Prediction of production decline and evaluation of the adsorbed/free gas ratio are critical for determining the lifespan and production status of shale gas wells.Traditional production prediction methods have some sho...Prediction of production decline and evaluation of the adsorbed/free gas ratio are critical for determining the lifespan and production status of shale gas wells.Traditional production prediction methods have some shortcomings because of the low permeability and tightness of shale,complex gas flow behavior of multi-scale gas transport regions and multiple gas transport mechanism superpositions,and complex and variable production regimes of shale gas wells.Recent research has demonstrated the existence of a multi-stage isotope fractionation phenomenon during shale gas production,with the fractionation characteristics of each stage associated with the pore structure,gas in place(GIP),adsorption/desorption,and gas production process.This study presents a new approach for estimating shale gas well production and evaluating the adsorbed/free gas ratio throughout production using isotope fractionation techniques.A reservoir-scale carbon isotope fractionation(CIF)model applicable to the production process of shale gas wells was developed for the first time in this research.In contrast to the traditional model,this model improves production prediction accuracy by simultaneously fitting the gas production rate and δ^(13)C_(1) data and provides a new evaluation method of the adsorbed/free gas ratio during shale gas production.The results indicate that the diffusion and adsorption/desorption properties of rock,bottom-hole flowing pressure(BHP)of gas well,and multi-scale gas transport regions of the reservoir all affect isotope fractionation,with the diffusion and adsorption/desorption parameters of rock having the greatest effect on isotope fractionation being D∗/D,PL,VL,α,and others in that order.We effectively tested the universality of the four-stage isotope fractionation feature and revealed a unique isotope fractionation mechanism caused by the superimposed coupling of multi-scale gas transport regions during shale gas well production.Finally,we applied the established CIF model to a shale gas well in the Sichuan Basin,China,and calculated the estimated ultimate recovery(EUR)of the well to be 3.33×10^(8) m^(3);the adsorbed gas ratio during shale gas production was 1.65%,10.03%,and 23.44%in the first,fifth,and tenth years,respectively.The findings are significant for understanding the isotope fractionation mechanism during natural gas transport in complex systems and for formulating and optimizing unconventional natural gas development strategies.展开更多
Based on the test and experimental data from exploration well cores of the Upper Paleozoic in the central-eastern Ordos Basin,combined with structural,burial depth and fluid geochemistry analyses,this study reveals th...Based on the test and experimental data from exploration well cores of the Upper Paleozoic in the central-eastern Ordos Basin,combined with structural,burial depth and fluid geochemistry analyses,this study reveals the fluid characteristics,gas accumulation control factors and accumulation modes in the Upper Paleozoic coal reservoirs.The study indicates findings in two aspects.First,the 1500-1800 m interval represents the critical transition zone between open fluid system in shallow-medium depths and closed fluid system in deep depths.The reservoirs above 1500 m reflect intense water invasion,with discrete pressure gradient distribution,and the presence of methane mixed with varying degrees of secondary biogenic gas,and they generally exhibit high water saturation and adsorbed gas undersaturation.The reservoirs deeper than 1800 m,with extremely low permeability,are self-sealed,and contains closed fluid systems formed jointly by the hydrodynamic lateral blocking and tight caprock confinement.Within these systems,surface runoff infiltration is weak,the degree of secondary fluid transformation is minimal,and the pressure gradient is relatively uniform.The adsorbed gas saturation exceeds 100%in most seams,and the free gas content primarily ranges from 1 m^(3)/t to 8 m^(3)/t(greater than 10 m^(3)/t in some seams).Second,the gas accumulation in deep coals is primarily controlled by coal quality,reservoir-caprock assemblage,and structural position governed storage,wettability and sealing properties,under the constraints of the underground temperature and pressure conditions.High-rank,low-ash yield coals with limestone and mudstone caprocks show superior gas accumulation potential.Positive structural highs and wide and gentle negative structural lows are favorable sites for gas enrichment,while slope belts of fold limbs exhibit relatively lower gas content.This research enhances understanding of gas accumulation mechanisms in coal reservoirs and provides effective parameter reference for precise zone evaluation and innovation of adaptive stimulation technologies for deep resources.展开更多
Taking deep coal-rock gas in the Yulin and Daning-Jixian areas of the Ordos Basin,NW China,as the research object,full-diameter coal rock samples with different cleat/fracture development degrees from the Carboniferou...Taking deep coal-rock gas in the Yulin and Daning-Jixian areas of the Ordos Basin,NW China,as the research object,full-diameter coal rock samples with different cleat/fracture development degrees from the Carboniferous Benxi Formation were selected to conduct physical simulation and isotope monitoring experiments of the full-life-cycle depletion development of coal-rock gas.Based on the experimental results,a dual-medium carbon isotope fractionation(CIF)model coupling cleats/fractures and matrix pores was constructed,and an evaluation method for free gas production patterns was established to elucidate the carbon isotope fractionation mechanism and adsorbed/free gas production characteristics during deep coal-rock gas development.The results show that the deep coal-rock gas development process exhibits a three-stage carbon isotope fractionation pattern:“Stable(Ⅰ)→Decrease(Ⅱ)→Increase(Ⅲ)”.A rapid decline in boundary pressure in stageⅢleads to fluctuations in isotope value,characterized by a“rapid decrease followed by continued increase”,with free gas being produced first and long-term supply of adsorbed gas.The CIF model can effectively match measured gas pressure,cumulative gas production,and δ^(13)C_(1) value of produced gas.During the first two stages of isotope fractionation,free gas dominated cumulative production.During the mid-late stages of slow depletion production,the staged pressure control development method can effectively increase the gas recovery.The production of adsorbed gas is primarily controlled by the rock's adsorption capacity and the presence of secondary flow channels.Effectively enhancing the recovery of adsorbed gas during the late stage remains crucial for maintaining stable production and improving the ultimate recovery factor of deep coal-rock gas.展开更多
