Identification and evaluation of shale oil micromigration and its petroleum geological significance

Taking the Lower Permian Fengcheng Formation shale in Mahu Sag of Junggar Basin,NW China,as an example,core observation,test analysis,geological analysis and numerical simulation were applied to identify the shale oil micro-migration phenomenon.The hydrocarbon micro-migration in shale oil was quantitatively evaluated and verified by a self-created hydrocarbon expulsion potential method,and the petroleum geological significance of shale oil micro-migration evaluation was determined.Results show that significant micro-migration can be recognized between the organic-rich lamina and organic-poor lamina.The organic-rich lamina has strong hydrocarbon generation ability.The heavy components of hydrocarbon preferentially retained by kerogen swelling or adsorption,while the light components of hydrocarbon were migrated and accumulated to the interbedded felsic or carbonate organic-poor laminae as free oil.About 69% of the Fengcheng Formation shale samples in Well MY1 exhibit hydrocarbon charging phenomenon,while 31% of those exhibit hydrocarbon expulsion phenomenon.The reliability of the micro-migration evaluation results was verified by combining the group components based on the geochromatography effect,two-dimension nuclear magnetic resonance analysis,and the geochemical behavior of inorganic manganese elements in the process of hydrocarbon migration.Micro-migration is a bridge connecting the hydrocarbon accumulation elements in shale formations,which reflects the whole process of shale oil generation,expulsion and accumulation,and controls the content and composition of shale oil.The identification and evaluation of shale oil micro-migration will provide new perspectives for dynamically differential enrichment mechanism of shale oil and establishing a“multi-peak model in oil generation”of shale.

Hydrocarbon accumulation and orderly distribution of whole petroleum system in marine carbonate rocks of Sichuan Basin,SW China

Based on the situation and progress of marine oil/gas exploration in the Sichuan Basin,SW China,the whole petroleum system is divided for marine carbonate rocks of the basin according to the combinations of hydrocarbon accumulation elements,especially the source rock.The hydrocarbon accumulation characteristics of each whole petroleum system are analyzed,the patterns of integrated conventional and unconventional hydrocarbon accumulation are summarized,and the favorable exploration targets are proposed.Under the control of multiple extensional-convergent tectonic cycles,the marine carbonate rocks of the Sichuan Basin contain three sets of regional source rocks and three sets of regional cap rocks,and can be divided into the Cambrian,Silurian and Permian whole petroleum systems.These whole petroleum systems present mainly independent hydrocarbon accumulation,containing natural gas of affinity individually.Locally,large fault zones run through multiple whole petroleum systems,forming a fault-controlled complex whole petroleum system.The hydrocarbon accumulation sequence of continental shelf facies shale gas accumulation,marginal platform facies-controlled gas reservoirs,and intra-platform fault-and facies-controlled gas reservoirs is common in the whole petroleum system,with a stereoscopic accumulation and orderly distribution pattern.High-quality source rock is fundamental to the formation of large gas fields,and natural gas in a whole petroleum system is generally enriched near and within the source rocks.The development and maintenance of large-scale reservoirs are essential for natural gas enrichment,multiple sources,oil and gas transformation,and dynamic adjustment are the characteristics of marine petroleum accumulation,and good preservation conditions are critical to natural gas accumulation.Large-scale marginal-platform reef-bank facies zones,deep shale gas,and large-scale lithological complexes related to source-connected faults are future marine hydrocarbon exploration targets in the Sichuan Basin.

