Determining the process of densification and tectonic evolution of tight sandstone can help to understand the distribution of reservoirs and find relatively high-permeability areas.Based on integrated approaches of th...Determining the process of densification and tectonic evolution of tight sandstone can help to understand the distribution of reservoirs and find relatively high-permeability areas.Based on integrated approaches of thin section,scanning electron microscopy(SEM),cathode luminescence(CL),nuclear magnetic resonance(NMR),X-ray diffraction(XRD),N2 porosity and permeability,micro-resistivity imaging log(MIL)and three-dimensional seismic data analysis,this work discussed the reservoir characteristics of the member 8 of the Permian Xiashihezi Formation(He 8 sandstones)in the Linxing area of eastern Ordos Basin,determined the factors affecting reservoir quality,and revealed the formation mechanism of relatively high-permeability areas.The results show that the He 8 sandstones in the Linxing area are mainly composed of feldspathic litharenites,and are typical tight sandstones(with porosity<10%and permeability<1 mD accounting for 80.3%of the total samples).Rapid burial is the main reason for reservoir densification,which resulted in 61%loss of the primary porosity.In this process,quartz protected the original porosity by resisting compaction.The cementation(including carbonate,clay mineral and siliceous cementation)further densified the sandstone reservoirs,reducing the primary porosity with an average value of 28%.The calcite formed in the eodiagenesis occupied intergranular pores and affected the formation of the secondary pores by preventing the later fluid intrusion,and the Fe-calcite formed in the mesodiagenetic stage densified the sandstones further by filling the residual intergranular pores.The clay minerals show negative effects on reservoir quality,however,the chlorite coatings protected the original porosity by preventing the overgrowth of quartz.The dissolution of feldspars provides extensive intergranular pores which constitute the main pore type,and improves the reservoir quality.The tectonic movements play an important role in improving the reservoir quality.The current tectonic traces of the study area are mainly controlled by the Himalayan movement,and the high-permeability reservoirs are mainly distributed in the anticline areas.Additionally,the improvement degree(by tectonic movements)of reservoir quality is partly controlled by the original composition of the sandstones.Thus,the selection of potential tight gas well locations in the study area should be focused on the anticline areas with relatively good original reservoir quality.And the phenomena can be referenced for other fluvial tight sandstone basins worldwide.展开更多
Tight sandstone gas reservoirs have poorer porosityepermeability relationships,so conventional reservoir classification schemes can hardly satisfy the classification and evaluation demand of this type of reservoirs.To...Tight sandstone gas reservoirs have poorer porosityepermeability relationships,so conventional reservoir classification schemes can hardly satisfy the classification and evaluation demand of this type of reservoirs.To solve this problem,this paper took the Permian tight sandstone gas reservoir in the Linxing Block along the eastern margin of the Ordos Basin as an example to describe the micro-structures of the tight sandstone reservoirs by means of high-pressure mercury injection,nuclear magnetic resonance(NMR),scanning electron mi-croscope(SEM)and so on.Then,the control effect of micro-structure parameters on the macrophysical properties was studied.Finally,classification and evaluation of tight sandstone reservoirs were carried out on this basis.And the following research results were obtained.First,NMR can identify the distribution of pores of different sizes,and high-pressure mercury injection can reflect the poreethroat configuration and percolation capacity of a reservoir.Second,both methods are better coincident in the description results.With an in-crease of the right peak of T2 spectra,the mercury intrusion curve presents a concave shape and the pore throat radius increases while the pore type gradually changes from intragranular dissolution pores and intercrystalline pores to intergranular pores and intergranular disso-lution pores and the reservoir quality gets better.Third,micro-pore structure controls reservoir physical properties andfluid mobility.And the porosity of large pores is best correlated with the effective porosity,so it can be used to evaluate the reservoir capacity of tight sandstone.Fourth,the throat radius R15 obtained by high pressure mercury injection is in the best correlation with porosity and permeability,so it can be used to evaluate the percolation capacity of tight sandstone.Fifth,by combining the porosity of large pores with the R15,the tight sandstone reservoirs in the Linxing Block are classified into 4 categories,and the classification results are in a good agreement with the on-site well test data.It is concluded that the combination of high-pressure mercury injection and NMR can effectively identify the key parameters which reflect the reservoir capacity and percolation capacity of tight sandstone,and improve the reliability and integrity of reservoir classification.And by selecting the key parameters that reflect reservoir capacity and percolation capacity,it can provide guidance for the classification and evaluation of tight sandstone reservoirs.展开更多
