| Although natural and artificial fractures are the key factors for shale gas production,a great deal of shale gas exploration and development experience shows that shale matrix permeability is the key parameter for shale gas production stabilization and economic development.Due to the heterogeneity of shale itself,the matrix permeability of shale has a strong anisotropy.However,little is known about the manifestation and influencing factors of the anisotropy of shale matrix permeability,especially the micro mechanism of the influence of pore structure on permeability anisotropy.In this paper,on the basis of the investigation in dense reservoir permeability characterization technology at home and abroad,we take the lower Paleozoic Marine organic-rich shale in the Yangtze region of China as the research object.The samples with fractures were removed by X-ray micron CT scanning technology,and the matrix permeability in the triaxial direction of shale was characterized by cubic samples and 3D-printed polymer membrane.In order to study the characteristics of shale rock fabric,organic matter pores and inorganic mineral pore structure by means of petrology and electron microscopy,we hope to reveal the genetic mechanism of shale matrix permeability anisotropy from the microscopic scale.It is found that the horizontal matrix permeability of shale is 1-2 orders of magnitude higher than the vertical matrix permeability,and the matrix permeability of two parallel bedding directions of the same sample also showed great difference.For example,the horizontal matrix permeability of the lower Cambrian Niutitang formation shale in the southeastern Chongqing of the eastern part in upper Yangtze is between42.8n D and 189.4n D,with an average of 99.4n D,while the vertical shale matrix permeability is only 5.4n D on average,and the ratio of horizontal to vertical matrix permeability ranged from 9.8 to 29.6,and the ratio of matrix permeability between the two horizontal directions was between 1.4 and 2.3.The average horizontal matrix permeability of the lower Cambrian Shuijingtuo formation shale in northeast Chongqing was 2861.8n D,and the vertical matrix permeability was 11.3n D?27.8n D,with an average of 19.2n D,and the ratio of horizontal to vertical permeability is between 55.5and 277.4,and the permeability ratios of the two parallel bedding directions are between2.6 and 4.4.The average horizontal matrix permeability of the lower Cambrian shale of Niutitang formation in central Hunan is 31.7n D,while the average vertical matrix permeability is only 7.6n D,and the ratio of horizontal to vertical matrix permeability is between 3.4 and 5.1,and the ratio of matrix permeability between the two horizontal directions is between 1.2 and 1.4.The average horizontal matrix permeability of the shale in Wufeng Formation of the well XY3 in southeastern Chongqing is 48.6n D and the average vertical matrix permeability is 11.9n D,and the ratio of horizontal to vertical matrix permeability is between 2.3 and 10.3,and the ratio of matrix permeability between the two parallel bedding directions is between 1.1 and 2.0.The average horizontal matrix permeability of the shale in the Longmaxi Formation of YC8 is79.9n D,while the vertical shale matrix permeability is 6.5n D on average,and the ratio of horizontal to vertical matrix permeability ranged from 6.9 to 20.5,and the permeability ratios of the two horizontal matrices were between 1.4 and 2.3.The matrix permeability of shale is mainly controlled by the microstructure of rock.It is found that the mineral composition and pore structure have a great influence on the matrix permeability of shale.The results show that the directional arrangement of mineral particles along the long axis greatly increases the matrix permeability of shale,and makes the anisotropy of the matrix permeability of shale(The ratio of matrix permeability between parallel bedding directions and vertical bedding directions can reach 29.6,and the ratio of matrix permeability between two parallel bedding directions can reach 2.3)much stronger than that of the shale with mineral particles unaligned(The ratio of matrix permeability between the parallel bedding direction and the vertical bedding direction is only 3.4,and the ratio of matrix permeability between the two parallel bedding directions is only 1.1).The increase of quartz content enhanced the disorder of shale mineral fabric,which decreases the matrix permeability of shale,especially when the quartz content is above 50%,the permeability of shale matrix decreases significantly.Low temperature gas adsorption study shows that the number and total volume of shale micropores and mesoporous are positively correlated with the matrix permeability of shale,which indicates that micropores and mesoporous are very important for gas flow in shale matrix.The more developed shale micropores and mesoporous,the higher the matrix permeability of shale.The seepage capacity of shale micro fractures is higher than pores,and the spatial connectivity of micro fractures and pores determines the horizontal matrix permeability.In addition,no correlation was found between the burial depth,porosity and TOC content of the sample and the matrix permeability.Focusing on the shale samples rich in biological fossils,the fabric relationship between biological fossils and minerals,the residual organic matter of graptolite and the pore development characteristics were mainly studied.Research results show that nanoscale pores are developed in the organic matter of fossils,and a large number of micro-fractures are also developed in the contact edge with minerals.The presence of organic matter in biological fossils greatly increases the matrix permeability in the direction of parallel shale bedding.The matrix permeability of the shale along the long axis of graptolite is much higher than that of the other direction.However,in the case of nondirectionally arranged shale with graptolite and shell fossils,the permeability difference between the two parallel bedding directions perpendicular to each other is small. |
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