Burial depth interval of the shale brittle–ductile transition zone and its implications in shale gas exploration and production

Brittleness and ductility of shale are closely related to shale gas exploration and production. How to predict brittleness and ductility of shale is one of the key issues in the study of shale gas preservation and hydraulic fracturing treatments. The magnitude of shale brittleness was often determined by brittle mineral content(for example, quartz and feldspars) in shale gas exploration.However, the shale brittleness is also controlled by burial depth. Shale brittle/ductile properties such as brittle, semibrittle and ductile can mutually transform with burial depth variation. We established a work flow of determining the burial depth interval of brittle–ductile transition zone for a given shale. Two boundaries were employed to divide the burial depth interval of shale brittle/ductile properties. One is the bottom boundary of the brittle zone(BZ), and the other is the top boundary of the ductile zone(DZ). The brittle–ductile transition zone(BDTZ) is between them.The bottom boundary of BZ was determined by the overconsolidation ratio(OCR) threshold value combined with pre-consolidation stress which the shale experienced over geological time. The top boundary of DZ was determined based on the critical confining pressure of brittle–ductile transition. The OCR threshold value and the critical confining pressure were obtained from uniaxial strain andtriaxial compression tests. The BZ, DZ and BDTZ of the Lower Silurian Longmaxi shale in some representative shale gas exploration wells in eastern Sichuan and western Hubei areas were determined according to the above work flow. The results show that the BZ varies with the maximum burial depth and the DZ varies with the density of the overlying rocks except for the critical confining pressure.Moreover, the BDTZ determined by the above work flow is probably the best burial depth interval for marine shale gas exploration and production in Southern China. Shale located in the BDTZ is semi-brittle and is not prone to be severely naturally fractured but likely to respond well to hydraulic fracturing. The depth interval of BDTZ determined by our work flow could be a valuable parameter of shale gas estimation in geology and engineering.

Distribution characteristics, exploration and development, geological theories research progress and exploration directions of shale gas in China

The shale gas resources in China have great potential and the geological resources of shale gas is over 100×10^(12)m^(3),which includes about 20×10^(12)m^(3) of recoverable resources.Organic-rich shales can be divided into three types according to their sedimentary environments,namely marine,marine-continental transitional,and continental shales,which are distributed in 13 stratigraphic systems from the Mesoproterozoic to the Cenozoic.The Sichuan Basin and its surrounding areas have the highest geological resources of shale gas,and the commercial development of shale gas has been achieved in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in these areas,with a shale gas production of up to 20×10^(9)m^(3) in 2020.China has seen rapid shale gas exploration and development over the last five years,successively achieving breakthroughs and important findings in many areas and strata.The details are as follows.(1)Large-scale development of middle-shallow shale gas(burial depth:less than 3500 m)has been realized,with the productivity having rapidly increased;(2)breakthroughs have been constantly made in the development of deep shale gas(burial depth:3500-4500 m),and the ultradeep shale gas(burial depth:greater than 4500 m)is under testing;(3)breakthroughs have been made in the development of normal-pressure shale gas,and the assessment of the shale gas in complex tectonic areas is being accelerated;(4)shale gas has been frequently discovered in new areas and new strata,exhibiting a great prospect.Based on the exploration and development practice,three aspects of consensus have been gradually reached on the research progress in the geological theories of shale gas achieved in China.(1)in terms of deep-water fine-grained sediments,organic-rich shales are the base for the formation of shale gas;(2)in terms of high-quality reservoirs,the development of micro-nano organic matter-hosted pores serves as the core of shale gas accumulation;(3)in terms of preservation conditions,weak structural transformation,a moderate degree of thermal evolution,and a high pressure coefficient are the key to shale gas enrichment.As a type of important low-carbon fossil energy,shale gas will play an increasingly important role in achieving the strategic goals of peak carbon dioxide emissions and carbon neutrality.Based on the in-depth study of shale gas geological conditions and current exploration progress,three important directions for shale gas exploration in China in the next five years are put forward.

A semi-analytical pressure and rate transient analysis model for inner boundary and propped fractures exhibiting dynamic behavior under long-term production conditions

