from rapid loading,with overburden weight transferred under undrained condition due to intense hydraulic fracturing,which significantly redistributes the weight-bearing role from the solid coal structure to the injected fluid and liberated gas within artificial pores over a brief timespan.Furthermore,field application indicates that the dissipation of EPWP and EPGP can be actually considered as the process of well production,where methane and water are extracted from deep coalbed methane wells,leading to consolidation for the artificial reservoirs.Moreover,history matching results demonstrate that the excess-pressure model established in this study provides a better explanation for the declining trends observed in both gas and water production curves,compared to conventional practices in coalbed methane reservoir engineering and petroleum engineering.This research not only enhances the understanding of DCBM reservoir behavior but also offers insights applicable to production analysis in other unconventional resources reliant on hydraulic fracturing.
基金Supported by the National Natural Science Foundation of China Project(52274014)Comprehensive Scientific Research Project of China National Offshore Oil Corporation(KJZH-2023-2303)。
文摘Deep coal seams show low permeability,low elastic modulus,high Poisson’s ratio,strong plasticity,high fracture initiation pressure,difficulty in fracture extension,and difficulty in proppants addition.We proposed the concept of large-scale stimulation by fracture network,balanced propagation and effective support of fracture network in fracturing design and developed the extreme massive hydraulic fracturing technique for deep coalbed methane(CBM)horizontal wells.This technique involves massive injection with high pumping rate+high-intensity proppant injection+perforation with equal apertures and limited flow+temporary plugging and diverting fractures+slick water with integrated variable viscosity+graded proppants with multiple sizes.The technique was applied in the pioneering test of a multi-stage fracturing horizontal well in deep CBM of Linxing Block,eastern margin of the Ordos Basin.The injection flow rate is 18 m^(3)/min,proppant intensity is 2.1 m^(3)/m,and fracturing fluid intensity is 16.5 m^(3)/m.After fracturing,a complex fracture network was formed,with an average fracture length of 205 m.The stimulated reservoir volume was 1987×10^(4)m^(3),and the peak gas production rate reached 6.0×10^(4)m^(3)/d,which achieved efficient development of deep CBM.
文摘Deep coalbed methane(CBM)resources are enormous and have become a hot topic in the unconventional exploration and development of natural gas.The fractures in CBM reservoirs are important channels for the storage and migration of CBM and control the high production and enrichment of CBM.Therefore,fracture prediction in deep CBM reservoirs is of great significance for the exploration and development of CBM.First,the basic principles of calculating texture attributes by gray-level cooccurrence matrix(GLCM)and gray-level run-length matrix(GLRLM)were introduced.A geological model of the deep CBM reservoirs with fractures was then constructed and subjected to seismic forward simulation.The seismic texture attributes were extracted using the GLCM and GLRLM.The research results indicate that the texture attributes calculated by both methods are responsive to fractures,with the 45°and 135°gray level inhomogeneity texture attributes based on the GLRLM showing better identification effects for fractures.Fracture prediction of a deep CBM reservoir in the Ordos Basin was carried out based on the GLRLM texture attributes,providing an important basis for the efficient development and utilization of deep CBM.
基金funded by the applied research and technology project titled“Research on Key Technologies for Optimization Design of Deep Coalbed Methane Development”conducted by PetroChina Company Limited(Grant No.2023ZZ1804).
