Physical Properties Of Wufeng-Longmaxi Shale Reservoir In North Guizhou-South Chongqing Area

Shale reservoirs are dominated by nanometer and micrometer pore throats.The complexity of micropore structure determines the complexity of physical properties of shale reservoirs.The unclear understanding of shale reservoir space and fluid occurrence characteristics restricts the evaluation of shale gas reservoirs and the study of reservoir formation mechanism.The northern Guizhou-southern Chongqing area is located in the transition zone of northern Guizhou slope zone,southern Sichuan low-steep fold zone,Eastern Sichuan comb-like fold zone and Western Hunan-Hubei trough-like fold zone.It is of great significance to study the microstructures and occurrence characteristics of different shale facies reservoirs controlled by different tectonic units and sedimentary microfacies.Based on the research status at home and abroad,the micro-physical characteristics of shale reservoirs in the study area are discussed through field exploration,organic geochemical testing,qualitative and quantitative observation of shale pore structure,and isothermal adsorption of methane.Organic shale is widely distributed in the study area.Silica shale,carbonaceous shale,calcareous shale and silty shale are commonly found.They have good geochemical indicators.The average content of organic matter is 3%,brittle minerals are more than 50%.On the plane,quartz in the study area decreases from the Northeast(Daozhen-Nanchuan line)to the southwest(Qijiang-Xishui line),while dolomite and calcite increase,and siliceous shale turns to calcareous shale.The mineral composition changes from binary to ternary,from deep-water shelf to shallow-water shelf,and from sedimentary center to margin.The heterogeneity of shale is strong.From bottom to top,TOC and quartz decrease,brittleness decreases and clay increases.The shale in the study area is mainly mesoporous,and the mesoporous and macroporous contribute to the main pore volume,while the mesoporous and microporous contribute to the main specific surface area.Micropore is dominated by organic pore,mesopore and macropore are dominated by inorganic pore.In the study area,pore type transits from cylindrical pore to slit pore,fracturability decreases and surface flexibility increases.In Qijiang area,slit pore is the main pore,and pore type is relatively single.Pulse experiments show that permeability decreases exponentially with the increase of confining pressure and effective pressure when the fluid pressure is constant.The permeability of low permeability shale is more sensitive to confining pressure.The permeability can not recover to the initial value when the pressure is unloaded,and the smaller the permeability,the lower the recovery rate.Pulse permeability is positively correlated with the most probable pore size,and micro-variation of pore tortuosity(2.05-2.25)will cause significant changes in porosity and permeability.Thrust faults with multiple layers of detachment often do not cut through anticline,resulting in secondary fractures,which can increase permeability and free gas will not be lost.The methane isothermal adsorption of shale depends on the specific surface area of the pore,and is influenced by the composition and pore structure of shale.The micropore and mesoporous are positively correlated with methane isothermal adsorption,but not with macropore.There was a relatively good positive correlation with TOC and quartz,but no significant correlation with carbonate and clay.The isothermal adsorption of methane increases first and then decreases with the decrease of shale particle size.The optimum range is about 60 meshes(250 um).This is consistent with the trend that micropore,mesoporous and macropore increase in the early stage of shale fragmentation,and all levels of pore change to a larger level in the later stage,which results in the increase of specific surface area of shale first and then decrease.