The loss of hydrocarbon production caused by the dynamic behavior of the inner boundary and propped fractures under long-term production conditions has been widely reported in recent studies.However,the quantitative relationships for the variations of the inner boundary and propped fractures have not been determined and incorporated in the semi-analytical models for the pressure and rate transient analysis.This work focuses on describing the variations of the inner boundary and propped fractures and capturing the typical characteristics from the pressure transient curves.A generalized semi-analytical model was developed to characterize the dynamic behavior of the inner boundary and propped fractures under long-term production conditions.The pressure-dependent length shrinkage coefficients,which quantify the length changes of the inner zone and propped fractures,are modified and incorporated into this multi-zone semi-analytical model.With simultaneous numerical iterations and numerical inversions in Laplace and real-time space,the transient solutions to pressure and rate behavior are determined in just a few seconds.The dynamic behavior of the inner boundary and propped fractures on transient pressure curves is divided into five periods:fracture bilinear flow(FR1),dynamic PFs flow(FR2),inner-area linear flow(FR3),dynamic inner boundary flow(FR4),and outer-area dominated linear flow(FR5).The early hump during FR2 period and a positive upward shift during FR4period are captured on the log-log pressure transient curves,reflecting the dynamic behavior of the inner boundary and propped fractures during the long-term production period.The transient pressure behavior will exhibit greater positive upward trend and the flow rate will be lower with the shrinkage of the inner boundary.The pressure derivative curve will be upward earlier as the inner boundary shrinks more rapidly.The lower permeability caused by the closure of un-propped fractures in the inner zone results in greater upward in pressure derivative curves.If the permeability loss for the dynamic behavior of the inner boundary caused by the closure of un-propped fractures is neglected,the flow rate will be overestimated in the later production period.

Differential Characteristics of the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Shale Reservoir and its Implications for Exploration and Development of Shale Gas in/around the Sichuan Basin

The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery,which is the key stratum for marine shale gas exploration and development(E&D)in China.Based on sedimentary environment,material basis,storage space,fracability and reservoir evolution data,the reservoir characteristics of the Wufeng-Longmaxi shale and their significance for shale gas E&D are systematically compared and analyzed in this paper.The results show that(1)the depocenter of the Wufeng(WF)-Longmaxi(LM)shale gradually migrates from east to west.The high-quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales,which are primarily distributed in the graptolite shale interval of WF2-LM5.The high-quality reservoirs in the southern Sichuan Basin are mainly calcareous-siliceous and organic-rich argillaceous shales,which are distributed in the graptolite shale interval of WF2-LM7.(2)Deep shale gas(the burial depth>3500 m)in the Sichuan Basin has high-ultrahigh pressure and superior physical properties.The organic-rich siliceous,calcareous-siliceous and organic-rich argillaceous shales have suitable reservoir properties.The marginal area of the Sichuan Basin has a higher degree of pressure relief,which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity.(3)Combining shale gas exploration practices and impacts of lithofacies,depth,pressure coefficient and brittle-ductile transition on the reservoir properties,it is concluded that the favorable depth interval of the Wufeng-Longmaxi shale gas is 2200~4000 m under current technical conditions.(4)Aiming at the differential reservoir properties of the Wufeng-Longmaxi shale in the Sichuan Basin and its periphery,several suggestions for future research directions and E&D of shale gas are formulated.

Enrichment characteristics and exploration directions of deep shale gas of Ordovician-Silurian in the Sichuan Basin and its surrounding areas,China

The enrichment characteristics of deep shale gas in the Ordovician Wufeng-Silurian Longmaxi formations in the Sichuan Basin and its surrounding areas are investigated through experiments under high temperature and high pressure,including petrophysical properties analyses,triaxial stress test and isothermal adsorption of methane experiment.(1)The deep shale reservoirs drop significantly in porosity and permeability compared with shallower shale reservoirs,and contain mainly free gas.(2)With higher deviatoric stress and axial strain,the deep shale reservoirs have higher difficulty fracturing.(3)Affected by structural location and morphology,fracture characteristics,geofluid activity stages and intensity,deep shale gas reservoirs have more complicated preservation conditions.(4)To achieve the commercial development of deep shale gas reservoirs,deepening geological understanding is the basis,and exploring reservoir simulation technology befitting the geological features is the key.(5)The siliceous shale and limestone-bearing siliceous shale in the Metabolograptus persculptus-Parakidograptus acuminatus zones(LM1-LM3 graptolite zones)are the high-production intervals for deep shale gas and the most favorable landing targets for horizontal drilling.Deeps water areas such as Jiaoshiba,Wulong,Luzhou and Changning with deep shale reservoirs over 10 m thickness are the most favorable areas for deep shale gas enrichment.It is recommended to carry out exploration and development practice in deep-water shale gas areas deposited deep with burial depth no more than 5000 m where the geological structure is simple and the shale thickness in the LM1-LM3 graptolite zone is greater than 10 m.It is better to increase the lateral length of horizontal wells,and apply techniques including high intensity of perforations,large volume of proppant,far-field and near-wellbore diversions to maximize the stimulated deep reservoir volume.