Shale needs to contain a sufficient amount of gas to make it viable for exploitation. The continental heterogeneous shale formation in the Yan-chang （YC） area is investigated by firstly measuring the shale gas conte...Shale needs to contain a sufficient amount of gas to make it viable for exploitation. The continental heterogeneous shale formation in the Yan-chang （YC） area is investigated by firstly measuring the shale gas content in a laboratory and then investigating use of a theoretical prediction model. Key factors controlling the shale gas content are determined, and a prediction model for free gas content is established according to the equation of gas state and a new petrophysical volume model. Application of the Langmuir volume constant and pressure constant obtained from results of adsorption isotherms is found to be limited because these constants are greatly affected by experimental temperature and pressures. Therefore, using measurements of adsorption isotherms and thermodynamic theory, the influence of temperature, total organic carbon （TOC）, and mineralogy on Langmuir volume constants and pressure constants are investigated in detail. A prediction model for the Langmuir pressure constant with a correction of temperatures is then established, and a prediction model for the Langmuir volume constant with correction of temperature, TOC, and quartz contents is also proposed. Using these corrected Langmuir constants, application of the Langmuir model determined using experimental adsorption isotherms is extrapolated to reservoir temperature, pressure, and lithological conditions, and a method for the prediction of shale gas content using well logs is established. Finally, this method is successfully applied to predict the shale gas content of the continental shale formation in the YC area, and practical application is shown to deliver good results with high precision.展开更多
Gas sorption and non-Darcy flow are two important issues for shale gas reservoirs. The sorption consists of dissolution and adsorption. Dissolved gas and adsorbed gas are different. The former is dissolved in the shal...Gas sorption and non-Darcy flow are two important issues for shale gas reservoirs. The sorption consists of dissolution and adsorption. Dissolved gas and adsorbed gas are different. The former is dissolved in the shale matrix, while the latter is concentrated near the solid walls of pores. In this paper, the Langmuir equation is used to describe adsorption and Henry’s law is used to describe dissolution. The K coefficient in Henry’s law of 0.052 mmol/(MPa g TOC) is obtained by matching experimental data. The amount of dissolved gas increases linearly when pressure increases. Using only the Langmuir equation without considering dissolution can lead to a significant underestimation of the amount of sorbed gas in shales. For non-Darcy gas flow, the apparent permeability model for free gas is established by combining slip flow and Knudsen flow. For adsorbed gas, the surface diffusion effect is also considered in this model. The surface diffu- sion coefficient is suggested to be of the same scale as the gas self-diffusion coefficient, and the corresponding effective permeability is derived. When 1/ increases,k/ kincreases, but the relationship is not linear as the Klinkenberg effect suggests. The effect of adsorption on the gas flow is significant in nanopores (r≤2 nm). Adsorption increases apparent permeability in shales at low pressures and decreases it at high pressures.展开更多
In recent years,great breakthroughs have been made in the exploration and development of natural gas in deep coal-rock reservoirs in Junggar,Ordos and other basins in China.In view of the inconsistency between the ind...In recent years,great breakthroughs have been made in the exploration and development of natural gas in deep coal-rock reservoirs in Junggar,Ordos and other basins in China.In view of the inconsistency between the industrial and academic circles on this new type of unconventional natural gas,this paper defines the concept of"coal-rock gas"on the basis of previous studies,and systematically analyzes its characteristics of occurrence state,transport and storage form,differential accumulation,and development law.Coal-rock gas,geologically unlike coalbed methane in the traditional sense,occurs in both free and adsorbed states,with free state in abundance.It is generated and stored in the same set of rocks through short distance migration,occasionally with the accumulation from other sources.Moreover,coal rock develops cleat fractures,and the free gas accumulates differentially.The coal-rock gas reservoirs deeper than 2000 m are high in pressure,temperature,gas content,gas saturation,and free-gas content.In terms of development,similar to shale gas and tight gas,coal-rock gas can be exploited by natural formation energy after the reservoirs connectivity is improved artificially,that is,the adsorbed gas is desorbed due to pressure drop after the high-potential free gas is recovered,so that the free gas and adsorbed gas are produced in succession for a long term without water drainage for pressure drop.According to buried depth,coal rank,pressure coefficient,reserves scale,reserves abundance and gas well production,the classification criteria and reserves/resources estimation method of coal-rock gas are presented.It is preliminarily estimated that the coal-rock gas in place deeper than 2000 m in China exceeds 30×10^(12)m^(3),indicating an important strategic resource for the country.The Ordos,Sichuan,Junggar and Bohai Bay basins are favorable areas for large-scale enrichment of coal-rock gas.The paper summarizes the technical and management challenges and points out the research directions,laying a foundation for the management,exploration,and development of coal-rock gas in China.展开更多