文摘Deep coalbed methane(CBM)has become one of the most significant potential sources of natural gas in China.However,the exploration and development of deep CBM in China is still in an initial stage,and its accumulation-forming characteristics require further study.Therefore,taking the No.8 deep coal seam in the central-eastern region of Ordos Basin as an example,this study investigated the geologic characteristics of CBM accumulations to establish a numerical model.The evolution of the burial and accumulation of CBM in the area was reconstructed.The modeling results suggest that the No.8 coal seam experienced continuous subsidence from the Late Cretaceous to the Triassic,alternating subsidence and uplift during the Jurassic,rapid burial throughout the Early Cretaceous,and continuous uplift since the Late Cretaceous.The coal reached its maximum maturity at the end of the Early Cretaceous.Furthermore,CBM generation in the region was divided into four stages of thermal eventsdbiogenic and early thermogenic gas,cracking of light oil into gas,cracking of the remaining kerogen into gas,and hydrocarbon generation ceasingdwhich accelerated coal maturity and generation.The adsorption capacity presented an overall declining trend prior to the end of the Cretaceous,followed by a rapid increase since the Late Cretaceous.As for adsorption mass evolution,the CBM successively un-derwent unsaturated minor adsorption,unsaturated rapid-rising adsorption,saturated decreasing adsorption,and saturated rising adsorption.The in-situ gas mass was found to be controlled by a combination of generation,adsorption,and expulsion of hydrocarbons,with its present-day value being 9-29×10^(4)t/km^(2)and the corresponding gas volume per ton of coal being 12-28 m^(3)/t.Moreover,free gas evolution initially showed an increasing trend,followed by a decline,ultimately accounting for 11%-28%of the total gas content.
基金supported by the National Natural Science Foundation of China(52074045,52274074)the Science Fund for Distinguished Young Scholars of Chongqing(CSTB2022NSCQ-JQX0028).
文摘Developing deep fragmented soft coalbed methane(CBM)can significantly enhance domestic natural gas supplies,reduce reliance on imported energy,and bolster national energy security.This manuscript provides a comprehensive review of commonly employed coalbed methane extraction technologies.It then delves into several critical issues in the current stage of CBM exploration and development in China,including the compatibility of existing technologies with CBM reservoirs,the characteristics and occurrence states of CBM reservoirs,critical desorption pressure,and gas generation mechanisms.Our research indicates that current CBM exploration and development technologies in China have reached an internationally advanced level,yet the industry is facing unprecedented challenges.Despite progress in low-permeability,high-value coal seams,significant breakthroughs have not been achieved in exploring other types of coal seams.For different coal reservoirs,integrated extraction technologies have been developed,such as surface pre-depressurisation and segmented hydraulic fracturing of coal seam roof strata.Additionally,techniques like large-scale volume fracturing in horizontal wells have been established,significantly enhancing reservoir stimulation effects and coalbed methane recovery rates.However,all of these technologies are fundamentally based on permeation.These technologies lack direct methods aimed at enhancing the diffusion rate of CBM,thereby failing to fully reflect the unique characteristics of CBM.Current CBM exploration and development theories and technologies are not universally applicable to all coal seams.They do not adequately account for the predominantly adsorbed state of CBM,and the complex and variable gas generation mechanisms further constrain CBM development in China.Finally,continuous exploration of new deep CBM exploration technologies is necessary.Integrating more effective reservoir stimulation technologies is essential to enhance technical adaptability concerning CBM reservoir characteristics,gas occurrence states,and gas generation mechanisms,ultimately achieving efficient CBM development.We conclude that while China possesses a substantial foundation of deep fractured CBM resources,industry development is constrained and requires continuous exploration of new CBM exploration and development technologies to utilize these resources effectively.
文摘The publisher regrets that the article type for this publication was incorrectly labeled as a Research Article.The correct designation should be Review Article.This correction does not affect the content or conclusions of the article.The publisher apologizes for any inconvenience caused.
基金Supported by the China National Science and Technology Major Project(2025ZD1405700)CNPC Science and Technology Project(2023YQX20117).