Characteristics of Triassic Petroleum Systems in the Longmenshan Foreland Basin,Sichuan Province,China

The Triassic in the Longmengshan foreland basin is rich in oil and gas resources. Its reservoirs feature low-porosity, low-permeability, small pore throat, high water saturation, and strong heterogeneity. The existence of abnormally high pressure and various reservoir-cap combinations developed at different times provide favorable conditions for trapping oil and gas. Taking the theory of petroleum systems as a guide, and beginning with research on tectonics, sedimentary history, distribution and evolution of source rocks, reservoir evolution, hydraulic force distribution, and hydrocarbon migration, analysis and study of static factors like source rocks, reservoirs and cap rocks, and dynamic factors such as hydrocarbon generation, migration, and accumulation revealed the characteristics of the Upper Triassic petroleum system in western Sichuan province. The deepbasin gas in the central hydrocarbon kitchen of the Upper Triassic, structural-lithological combination traps on the surrounding slopes, and the structural traps of the Indo-Chinese-Yangshan paleohighs, are potential plays. The relatively well- developed fault zones in the southern segment of the Longmengshan foothill belt are favorable Jurassic gas plays. Pengshan-Xinjin, Qiongxi, and Dayi are recent exploration targets for Jurassic oil/gas reservoirs.

An overview of the geology and production of the Fuling shale gas field,Sichuan Basin,China

The Fuling shale gas field in China is the largest shale gas field as well as the largest of its type discovered in any Lower Paleozoic formation.In this study,the geology and production of the upper and lower gas layers in the Fuling shale gas field are evaluated in terms of structure,shale quality,fault,initial production,and estimated ultimate recovery(EUR).The shale in the lower gas layer of the Jiaoshiba anticline is a high-quality reservoir,where the space is dominated by organic pores in kerogen,and the gas content is high.The shale gas wells reveal relatively high initial production and EUR.However,the shale in the upper gas layer of the Jiaoshiba anticline has reservoir space mainly composed of clay mineral pores and organic pores within bitumen,and the gas content is low.In terms of structure,primary gas migration may occur in the upper gas layer,resulting in free gas accumulation in the structural high,where the development effects are generally better than those in the structural low.The lower gas layer in the Pingqiao anticline,is the main interval for shale gas accumulation and development due to the high-quality shale.Under the influence of faults,the efficiency of exploration wells emplaced on top of the anticline is much lower a compared with those in the flanks.The residual synclines close to the Sichuan Basin,including the Baima and Baitao anticlines,are characterized by more recent uplifts,larger area,greater distance from the deep and large faults,and early fracture closure.Therefore,we recommend that the shale gas exploration and development should be carried out preferentially in areas close to the center of the residual synclines,featuring relatively high-pressure coefficient and moderate burial depth.

Relationship between radiogenic heat production in granitic rocks and emplacement age