The Upper Shihezi sedimentary rocks in the Linxing region has been estimated with a significant volume of tight sandstone gas.However,lateral distribution of the present-day stress magnitude is poorly understood,which...The Upper Shihezi sedimentary rocks in the Linxing region has been estimated with a significant volume of tight sandstone gas.However,lateral distribution of the present-day stress magnitude is poorly understood,which limits further gas production.Hence,a one-dimensional mechanical earth model and a three-dimensional heterogeneous geomechanical model are built to address this issue.The results indicate that the strike-slip stress regime is dominant in the Upper Shihezi Formation.Relatively low stresses are mainly located around wells L-60,L-22,L-40,L-90,etc,and stress distributions exhibit the similarity in the Members H2 and H4.The differential stresses are relatively low in the Upper Shihezi Formation,suggesting that complex hydraulic fracture networks may be produced.Natural fractures in the Upper Shihezi Formation contribute little to the overall gas production in the Linxing region.In addition,the minimum principal stress gradient increases with Young's modulus,suggesting that the stiffer rocks commonly convey higher stress magnitudes.There is a strong interplay between stress distribution and heterogeneity in rock mechanics.Overall,the relative error between the predicted and measured results is less than 10%,implying that the predicted stress distribution is reliable and can be used for subsequent analysis in the Linxing region.展开更多
Understanding the distribution of in-situ stresses is extremely important in a wide range of fields such as oil and gas exploration and development, CO2 sequestration, borehole stability, and stress-related geohazards...Understanding the distribution of in-situ stresses is extremely important in a wide range of fields such as oil and gas exploration and development, CO2 sequestration, borehole stability, and stress-related geohazards assessment. In the present study, the in-situ stress distribution in the Linxing area of eastern Ordos Basin, China, was analyzed based on well tested parameters. The maximum horizontal principal stress （SHmax）, minimum horizontal principal stress （Shmin）, and vertical stress （Sv） were calculated, and they were linearly correlated with burial depth. In general, two types of in-situ stress fields were determined in the Linxing area： （i） the in-situ stress state followed the relation Sv 〉 Snmax 〉 Shmin in shallow layers with burial depths of less than about 940 m, indicating a normal faulting stress regime; （ii） the Snmax magnitude increased conspicuously and was greater than the Sv magnitude in deep layers with depths more than about 940 m, and the in-situ stress state followed the relation Snmax 〉 Sv 〉 Shmin, demonstrating a strike-slip faulting stress regime. The horizontal differential stress （Snmax-Shmtn） increased with burial depth, indicating that wellbore instability may be a potentially significant problem when drilling deep vertical wells. The lateral stress coefficient ranged from 0.73 to 1.08 with an average of 0.93 in the Linxing area. The coalbed methane （CBM） reservoir permeability was also analyzed. No obvious exponential relationship was found between coal permeability and effective in-situ stress magnitude. Coal permeability was relatively high under a larger effective in-situ stress magnitude. Multiple factors, including fracture development, contribute to the variation of CBM reservoir permeability in the Linxing area of eastern Ordos Basin.展开更多
Unconventional reservoirs are generally characterized by low matrix porosity and permeability,in which natural fractures are important factors for gas production.In this study,we analyzed characteristics of natural fr...Unconventional reservoirs are generally characterized by low matrix porosity and permeability,in which natural fractures are important factors for gas production.In this study,we analyzed characteristics of natural fractures,and their influencing factors based on observations from outcrops,cores and image logs.The orientations of natural fractures were mainly in the∼N-S,WNW-ESE and NE-SW directions with relatively high fracture dip angles.Fracture densities were calculated based on fracture measurements within cores,indicating that natural fractures were not well-developed in the Benxi-Upper Shihezi Formations of Linxing Block.The majority of natural fractures were open fractures and unfilled.According to the characteristics of fracture sets and tectonic evolution of the study area,natural fractures in the Linxing Block were mainly formed in the Yanshanian and Himalayan periods.The lithology and layer thickness influenced the development of natural fractures,and more natural fractures were generated in carbonate rocks and thin layers in the study area.In addition,in the Linxing Block,natural fractures with∼N-S-trending strikes contributed little to the overall subsurface fluid flow under the present-day stress state.These study results provide a geological basis for gas exploration and development in the Linxing unconventional reservoirs of Ordos Basin.展开更多
基金financially supported by the National Science and Technology Major Project of China (grant No. 2016ZX05066001–002)the National Science Foundation for Young Scientists of China (grant No. 41702171)the Program for Excellent Talents in Beijing (grant No. 2017000020124G107)