Based on the drilling, logging, experimental and testing data of Well PD1, a shallow normal-pressure shale gas well in the Laochangping anticline in southeastern Sichuan Basin, the shallow shale gas reservoirs of the ...Based on the drilling, logging, experimental and testing data of Well PD1, a shallow normal-pressure shale gas well in the Laochangping anticline in southeastern Sichuan Basin, the shallow shale gas reservoirs of the Ordovician Wufeng Formation to Silurian Longmaxi Formation (Wufeng-Longmaxi) were investigated in terms of geological characteristics, occurrence mechanism, and adsorption-desorption characteristics, to reveal the enrichment laws and high-yield mechanism of shallow normal-pressure shale gas in complex structure areas. First, the shallow shale gas reservoirs are similar to the medium-deep shale gas reservoirs in static indicators such as high-quality shale thickness, geochemistry, physical properties and mineral composition, but the former is geologically characterized by low formation pressure coefficient, low gas content, high proportion of adsorbed gas, low in-situ stress, and big difference between principal stresses. Second, shallow shales in the complex structure areas have the gas occurrence characteristics including low total gas content (1.1-4.8 m3/t), high adsorbed gas content (2.5-2.8 m3/t), low sensitive desorption pressure (1.7-2.5 MPa), and good self-sealing. Third, the adsorbed gas enrichment of shales is mainly controlled by organic matter abundance, formation temperature and formation pressure: the higher the organic matter abundance and formation pressure, the lower the formation temperature and the higher the adsorption capacity, which is more beneficial for the adsorbed gas occurrence. Fourth, the shallow normal-pressure shale gas corresponds to low sensitive desorption pressure. The adsorbed gas can be rapidly desorbed and recovered when the flowing pressure is reduced below the sensitive desorption pressure. Fifth, the exploration breakthrough of Well PD1 demonstrates that the shallow complex structure areas with adsorbed gas in dominance can form large-scale shale reservoirs, and confirms the good exploration potential of shallow normal-pressure shale gas in the margin of the Sichuan Basin.展开更多
Nanoporous carbons were synthesized using furfuryl alcohol and sucrose as precursors and MCM-41 and mordenite as nanoporous templates.The produced nanoporous carbons were used as adsorbent for methane storage.The aver...Nanoporous carbons were synthesized using furfuryl alcohol and sucrose as precursors and MCM-41 and mordenite as nanoporous templates.The produced nanoporous carbons were used as adsorbent for methane storage.The average pore diameter of the samples varied from 3.9 nm to 5.9 nm and the BET surface area varied from 320m2/g to 824m2/g.The volumetric adsorption experiments revealed that MCM-41 and sucrose had better performance compared with mordenite and furfuryl alcohol,correspondingly.Also,the effect of precursor to template ratio on the structure of nanoporous carbons and their adsorption capacities was investigated.The nanoporous carbon produced from MCM-41 mesoporous molecular sieve partially filled by sucrose shows the best methane adsorption capacity among the tested samples.展开更多
The acidolysis gas in source rocks is the desorbed hydrocarbon gas in non-connected pores and carbonate crystal lattice,while the absorbed gas lies in the connected pores and is absorbed by surface.It is controversial...The acidolysis gas in source rocks is the desorbed hydrocarbon gas in non-connected pores and carbonate crystal lattice,while the absorbed gas lies in the connected pores and is absorbed by surface.It is controversial about whether to use absorbed gas or acidolysis gas in gas source correlation.Using the absorbed gas and acidolysis gas in the first member of the Middle Permian Maokou Fm(hereinafter referred to as Mao 1 member)of Fuling area in the Sichuan Basin,the differences of components and carbon isotopes between absorbed gas and acidolysis gas and their applicable conditions in gas source correlation were discussed.The results show that:(1)there are larger differences of components and isotopes between absorbed gas and acidolysis gas.The absorbed gas has more heavy hydrocarbons and heavier carbon isotope.This difference can be attributed to the easier diffusion of methane and light hydrocarbons in absorbed gas,leading to relatively enriched heavy hydrocarbons and heavier carbon isotope in absorbed gas;(2)acidolysis gas is a mixture of hydrocarbon gas formed during different stages,while absorbed gas is the result of equilibrium between diffusion and supply of hydrocarbon gas,which can result in the differences on carbon isotope sequences between them;(3)the characteristics of natural gas produced after acid fracturing in the Mao 1 member in Fuling area resembles those of acidolysis gas in source rocks,while the characteristics of natural gas produced without acid fracturing are more similar to those of adsorbed gas;(4)the natural gas pool of the Mao 1 member is a carbonate gas pool with self-generation and self-storage.It is concluded that for the gas pool formed after migration,its producing pays have not been processed by acid fracturing,the produced gas should be correlated with the adsorbed gas in source rocks;for the carbonate gas pool with self-generation and self-storage,acid fracturing is usually needed,and the produced gas should be correlated with the acidolysis gas in source rocks.展开更多
The long-term seepage of hydrocarbons, either as macroseepage or microseepage, can set up near-surface oxidation reduction zones that favor the development of a diverse array of chemical and mineralogical changes. The...The long-term seepage of hydrocarbons, either as macroseepage or microseepage, can set up near-surface oxidation reduction zones that favor the development of a diverse array of chemical and mineralogical changes. The bacterial oxidation of light hydrocarbons can directly or indirectly bring about significant changes in the values of pH and Eh of the surrounding environment, thereby also changing the stability fields of the different mineral species present in that environment. The paper reports the role of hydrocarbon microseepage in surface alterations of trace metal concentrations. In this study trace metal alterations were mapped that appear to be associated with hydrocarbon microseepages in the oil/ gas fields. A total of 50 soil samples were collected near oil