文摘Based on the coalbed methane(CBM)/coal-rock gas(CRG)geological,geophysical,and experimental testing data from the Daji block in the Ordos Basin,the coal-forming and hydrocarbon generation&accumulation characteristics across different zones were dissected,and the key factors controlling the differential CBM/CRG enrichment were identified.The No.8 coal seam of the Carboniferous Benxi Formation in the Daji block is 8-10 m thick,typically overlain by limestone.The primary hydrocarbon generation phase occurred during the Early Cretaceous.Based on the differences in tectonic evolution and CRG occurrence,and with the maximum vitrinite reflectance of 2.0%and burial depth of 1800 m as boundaries,the study area is divided into deeply buried and deeply preserved,deeply buried and shallowly preserved,and shallowly buried and shallowly preserved zones.The deeply buried and deeply preserved zone contains gas content of 22-35 m^(3)/t,adsorbed gas saturation of 95%-100%,and formation water with total dissolved solid(TDS)higher than 50000 mg/L.This zone features structural stability and strong sealing capacity,with high gas production rates.The deeply buried and shallowly preserved zone contains gas content of 16-20 m^(3)/t,adsorbed gas saturation of 80%-95%,and formation water with TDS of 5000-50000 mg/L.This zone exhibits localized structural modification and hydrodynamic sealing,with moderate gas production rate.The shallowly buried and shallowly preserved zone contains gas content of 8-16 m^(3)/t,adsorbed gas saturation of 50%-70%,and formation water with TDS lower than 5000 mg/L.This zone experienced intense uplift,resulting in poor sealing and secondary alteration of the primary gas reservoir,with partial adsorbed gas loss,and low gas production rate.A depositional unification and structural divergence model is proposed,that is,although coal seams across the basin experienced broadly similar depositional and tectonic histories,differences in tectonic intensity have led to spatial heterogeneity in the maximum burial depth(i.e.,thermal maturity of coal)and current burial depth and occurrence of CRG(i.e.,gas content and occurrence state).The research results provide valuable guidance for advancing the theoretical understanding of CBM/CRG enrichment and for improving exploration and development practices.
基金Supported by the National Natural Science Foundation of China(NSFC)-Enterprise Innovation and Development Joint Fund Key Project(U24B2018)the Distinguished Young Scholars Program of the National Natural Science Foundation of China(42125205).
文摘Low-salinity fracturing fluids tend to induce ion migration,alter wettability,and cause fluctuations in gas desorption efficiency when penetrating deep coal seams.Taking the No.8 coal from the Daning-Jixian area in the Ordos Basin,NW China,as a representative example,this study employs physical simulation experiments to reveal the coupled control mechanism of salinity gradient on the ion-coal matrix-gas/water interfacial system and its key role in the imbibition-desorption process.The increasing ionic concentration improves the hydrophobicity of coal,with multivalent ions exhibiting particularly significant effects.The imbibition and ion diffusion occur in opposite directions,with imbibition equilibrium being achieved earlier than ionic equilibrium.Water-coal interactions induce both mineral dissolution and secondary precipitation.When a low-salinity fracturing fluid is injected into a high-salinity reservoir,the osmotic-pressure difference drives imbibition,promotes CH4 desorption,but results in higher fluid loss.Conversely,injecting high-salinity fracturing fluid into a low-salinity reservoir creates a reverse osmotic gradient that suppresses leak-off while improving flowback efficiency.Based on these findings,a high-low salinity sequential injection strategy is proposed for deep coal seams:high-salinity fluid is first injected to form stable fracture networks,followed by low-salinity fluid to enlarge the imbibition zone and enhance CH4 desorption and diffusion.Moderate well soaking is recommended to increase the imbibition volume,thereby achieving multiple positive effects such as maintaining reservoir pressure,preserving formation energy,and promoting imbibition-driven displacement.
基金This research was funded by the National Natural Science Foundation of China (Grant No. 41902178)National Science and Technology Major Project (Oil & Gas) (No. 2016ZX05065)+1 种基金Natural Science Foundation of Shanxi Province, China (No. 20210302123165)Open Fund of Beijing Key Laboratory of Unconventional Natural Gas Geological Evaluation and Development Engineering, China University of Geosciences (Beijing) (No. 2019BJ02001).