Granites play a crucial role in the Earth's thermal regime and its evolution.Radiogenic heat production(RHP)by the decay of radioactive elements(U,Th,K)in granites is a significant parameter in estimating the thermal structure of the lithosphere.RHP variability of granites with their emplacement ages could provide insights for thermal modeling in different geological epochs.An aggregated RHP from 2877 globally-distributed granitic samples of continental crust are analyzed for this study;these sample cover the entire geological history.The average bulk RHP in all types of granitic rocks of all ages is 2.92±1.86μW/m^(3).The RHP tends to increase gradually with progressively younger geologic emplacement age,based on a statistical analysis of the data.However,the youngest granites do not necessarily have the highest RHP.The mean RHP in 181 representative Cenozoic Himalayan leucogranitesdwhich are the youngest granites found on Earth,is as low as 1.84μW/m^(3).This is probably related to the initial conditions of magma formation,magmatic source material,and differentiation processes in the HimalayaneTibetan plateau.By correcting the decay factor,variations of the RHP in the emplaced granitic rocks are obtained,indicating the changing levels of heat production and different thermal regimes on Earth in various geological epochs.The highest RHP in granitic rocks emplaced in the Archean and Early Proterozoic corresponds to two global-scale collisional events during supercontinent cycles,at 2.7 and 1.9 Ga respectively.RHPs of granites can be an important indicator in the study of Earth's thermal regime and its evolution.

Production evolution patterns and development stage division of waterflooding oilfields

The continuous growth of recoverable reserves in a waterflooding oilfield has a significant impact on the patterns of production evolution. A new production evolution model is established by improving the Weng Cycle model. With the new model, the statistical correspondence between the production decline stage and the reserve-production imbalance is clarified,and the correlation of water cut with the recovery percent of recoverable reserves is discussed, providing quantitative basis of reservoir engineering for dividing development stages of oilfield and defining mature oilfields. According to the statistics of oilfields in eastern China, the time point corresponding to the reserve-production balance coefficient dropping to less than 1dramatically is well correlated the beginning point of production decline, thus the time when the reserve-production balance coefficient drops dramatically can be taken as the initiation point of production decline stage. The research results show that the water cut and the recovery percent of recoverable reserves have a good statistical match in the high water cut stage, and it is more rational to take both the start point of production decline stage and the water cut of 90%(or the recovery percent of recoverable reserves of 80%) as the critical criteria for defining a mature oilfield. Five production evolution patterns can be summarized as follows: growth–peak plateau–stepped decline, growth–stepped stabilizing–stepped decline, growth–stepped stabilizing–rapid decline, growth–peak plateau–rapid decline, and growth–continuous decline.

Progress and Prospects of Oil and Gas Exploration and Development in the World

2017 has seen the recovery in the global oil and gas market.Both Brent and WTI were priced at over USD 50 per barrel.Oil companies have basically adapted to the relatively low oil price environment through a series of cost-cutting and efficiency measures,and the number of projects making final investment decisions (FID) has increased for the second consecutive year.Global annual new conventional oil and gas reserves hit new lows in 2016 and 2017.However,gratifying progress has been made in the area of unconventional oil and gas.Shale oil and gas in the US has become the biggest bright spot in the oil and gas industry in the past two years.There have also been new developments in the extraction of natural gas hydrates,which may lead to a greater role for natural gas in energy consumption.Recovery will be the main tone of the oil and gas market in 2018.

Challenges and Strategies of Shale Oil Exploration and Development in China

Encouraged by the great success of shale oil development in the US, China began its shale oil exploration and development activities in 2010. During the past decade, progresses had been made in many shale-oil-bearing basins, including Ordos, Junggar and Bohai Bay. This article reviewed in detail the current situations of shale oil exploration and development in these basins. Issues such as shale oil play geological conditions that quite different from that of the U.S., lack of specific technologies, impact of a low oil price, absence of sufficient fiscal support from the central government and room for further institutional improvement, were sorted out to be the major challenges in the path of shale oil exploration and development in the basins. To deal with the problems, the authors proposed a nationallevel research for cost-reduction and efficiency-increasing strategy and painted a preliminary roadmap of technology research for shale oil extraction. Four national-level pilot projects of shale oil development were also suggested in the paper.