and gas fields of the Tatipaka and Pasarlapudi areas of the Krishna Godavari Basin, Andhra Pradesh. The soil samples were collected from a depth of 2-2.5 m. The paper reports the chemical alterations associated with trace metals in soils that are related to hydrocarbon microseepages above some of the major oil and gas fields of this petroliferous region. Trace metals, such as scandium （Sc）, vanadium （V）, chromium （Cr）, cobalt （Co）, nickel （Ni）, copper （Cu）, zinc （Zn）, barium （Ba） and strontium （Sr）, in soil samples were analyzed using inductively coupled plasma- mass spectrometry （ICP-MS）. The concentrations of Sc （8 to 40 mg/kg）, V （197 to 489 mg/kg）, Cr （106 to 287 mg/kg）, Co （31 to 52 mg/kg）, Ni （65 to 110 mg/kg）, Cu （88 to 131 mg/kg）, Zn （88 to 471 mg/kg）, Ba （263 to 3,091 mg/kg） and Sr （119 to 218 mg/kg） were obtained. It was observed that the concentrations of trace elements were tremendously increased when they were compared with their normal concentrations in soils. The analysis of adsorbed soil gas showed the presence of high concentrations of YC2＋ （C2H6, C3H8 and n-C4H10） ranging from 7 to 222 μg/kg respectively. Integrated studies of trace elements over adsorbed light gaseous hydrocarbons （2C2＋） anomalies showed good correlation with the existing oil and gas wells. The carbon isotopic composition of δ13C1 of the samples ranges between -36.6%o to -22.7‰ （Pee Dee Belemnite） values indicate thermogenic origin, which presents convincing evidence that the adsorbed soil gases collected from these sediments are of catagenetic origin. The increase in the concentrations of trace metals near oil/gas producing areas, suggests a soil chemical change to a reducing environment, presumably due to the influence of hydrocarbon microseepage, which could be applied with other geoscientific data to identify areas of future hydrocarbon exploration in frontier areas.展开更多
The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional explorati...The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study, near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area. Soil samples were collected from favorable areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons （methane through butane） were used as surface indicators of pe- troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons （C_1-C_4） and their carbon isotopes from the western part of Tapti graben, Deccan Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane （C1 518 ppb） and ethane plus higher hydrocarbons （∑C2＋ = 977 ppb） were observed. Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents. This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks, beneath Deccan Traps. The carbon isotopic composition of methane, ethane and propane ranges are from 22.5‰to -30.2‰ PDB, -18.0‰ to 27.1‰,, PDB and 16.9‰-32.1‰ PDB respectively, which are in thermogenic source. Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks. Prominent hydrocarbon concentra- tions were associated with dykes, lineaments and presented on thinner basaltic cover in the study area, which probably acts as channel for the micro-seepage of hydrocarbons.展开更多
Adsorbed gas content is an important parameter in shale gas reservoir evaluations,and its common calculation method is based on core experiments.However,in different areas,the correlations between the adsorbed gas con...Adsorbed gas content is an important parameter in shale gas reservoir evaluations,and its common calculation method is based on core experiments.However,in different areas,the correlations between the adsorbed gas content and well logging data might differ.Therefore,a model developed for one specific area cannot be considered universal.Based on previous studies,we studied the relationships between temperature,TOC,organic matter maturity and adsorbed gas content and revealed qualitative equations between these parameters.Then,the equations were combined to establish a new adsorbed gas content calculation model based on depth and total organic carbon(TOC).This model can be used to estimate the adsorbed gas content using only conventional well logging data when core experimental data are rare or even unavailable.The method was applied in the southern Sichuan Basin,and the adsorbed gas content results agree well with those calculated using the Langmuir isothermal model and core experimental data.The actual data processing results show that the adsorbed gas content model is reliable.展开更多
Shale gas is being hailed as the green energy of the future due to high heating value,low carbon emissions,and large reserves.Gas content of shale is a key parameter for evaluating the shale gas potential and screenin...Shale gas is being hailed as the green energy of the future due to high heating value,low carbon emissions,and large reserves.Gas content of shale is a key parameter for evaluating the shale gas potential and screening for the shale gas sweet spots.Although the concept of gas content has been well defined,obtaining a reliable gas content data still remains a challenge.A significant barrier is the method for evaluating the gas content.In this paper,we provide a review of the long-established and recently developed gas content evaluation methods.In the first part of this review article,the history of gas content evaluation methods is summarized since 1910s,relied on published and unpublished literatures as well as our own experiences.Then,the fundamental contents and concepts involved in gas content evaluation are introduced to provide a clear theoretical foundation for the methods.In the third part,eleven evaluation methods,including four direct methods and seven indirect methods,are systematically reviewed.In each method,its application to evaluating the gas content is presented,the key advances are highlighted,and the advantages and limitations are discussed.Finally,future directions are discussed to promote creative thinking across disciplines to develop new methods or improve current methods for evaluating the gas content more accurately and efficiently.展开更多
基金supported by the Natural Science Foundation of China(Grant No.42302170)National Postdoctoral Innovative Talent Support Program(Grant No.BX20220062)+3 种基金CNPC Innovation Found(Grant No.2022DQ02-0104)National Science Foundation of Heilongjiang Province of China(Grant No.YQ2023D001)Postdoctoral Science Foundation of Heilongjiang Province of China(Grant No.LBH-Z22091)the Natural Science Foundation of Shandong Province(Grant No.ZR2022YQ30).