文摘Commercial exploration and development of deep buried coalbed methane (CBM) in Daning-Jixian Block, eastern margin of Ordos Basin, have rapidly increased in recent decades. Gas content, saturation and well productivity show significant heterogeneity in this area. To better understand the geological controlling mechanism on gas distribution heterogeneity, the burial history, hydrocarbon generation history and tectonic evolution history were studied by numerical simulation and experimental simulation, which could provide guidance for further development of CBM in this area. The burial history of coal reservoir can be classified into six stages, i.e., shallowly buried stage, deeply burial stage, uplifting stage, short-term tectonic subsidence stage, large-scale uplifting stage, sustaining uplifting and structural inversion stage. The organic matter in coal reservoir experienced twice hydrocarbon generation. Primary and secondary hydrocarbon generation processes were formed by the Early and Middle Triassic plutonic metamorphism and Early Cretaceous regional magmatic thermal metamorphism, respectively. Five critical tectonic events of the Indosinian, Yanshanian and Himalayan orogenies affect different stages of the CBM reservoir accumulation process. The Indosinian orogeny mainly controls the primary CBM generation. The Yanshanian Orogeny dominates the second gas generation and migration processes. The Himalayan orogeny mainly affects the gas dissipation process and current CBM distribution heterogeneity.
基金funded by the National Key R&D Program of China(2024YFC2909400)the National Natural Science Foundation of China(42402180,42202195)the tackling applied science and technology projects of China National Petroleum Corporation(2023ZZ18)。
文摘Deep coal reservoirs(buried depth>2000 m)represent a significant yet underexploited resource for coalbed methane(CBM)production.In these reservoirs,CBM primarily exists in adsorbed and free phase,with the pore structure playing a critical role in gas storage and migration.The Jiaxian block in the northeastern Ordos Basin,has emerged as a key area for deep CBM exploration due to its promising resource potential.However,the pore structure characteristics of the No.8 coal seam in Jiaxian block and their implications for gas storage and production remain poorly understood.A comprehensive characterization of the No.8 coal seam's pore structure is conducted in the study using multiple methods including high-pressure mercury injection,N2/CO_(2)adsorption experiments,and integration of measured core gas content data and production history.The study results reveal that the pores can be mainly classified as vesicles and cellular pores,and the fractures are mainly static pressure fractures.Micropores(pore diameter<10 nm)dominate the pore system(accounting for more than 99%of the total specific surface area),providing important adsorption sites for gas storage.Although mesopores(pore diameter of 100-1000 nm)and macropores(pore diameter>1000 nm)account for a small proportion,they feature effective storage spaces and interconnectivity,resulting in a high proportion of free gas.Therefore,the reservoirs shows great development potential after stimulation(such as hydraulic fracturing).These findings emphasize the feasibility of large-scale and long-term development of CBM in the Jiaxian block in terms of reservoir space,gas content and production characteristics.This study serves to lay a scientific basis for its efficient exploitation.
基金supported by the National Natural Science Foundation of China(Grant No.42130802)PetroChina Company Limited“14th Five Year Plan”Science and Technology Major Project(No.2021DJ2301).
文摘An improved evaluation method for estimating gas content during the inversion process of deep-burial coal was established based on the on-site natural desorption curves.The accuracy of the US Bureau of Mines(USBM),Polynomial fitting,Amoco,and the improved evaluation methods in the predicting of lost gas volume in deep seams in the Mabidong Block of the Qinshui Basin were then compared.Furthermore,the calculation errors of these different methods in simulating lost gas content based on coring time were compared.A newly established nonlinear equation was developed to estimate the minimum error value,by controlling the lost time within 16 min,the related errors can be reduced.The improved evaluation was shown to accurately and rapidly predict the gas content in deep seams.The results show that the deep coal bed methane accumulation is influenced by various factors,including geological structure,hydrodynamic conditions,roof lithology,and coalification.Reverse faults and weak groundwater runoff can hinder the escape of methane,and these factors should be considered in the future exploration and development of coalbed methane.