Prospects of China's Petroleum Refining and Petrochemical Industry under “Dual Carbon” Goals

The essence of achieving“carbon peaking and carbon neutrality”,that is,China’s“dual carbon”goals as unveiled by a guiding document on the country’s work by the Communist Party of China Central Committee and the State Council,lies in accelerating the transition from fossil energy consumption to non-fossil energy consumption.The petroleum refining and petrochemical industry,as one of the industries with the highest level of carbon emissions in China’s industrial system,faces enormous challenges as thus.In this regard,refineries and petrochemical manufacturers have to propose and implement even more effective strategies and measures to reduce carbon emissions against the background of the“dual carbon”goals.The focuses will be put on strengthening energy management during production,speeding up industrial adjustment and transformation,accelerating the recycling of multiple resources including waste plastics,and developing innovative petrochemical processes,with a view to enabling gradual replacement of new energy and vigorously developing renewables.These are to usher in an era of great change,and contribute to the realization of China’s goals in carbon dioxide emission peaking by 2030 and carbon neutrality by 2060.

Sinopec’s Hydrogen Ambition and Challenge

Hydrogen is one the clean energy forms or maybe the most clean energy form of all as it produces only water,electricity and heat when consumed in a fuel cell.With more frequent extreme climate events that caused damage and cost lives around world in the past few years,the call for replacing fossil energy including coal,oil and even natural gas(that are hold liable for these losses)with more sustainable and cleaner energy forms is loud.Hydrogen has been regarded as one of the viable and important solutions to fight against climate changes.

SINOPEC Oilfield Development Progress and Next Key Work Deployment

Since SINOPEC carried out the action plan of dramatically improving oil and gas exploration and development in 2019,oilfield development front has actively overcome the impact of disadvantage factors such as sharp fluctuations in oil prices,recurrent COVID-19 and withdrawal of nature reserves,fully implemented the decision and deployment of company group’s party leadership,focused on improving productivity,controlling decline,increasing recoverable capacity and reducing cost,adhered to the detailed reservoir understanding,precise plan deployment,detailed supporting processes and actuarial development benefits for strengthening the construction of production efficiency,paid close attention to the detailed development of old oilfields,enhanced the promotion and application of significantly improved oil recovery technology,put forth effort on consolidating the foundation of stable production,to make sure the production stable and rising,and oil production exceeded the planned target during the period from 2019 to 2022.In 2022,35.32 million tons oil were produced,with an increase of 0.16 million tons,marking the largest increase in the nearly past four years.

Probing the interaction between asphaltene-wax and its effects on the crystallization behavior of waxes in heavy oil via molecular dynamics simulation

High content of asphaltenes and waxes leads to the high pour point and the poor flowability of heavy oil,which is adverse to its efficient development and its transportation in pipe.Understanding the interaction mechanism between asphaltene-wax is crucial to solve these problems,but it is still unclear.In this paper,molecular dynamics simulation was used to investigate the interaction between asphaltenewax and its effects on the crystallization behavior of waxes in heavy oil.Results show that molecules in pure wax are arranged in a paralleled geometry.But wax molecules in heavy oil,which are close to the surface of asphaltene aggregates,are bent and arranged irregularly.When the mass fraction of asphaltenes in asphaltene-wax system(ω_(asp))is 0-25 wt%,the attraction among wax molecules decreases and the bend degree of wax molecules increases with the increase ofω_(asp).Theω_(asp)increases from 0 to 25 wt%,and the attraction between asphaltene-wax is stronger than that among waxes.This causes that the wax precipitation point changes from 353 to 333 K.While theω_(asp)increases to 50 wt%,wax molecules are more dispersed owing to the steric hindrance of asphaltene aggregates,and the interaction among wax molecules transforms from attraction to repulsion.It causes that the ordered crystal structure of waxes can't be formed at normal temperature.Simultaneously,the asphaltene,with the higher molecular weight or the more hetero atoms,has more obvious inhibition to the formation of wax crystals.Besides,resins also have an obvious inhibition on the wax crystal due to the formation of asphalteneresin aggregates with a larger radius.Our results reveal the interaction mechanism between asphaltene-wax,and provide useful guidelines for the development of heavy oil.