文摘Prediction of production decline and evaluation of the adsorbed/free gas ratio are critical for determining the lifespan and production status of shale gas wells.Traditional production prediction methods have some shortcomings because of the low permeability and tightness of shale,complex gas flow behavior of multi-scale gas transport regions and multiple gas transport mechanism superpositions,and complex and variable production regimes of shale gas wells.Recent research has demonstrated the existence of a multi-stage isotope fractionation phenomenon during shale gas production,with the fractionation characteristics of each stage associated with the pore structure,gas in place(GIP),adsorption/desorption,and gas production process.This study presents a new approach for estimating shale gas well production and evaluating the adsorbed/free gas ratio throughout production using isotope fractionation techniques.A reservoir-scale carbon isotope fractionation(CIF)model applicable to the production process of shale gas wells was developed for the first time in this research.In contrast to the traditional model,this model improves production prediction accuracy by simultaneously fitting the gas production rate and δ^(13)C_(1) data and provides a new evaluation method of the adsorbed/free gas ratio during shale gas production.The results indicate that the diffusion and adsorption/desorption properties of rock,bottom-hole flowing pressure(BHP)of gas well,and multi-scale gas transport regions of the reservoir all affect isotope fractionation,with the diffusion and adsorption/desorption parameters of rock having the greatest effect on isotope fractionation being D∗/D,PL,VL,α,and others in that order.We effectively tested the universality of the four-stage isotope fractionation feature and revealed a unique isotope fractionation mechanism caused by the superimposed coupling of multi-scale gas transport regions during shale gas well production.Finally,we applied the established CIF model to a shale gas well in the Sichuan Basin,China,and calculated the estimated ultimate recovery(EUR)of the well to be 3.33×10^(8) m^(3);the adsorbed gas ratio during shale gas production was 1.65%,10.03%,and 23.44%in the first,fifth,and tenth years,respectively.The findings are significant for understanding the isotope fractionation mechanism during natural gas transport in complex systems and for formulating and optimizing unconventional natural gas development strategies.
基金Supported by the National Natural Science Foundation of China(42130802,42272200)CNPC Science and Technology Major Project(2023ZZ18)+1 种基金PetroChina Changqing Oilfield Major Science and Technology Project(2023DZZ01)Technology Project of PetroChina Coalbed Methane Company Limited(2023-KJ-18)。
文摘Based on the test and experimental data from exploration well cores of the Upper Paleozoic in the central-eastern Ordos Basin,combined with structural,burial depth and fluid geochemistry analyses,this study reveals the fluid characteristics,gas accumulation control factors and accumulation modes in the Upper Paleozoic coal reservoirs.The study indicates findings in two aspects.First,the 1500-1800 m interval represents the critical transition zone between open fluid system in shallow-medium depths and closed fluid system in deep depths.The reservoirs above 1500 m reflect intense water invasion,with discrete pressure gradient distribution,and the presence of methane mixed with varying degrees of secondary biogenic gas,and they generally exhibit high water saturation and adsorbed gas undersaturation.The reservoirs deeper than 1800 m,with extremely low permeability,are self-sealed,and contains closed fluid systems formed jointly by the hydrodynamic lateral blocking and tight caprock confinement.Within these systems,surface runoff infiltration is weak,the degree of secondary fluid transformation is minimal,and the pressure gradient is relatively uniform.The adsorbed gas saturation exceeds 100%in most seams,and the free gas content primarily ranges from 1 m^(3)/t to 8 m^(3)/t(greater than 10 m^(3)/t in some seams).Second,the gas accumulation in deep coals is primarily controlled by coal quality,reservoir-caprock assemblage,and structural position governed storage,wettability and sealing properties,under the constraints of the underground temperature and pressure conditions.High-rank,low-ash yield coals with limestone and mudstone caprocks show superior gas accumulation potential.Positive structural highs and wide and gentle negative structural lows are favorable sites for gas enrichment,while slope belts of fold limbs exhibit relatively lower gas content.This research enhances understanding of gas accumulation mechanisms in coal reservoirs and provides effective parameter reference for precise zone evaluation and innovation of adaptive stimulation technologies for deep resources.
基金Youth Fund of National Natural Science Foundation of China(42302170)CNPC Scientific and Technological Innovation Fund(2022DQ02-0104)RIPED Open Project Fund(2024-KFKT-31).
文摘Taking deep coal-rock gas in the Yulin and Daning-Jixian areas of the Ordos Basin,NW China,as the research object,full-diameter coal rock samples with different cleat/fracture development degrees from the Carboniferous Benxi Formation were selected to conduct physical simulation and isotope monitoring experiments of the full-life-cycle depletion development of coal-rock gas.Based on the experimental results,a dual-medium carbon isotope fractionation(CIF)model coupling cleats/fractures and matrix pores was constructed,and an evaluation method for free gas production patterns was established to elucidate the carbon isotope fractionation mechanism and adsorbed/free gas production characteristics during deep coal-rock gas development.The results show that the deep coal-rock gas development process exhibits a three-stage carbon isotope fractionation pattern:“Stable(Ⅰ)→Decrease(Ⅱ)→Increase(Ⅲ)”.A rapid decline in boundary pressure in stageⅢleads to fluctuations in isotope value,characterized by a“rapid decrease followed by continued increase”,with free gas being produced first and long-term supply of adsorbed gas.The CIF model can effectively match measured gas pressure,cumulative gas production,and δ^(13)C_(1) value of produced gas.During the first two stages of isotope fractionation,free gas dominated cumulative production.During the mid-late stages of slow depletion production,the staged pressure control development method can effectively increase the gas recovery.The production of adsorbed gas is primarily controlled by the rock's adsorption capacity and the presence of secondary flow channels.Effectively enhancing the recovery of adsorbed gas during the late stage remains crucial for maintaining stable production and improving the ultimate recovery factor of deep coal-rock gas.