文摘Determining the process of densification and tectonic evolution of tight sandstone can help to understand the distribution of reservoirs and find relatively high-permeability areas.Based on integrated approaches of thin section,scanning electron microscopy(SEM),cathode luminescence(CL),nuclear magnetic resonance(NMR),X-ray diffraction(XRD),N2 porosity and permeability,micro-resistivity imaging log(MIL)and three-dimensional seismic data analysis,this work discussed the reservoir characteristics of the member 8 of the Permian Xiashihezi Formation(He 8 sandstones)in the Linxing area of eastern Ordos Basin,determined the factors affecting reservoir quality,and revealed the formation mechanism of relatively high-permeability areas.The results show that the He 8 sandstones in the Linxing area are mainly composed of feldspathic litharenites,and are typical tight sandstones(with porosity<10%and permeability<1 mD accounting for 80.3%of the total samples).Rapid burial is the main reason for reservoir densification,which resulted in 61%loss of the primary porosity.In this process,quartz protected the original porosity by resisting compaction.The cementation(including carbonate,clay mineral and siliceous cementation)further densified the sandstone reservoirs,reducing the primary porosity with an average value of 28%.The calcite formed in the eodiagenesis occupied intergranular pores and affected the formation of the secondary pores by preventing the later fluid intrusion,and the Fe-calcite formed in the mesodiagenetic stage densified the sandstones further by filling the residual intergranular pores.The clay minerals show negative effects on reservoir quality,however,the chlorite coatings protected the original porosity by preventing the overgrowth of quartz.The dissolution of feldspars provides extensive intergranular pores which constitute the main pore type,and improves the reservoir quality.The tectonic movements play an important role in improving the reservoir quality.The current tectonic traces of the study area are mainly controlled by the Himalayan movement,and the high-permeability reservoirs are mainly distributed in the anticline areas.Additionally,the improvement degree(by tectonic movements)of reservoir quality is partly controlled by the original composition of the sandstones.Thus,the selection of potential tight gas well locations in the study area should be focused on the anticline areas with relatively good original reservoir quality.And the phenomena can be referenced for other fluvial tight sandstone basins worldwide.
基金supported by the National Natural Science Foundation of China"Full-borchole pore throat characterization and modeling of tight sandstone based on the combination of image method and fluid method"(No.41602141)"Mechanism and effects of imbibition and retention of fracturing fluid in in-situ shale gas reservoirs"(No.41972139)the Independent Innovation Foundation of Universities(No.18CX02069A).
文摘Tight sandstone gas reservoirs have poorer porosityepermeability relationships,so conventional reservoir classification schemes can hardly satisfy the classification and evaluation demand of this type of reservoirs.To solve this problem,this paper took the Permian tight sandstone gas reservoir in the Linxing Block along the eastern margin of the Ordos Basin as an example to describe the micro-structures of the tight sandstone reservoirs by means of high-pressure mercury injection,nuclear magnetic resonance(NMR),scanning electron mi-croscope(SEM)and so on.Then,the control effect of micro-structure parameters on the macrophysical properties was studied.Finally,classification and evaluation of tight sandstone reservoirs were carried out on this basis.And the following research results were obtained.First,NMR can identify the distribution of pores of different sizes,and high-pressure mercury injection can reflect the poreethroat configuration and percolation capacity of a reservoir.Second,both methods are better coincident in the description results.With an in-crease of the right peak of T2 spectra,the mercury intrusion curve presents a concave shape and the pore throat radius increases while the pore type gradually changes from intragranular dissolution pores and intercrystalline pores to intergranular pores and intergranular disso-lution pores and the reservoir quality gets better.Third,micro-pore structure controls reservoir physical properties andfluid mobility.And the porosity of large pores is best correlated with the effective porosity,so it can be used to evaluate the reservoir capacity of tight sandstone.Fourth,the throat radius R15 obtained by high pressure mercury injection is in the best correlation with porosity and permeability,so it can be used to evaluate the percolation capacity of tight sandstone.Fifth,by combining the porosity of large pores with the R15,the tight sandstone reservoirs in the Linxing Block are classified into 4 categories,and the classification results are in a good agreement with the on-site well test data.It is concluded that the combination of high-pressure mercury injection and NMR can effectively identify the key parameters which reflect the reservoir capacity and percolation capacity of tight sandstone,and improve the reliability and integrity of reservoir classification.And by selecting the key parameters that reflect reservoir capacity and percolation capacity,it can provide guidance for the classification and evaluation of tight sandstone reservoirs.