Iteration and evaluation of shale oil development technology for continental rift lake basins

By benchmarking with the iteration of drilling technology,fracturing technology and well placement mode for shale oil and gas development in the United States and considering the geological characteristics and development difficulties of shale oil in the Jiyang continental rift lake basin,East China,the development technology system suitable for the geological characteristics of shale oil in continental rift lake basins has been primarily formed through innovation and iteration of the development,drilling and fracturing technologies.The technology system supports the rapid growth of shale oil production and reduces the development investment cost.By comparing it with the shale oil development technology in the United States,the prospect of the shale oil development technology iteration in continental rift lake basins is proposed.It is suggested to continuously strengthen the overall three-dimensional development,improve the precision level of engineering technology,upgrade the engineering technical indicator system,accelerate the intelligent optimization of engineering equipment,explore the application of complex structure wells,form a whole-process integrated quality management system from design to implementation,and constantly innovate the concept and technology of shale oil development,so as to promote the realization of extensive,beneficial and high-quality development of shale oil in continental rift lake basins.

Distribution patterns of tight sandstone gas and shale gas

Based on an elaboration of the resource potential and annual production of tight sandstone gas and shale gas in the United States and China,this paper reviews the researches on the distribution of tight sandstone gas and shale gas reservoirs,and analyzes the distribution characteristics and genetic types of tight sandstone gas reservoirs.In the United States,the proportion of tight sandstone gas in the total gas production declined from 20%-35%in 2008 to about 8%in 2023,and the shale gas production was 8310×10^(8)m^(3)in 2023,about 80%of the total gas production,in contrast to the range of 5%-17%during 2000-2008.In China,the proportion of tight sandstone gas in the total gas production increased from 16%in 2010 to 28%or higher in 2023.China began to produce shale gas in 2012,with the production reaching 250×10^(8)m^(3)in 2023,about 11%of the total gas production of the country.The distribution of shale gas reservoirs is continuous.According to the fault presence,fault displacement and gas layer thickness,the continuous shale gas reservoirs can be divided into two types:continuity and intermittency.Most previous studies believed that both tight sandstone gas reservoirs and shale gas reservoirs are continuous,but this paper holds that the distribution of tight sandstone gas reservoirs is not continuous.According to the trap types,tight sandstone gas reservoirs can be divided into lithologic,anticlinal,and synclinal reservoirs.The tight sandstone gas is coal-derived in typical basins in China and Egypt,but oil-type gas in typical basins in the United States and Oman.

Fine quantitative characterization of high-H2S gas reservoirs under the influence of liquid sulfur deposition and adsorption

In order to clarify the influence of liquid sulfur deposition and adsorption to high-H2S gas reservoirs,three types of natural cores with typical carbonate pore structures were selected for high-temperature and high-pressure core displacement experiments.Fine quantitative characterization of the cores in three steady states(original,after sulfur injection,and after gas flooding)was carried out using the nuclear magnetic resonance(NMR)transverse relaxation time spectrum and imaging,X-ray computer tomography(CT)of full-diameter cores,basic physical property testing,and field emission scanning electron microscopy imaging.The loss of pore volume caused by sulfur deposition and adsorption mainly comes from the medium and large pores with sizes bigger than 1000μm.Liquid sulfur has a stronger adsorption and deposition ability in smaller pore spaces,and causes greater damage to reservoirs with poor original pore structures.The pore structure of the three types of carbonate reservoirs shows multiple fractal characteristics.The worse the pore structure,the greater the change of internal pore distribution caused by liquid sulfur deposition and adsorption,and the stronger the heterogeneity.Liquid sulfur deposition and adsorption change the pore size distribution,pore connectivity,and heterogeneity of the rock,which further changes the physical properties of the reservoir.After sulfur injection and gas flooding,the permeability of TypeⅠreservoirs with good physical properties decreased by 16%,and that of TypesⅡandⅢreservoirs with poor physical properties decreased by 90%or more,suggesting an extremely high damage.This indicates that the worse the initial physical properties,the greater the damage of liquid sulfur deposition and adsorption.Liquid sulfur is adsorbed and deposited in different types of pore space in the forms of flocculence,cobweb,or retinitis,causing different changes in the pore structure and physical property of the reservoir.