基金sponsored by National Natural Science Foundation(Project number:41274136)
文摘Shale needs to contain a sufficient amount of gas to make it viable for exploitation. The continental heterogeneous shale formation in the Yan-chang （YC） area is investigated by firstly measuring the shale gas content in a laboratory and then investigating use of a theoretical prediction model. Key factors controlling the shale gas content are determined, and a prediction model for free gas content is established according to the equation of gas state and a new petrophysical volume model. Application of the Langmuir volume constant and pressure constant obtained from results of adsorption isotherms is found to be limited because these constants are greatly affected by experimental temperature and pressures. Therefore, using measurements of adsorption isotherms and thermodynamic theory, the influence of temperature, total organic carbon （TOC）, and mineralogy on Langmuir volume constants and pressure constants are investigated in detail. A prediction model for the Langmuir pressure constant with a correction of temperatures is then established, and a prediction model for the Langmuir volume constant with correction of temperature, TOC, and quartz contents is also proposed. Using these corrected Langmuir constants, application of the Langmuir model determined using experimental adsorption isotherms is extrapolated to reservoir temperature, pressure, and lithological conditions, and a method for the prediction of shale gas content using well logs is established. Finally, this method is successfully applied to predict the shale gas content of the continental shale formation in the YC area, and practical application is shown to deliver good results with high precision.
基金supported by the Department of Energy under Award Number DE-FE0024311
文摘Gas sorption and non-Darcy flow are two important issues for shale gas reservoirs. The sorption consists of dissolution and adsorption. Dissolved gas and adsorbed gas are different. The former is dissolved in the shale matrix, while the latter is concentrated near the solid walls of pores. In this paper, the Langmuir equation is used to describe adsorption and Henry’s law is used to describe dissolution. The K coefficient in Henry’s law of 0.052 mmol/(MPa g TOC) is obtained by matching experimental data. The amount of dissolved gas increases linearly when pressure increases. Using only the Langmuir equation without considering dissolution can lead to a significant underestimation of the amount of sorbed gas in shales. For non-Darcy gas flow, the apparent permeability model for free gas is established by combining slip flow and Knudsen flow. For adsorbed gas, the surface diffusion effect is also considered in this model. The surface diffu- sion coefficient is suggested to be of the same scale as the gas self-diffusion coefficient, and the corresponding effective permeability is derived. When 1/ increases,k/ kincreases, but the relationship is not linear as the Klinkenberg effect suggests. The effect of adsorption on the gas flow is significant in nanopores (r≤2 nm). Adsorption increases apparent permeability in shales at low pressures and decreases it at high pressures.
基金Supported by the Prospective and Basic Research Project of PetroChina(2021DJ23)。
文摘In recent years,great breakthroughs have been made in the exploration and development of natural gas in deep coal-rock reservoirs in Junggar,Ordos and other basins in China.In view of the inconsistency between the industrial and academic circles on this new type of unconventional natural gas,this paper defines the concept of"coal-rock gas"on the basis of previous studies,and systematically analyzes its characteristics of occurrence state,transport and storage form,differential accumulation,and development law.Coal-rock gas,geologically unlike coalbed methane in the traditional sense,occurs in both free and adsorbed states,with free state in abundance.It is generated and stored in the same set of rocks through short distance migration,occasionally with the accumulation from other sources.Moreover,coal rock develops cleat fractures,and the free gas accumulates differentially.The coal-rock gas reservoirs deeper than 2000 m are high in pressure,temperature,gas content,gas saturation,and free-gas content.In terms of development,similar to shale gas and tight gas,coal-rock gas can be exploited by natural formation energy after the reservoirs connectivity is improved artificially,that is,the adsorbed gas is desorbed due to pressure drop after the high-potential free gas is recovered,so that the free gas and adsorbed gas are produced in succession for a long term without water drainage for pressure drop.According to buried depth,coal rank,pressure coefficient,reserves scale,reserves abundance and gas well production,the classification criteria and reserves/resources estimation method of coal-rock gas are presented.It is preliminarily estimated that the coal-rock gas in place deeper than 2000 m in China exceeds 30×10^(12)m^(3),indicating an important strategic resource for the country.The Ordos,Sichuan,Junggar and Bohai Bay basins are favorable areas for large-scale enrichment of coal-rock gas.The paper summarizes the technical and management challenges and points out the research directions,laying a foundation for the management,exploration,and development of coal-rock gas in China.
基金Supported by the Sinopec Scientific Research Project(P21087-6).