基金The authors would like to thank the financial support from the National Natural Science Foundation of China(41702130,41872171 and 41672146)National Science and Technology Major Project(2016ZX05066)Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD).
文摘The Upper Shihezi sedimentary rocks in the Linxing region has been estimated with a significant volume of tight sandstone gas.However,lateral distribution of the present-day stress magnitude is poorly understood,which limits further gas production.Hence,a one-dimensional mechanical earth model and a three-dimensional heterogeneous geomechanical model are built to address this issue.The results indicate that the strike-slip stress regime is dominant in the Upper Shihezi Formation.Relatively low stresses are mainly located around wells L-60,L-22,L-40,L-90,etc,and stress distributions exhibit the similarity in the Members H2 and H4.The differential stresses are relatively low in the Upper Shihezi Formation,suggesting that complex hydraulic fracture networks may be produced.Natural fractures in the Upper Shihezi Formation contribute little to the overall gas production in the Linxing region.In addition,the minimum principal stress gradient increases with Young's modulus,suggesting that the stiffer rocks commonly convey higher stress magnitudes.There is a strong interplay between stress distribution and heterogeneity in rock mechanics.Overall,the relative error between the predicted and measured results is less than 10%,implying that the predicted stress distribution is reliable and can be used for subsequent analysis in the Linxing region.
基金We would like to express our gratitude to the reviewers for offering constructive suggestions and comments which improved this manuscript in many aspects. This work was supported by the National Science and Technology Major Project （No. 2016ZX05066）, the National Natural Science Foundation of China （Grant Nos. 41702130, 41672149, and 41672146）, the Fundamental Research Funds for the Central Universities （2015XKZD07）, and Priority Academic Program Development of Jiangsu Higher Education Institutions （PAPD）.
文摘Understanding the distribution of in-situ stresses is extremely important in a wide range of fields such as oil and gas exploration and development, CO2 sequestration, borehole stability, and stress-related geohazards assessment. In the present study, the in-situ stress distribution in the Linxing area of eastern Ordos Basin, China, was analyzed based on well tested parameters. The maximum horizontal principal stress （SHmax）, minimum horizontal principal stress （Shmin）, and vertical stress （Sv） were calculated, and they were linearly correlated with burial depth. In general, two types of in-situ stress fields were determined in the Linxing area： （i） the in-situ stress state followed the relation Sv 〉 Snmax 〉 Shmin in shallow layers with burial depths of less than about 940 m, indicating a normal faulting stress regime; （ii） the Snmax magnitude increased conspicuously and was greater than the Sv magnitude in deep layers with depths more than about 940 m, and the in-situ stress state followed the relation Snmax 〉 Sv 〉 Shmin, demonstrating a strike-slip faulting stress regime. The horizontal differential stress （Snmax-Shmtn） increased with burial depth, indicating that wellbore instability may be a potentially significant problem when drilling deep vertical wells. The lateral stress coefficient ranged from 0.73 to 1.08 with an average of 0.93 in the Linxing area. The coalbed methane （CBM） reservoir permeability was also analyzed. No obvious exponential relationship was found between coal permeability and effective in-situ stress magnitude. Coal permeability was relatively high under a larger effective in-situ stress magnitude. Multiple factors, including fracture development, contribute to the variation of CBM reservoir permeability in the Linxing area of eastern Ordos Basin.
基金Many thanks to the financial support from National Natural Science Foundation of China(Grant Nos.41702130 and 41872171)National Science and Technology Major Project(2016ZX05066)Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD).
文摘Unconventional reservoirs are generally characterized by low matrix porosity and permeability,in which natural fractures are important factors for gas production.In this study,we analyzed characteristics of natural fractures,and their influencing factors based on observations from outcrops,cores and image logs.The orientations of natural fractures were mainly in the∼N-S,WNW-ESE and NE-SW directions with relatively high fracture dip angles.Fracture densities were calculated based on fracture measurements within cores,indicating that natural fractures were not well-developed in the Benxi-Upper Shihezi Formations of Linxing Block.The majority of natural fractures were open fractures and unfilled.According to the characteristics of fracture sets and tectonic evolution of the study area,natural fractures in the Linxing Block were mainly formed in the Yanshanian and Himalayan periods.The lithology and layer thickness influenced the development of natural fractures,and more natural fractures were generated in carbonate rocks and thin layers in the study area.In addition,in the Linxing Block,natural fractures with∼N-S-trending strikes contributed little to the overall subsurface fluid flow under the present-day stress state.These study results provide a geological basis for gas exploration and development in the Linxing unconventional reservoirs of Ordos Basin.