文摘Based on the drilling, logging, experimental and testing data of Well PD1, a shallow normal-pressure shale gas well in the Laochangping anticline in southeastern Sichuan Basin, the shallow shale gas reservoirs of the Ordovician Wufeng Formation to Silurian Longmaxi Formation (Wufeng-Longmaxi) were investigated in terms of geological characteristics, occurrence mechanism, and adsorption-desorption characteristics, to reveal the enrichment laws and high-yield mechanism of shallow normal-pressure shale gas in complex structure areas. First, the shallow shale gas reservoirs are similar to the medium-deep shale gas reservoirs in static indicators such as high-quality shale thickness, geochemistry, physical properties and mineral composition, but the former is geologically characterized by low formation pressure coefficient, low gas content, high proportion of adsorbed gas, low in-situ stress, and big difference between principal stresses. Second, shallow shales in the complex structure areas have the gas occurrence characteristics including low total gas content (1.1-4.8 m3/t), high adsorbed gas content (2.5-2.8 m3/t), low sensitive desorption pressure (1.7-2.5 MPa), and good self-sealing. Third, the adsorbed gas enrichment of shales is mainly controlled by organic matter abundance, formation temperature and formation pressure: the higher the organic matter abundance and formation pressure, the lower the formation temperature and the higher the adsorption capacity, which is more beneficial for the adsorbed gas occurrence. Fourth, the shallow normal-pressure shale gas corresponds to low sensitive desorption pressure. The adsorbed gas can be rapidly desorbed and recovered when the flowing pressure is reduced below the sensitive desorption pressure. Fifth, the exploration breakthrough of Well PD1 demonstrates that the shallow complex structure areas with adsorbed gas in dominance can form large-scale shale reservoirs, and confirms the good exploration potential of shallow normal-pressure shale gas in the margin of the Sichuan Basin.
文摘Nanoporous carbons were synthesized using furfuryl alcohol and sucrose as precursors and MCM-41 and mordenite as nanoporous templates.The produced nanoporous carbons were used as adsorbent for methane storage.The average pore diameter of the samples varied from 3.9 nm to 5.9 nm and the BET surface area varied from 320m2/g to 824m2/g.The volumetric adsorption experiments revealed that MCM-41 and sucrose had better performance compared with mordenite and furfuryl alcohol,correspondingly.Also,the effect of precursor to template ratio on the structure of nanoporous carbons and their adsorption capacities was investigated.The nanoporous carbon produced from MCM-41 mesoporous molecular sieve partially filled by sucrose shows the best methane adsorption capacity among the tested samples.
基金supported by the National Major Science and Technology Project“Hydrocarbon generation and expulsion mechanism of marine source strata and evaluation technology”(No.:2017ZX05005-001-003)the Science&Technology Project of Sinopec Exploration Company“Evaluation on hydrocarbon generation potential in Mao 1 Member,Fuling area”(No.:35450003-17-ZC0607-0032).
文摘The acidolysis gas in source rocks is the desorbed hydrocarbon gas in non-connected pores and carbonate crystal lattice,while the absorbed gas lies in the connected pores and is absorbed by surface.It is controversial about whether to use absorbed gas or acidolysis gas in gas source correlation.Using the absorbed gas and acidolysis gas in the first member of the Middle Permian Maokou Fm(hereinafter referred to as Mao 1 member)of Fuling area in the Sichuan Basin,the differences of components and carbon isotopes between absorbed gas and acidolysis gas and their applicable conditions in gas source correlation were discussed.The results show that:(1)there are larger differences of components and isotopes between absorbed gas and acidolysis gas.The absorbed gas has more heavy hydrocarbons and heavier carbon isotope.This difference can be attributed to the easier diffusion of methane and light hydrocarbons in absorbed gas,leading to relatively enriched heavy hydrocarbons and heavier carbon isotope in absorbed gas;(2)acidolysis gas is a mixture of hydrocarbon gas formed during different stages,while absorbed gas is the result of equilibrium between diffusion and supply of hydrocarbon gas,which can result in the differences on carbon isotope sequences between them;(3)the characteristics of natural gas produced after acid fracturing in the Mao 1 member in Fuling area resembles those of acidolysis gas in source rocks,while the characteristics of natural gas produced without acid fracturing are more similar to those of adsorbed gas;(4)the natural gas pool of the Mao 1 member is a carbonate gas pool with self-generation and self-storage.It is concluded that for the gas pool formed after migration,its producing pays have not been processed by acid fracturing,the produced gas should be correlated with the adsorbed gas in source rocks;for the carbonate gas pool with self-generation and self-storage,acid fracturing is usually needed,and the produced gas should be correlated with the acidolysis gas in source rocks.
文摘The long-term seepage of hydrocarbons, either as macroseepage or microseepage, can set up near-surface oxidation reduction zones that favor the development of a diverse array of chemical and mineralogical changes. The bacterial oxidation of light hydrocarbons can directly or indirectly bring about significant changes in the values of pH and Eh of the surrounding environment, thereby also changing the stability fields of the different mineral species present in that environment. The paper reports the role of hydrocarbon microseepage in surface alterations of trace metal concentrations. In this study trace metal alterations were mapped that appear to be associated with hydrocarbon microseepages in the oil/ gas fields. A total of 50 soil samples were collected near oil and gas fields of the Tatipaka and Pasarlapudi areas of the Krishna Godavari Basin, Andhra Pradesh. The soil samples were collected from a depth of 2-2.5 m. The paper reports the chemical alterations associated with trace metals in soils that are related to hydrocarbon microseepages above some of the major oil and gas fields of this petroliferous region. Trace metals, such as scandium （Sc）, vanadium （V）, chromium （Cr）, cobalt （Co）, nickel （Ni）, copper （Cu）, zinc （Zn）, barium （Ba） and strontium （Sr）, in soil samples were analyzed using inductively coupled plasma- mass spectrometry （ICP-MS）. The concentrations of Sc （8 to 40 mg/kg）, V （197 to 489 mg/kg）, Cr （106 to 287 mg/kg）, Co （31 to 52 mg/kg）, Ni （65 to 110 mg/kg）, Cu （88 to 131 mg/kg）, Zn （88 to 471 mg/kg）, Ba （263 to 3,091 mg/kg） and Sr （119 to 218 mg/kg） were obtained. It was observed that the concentrations of trace elements were tremendously increased when they were compared with their normal concentrations in soils. The analysis of adsorbed soil gas showed the presence of high concentrations of YC2＋ （C2H6, C3H8 and n-C4H10） ranging from 7 to 222 μg/kg respectively. Integrated studies of trace elements over adsorbed light gaseous hydrocarbons （2C2＋） anomalies showed good correlation with the existing oil and gas wells. The carbon isotopic composition of δ13C1 of the samples ranges between -36.6%o to -22.7‰ （Pee Dee Belemnite） values indicate thermogenic origin, which presents convincing evidence that the adsorbed soil gases collected from these sediments are of catagenetic origin. The increase in the concentrations of trace metals near oil/gas producing areas, suggests a soil chemical change to a reducing environment, presumably due to the influence of hydrocarbon microseepage, which could be applied with other geoscientific data to identify areas of future hydrocarbon exploration in frontier areas.
文摘The Deccan Syneclise is considered to have significant hydrocarbon potential. However, significant hydrocarbon discoveries, particularly for Mesozoic sequences, have not been established through conventional exploration due to the thick basalt cover over Mesozoic sedimentary rocks. In this study, near-surface geochemical data are used to understand the petroleum system and also investigate type of source for hydrocarbons generation of the study area. Soil samples were collected from favorable areas identified by integrated geophysical studies. The compositional and isotopic signatures of adsorbed gaseous hydrocarbons （methane through butane） were used as surface indicators of pe- troleum micro-seepages. An analysis of 75 near-surface soil-gas samples was carried out for light hydrocarbons （C_1-C_4） and their carbon isotopes from the western part of Tapti graben, Deccan Syneclise, India. The geochemical results reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases. High concentrations of adsorbed thermogenic methane （C1 518 ppb） and ethane plus higher hydrocarbons （∑C2＋ = 977 ppb） were observed. Statistical analysis shows that samples from 13% of the samples contain anomalously high concentrations of light hydrocarbons in the soil-gas constituents. This seepage suggests largest magnitude of soil gas anomalies might be generated/source from Mesozoic sedimentary rocks, beneath Deccan Traps. The carbon isotopic composition of methane, ethane and propane ranges are from 22.5‰to -30.2‰ PDB, -18.0‰ to 27.1‰,, PDB and 16.9‰-32.1‰ PDB respectively, which are in thermogenic source. Surface soil sample represents the intersection of a migration conduit from the deep subsurface to the surface connected to sub-trappean Mesozoic sedimentary rocks. Prominent hydrocarbon concentra- tions were associated with dykes, lineaments and presented on thinner basaltic cover in the study area, which probably acts as channel for the micro-seepage of hydrocarbons.
基金supported by the National Natural Science Foundation of China(Grant No.41504094)the Open Foundation of Top Disciplines at Yangtze Unive rsity(No.2019KFJJ0818009)。
文摘Adsorbed gas content is an important parameter in shale gas reservoir evaluations,and its common calculation method is based on core experiments.However,in different areas,the correlations between the adsorbed gas content and well logging data might differ.Therefore,a model developed for one specific area cannot be considered universal.Based on previous studies,we studied the relationships between temperature,TOC,organic matter maturity and adsorbed gas content and revealed qualitative equations between these parameters.Then,the equations were combined to establish a new adsorbed gas content calculation model based on depth and total organic carbon(TOC).This model can be used to estimate the adsorbed gas content using only conventional well logging data when core experimental data are rare or even unavailable.The method was applied in the southern Sichuan Basin,and the adsorbed gas content results agree well with those calculated using the Langmuir isothermal model and core experimental data.The actual data processing results show that the adsorbed gas content model is reliable.
基金supported by the National Natural Science Foundation of China(42202175,41927801,and 42102128)the Open Foundation of State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development(33550000-22-ZC0613-0296)。
文摘Shale gas is being hailed as the green energy of the future due to high heating value,low carbon emissions,and large reserves.Gas content of shale is a key parameter for evaluating the shale gas potential and screening for the shale gas sweet spots.Although the concept of gas content has been well defined,obtaining a reliable gas content data still remains a challenge.A significant barrier is the method for evaluating the gas content.In this paper,we provide a review of the long-established and recently developed gas content evaluation methods.In the first part of this review article,the history of gas content evaluation methods is summarized since 1910s,relied on published and unpublished literatures as well as our own experiences.Then,the fundamental contents and concepts involved in gas content evaluation are introduced to provide a clear theoretical foundation for the methods.In the third part,eleven evaluation methods,including four direct methods and seven indirect methods,are systematically reviewed.In each method,its application to evaluating the gas content is presented,the key advances are highlighted,and the advantages and limitations are discussed.Finally,future directions are discussed to promote creative thinking across disciplines to develop new methods or improve current methods for evaluating the gas content more accurately and efficiently.
