Study On The Shale Gas Seepage Characteristics And Control Mechanism

Shale gas is currently a hotspot for unconventional natural gas exploration and development.The characteristics of shale gas seepage directly affect shale gas production and mining efficiency,which is a hot and difficult problem in current research.In this paper,29outcrop shale samples,which came from the Wufeng Formation and the Longmaxi Formation in the southeastern Sichuan Basin,were carried out for a series of laboratory experiments,including gas permeability measurement under variable confining pressure and gas pressure conditions,high-pressure mercury injection,nitrogen adsorption,scanning electron microscopy(SEM),nuclear magnetic analysis(NMR)and XRD.According to the experiments,the characteristics and controlling mechanism of dynamic change in shale gas permeability to gas(CH₄),were investigated and researched.Results are as follows.(1)Based on high-pressure mercury intrusion experiment,combined with nitrogen adsorption,SEM and XRD experiments,the classification standard of shale gas reservoir in the study area was established.Based on the inflection point distribution characteristics of the corrected mercury intrusion curve,the connectivity pores and fractures of the shale gas reservoir are divided into fractures(>5?m),macropores(5?m-100 nm),mesopores(100nm-25 nm),and small Holes(25 nm-10 nm)and micropores(<10 nm).The shale gas reservoirs in the study area are divided into three types:good reservoir(type I reservoir),transition reservoir(type II reservoir)and differential reservoir(type III reservoir).(2)According to the non-adsorbed gas(He)and adsorbed gas(CH₄)permeability tests under the same test conditions(variable confining pressure and variable inlet pressure),the influencing factors of permeability change are quantitatively analyzed.In addition,under a constant confining stress,the permeability change is the superimposed result of effective stress increase,matrix shrinkage and gas slippage that occur during the gas pressure depletion process.Based on the controlling mechanism of the permeability change,a prediction model for the gas permeability change in gas-shale with different metamorphic grade was established.(3)Based on the three indices of rebound pressure,permeability damage rate and total flow loss,the types of permeability change during the process of gas pressure reduction were classified as ascending,rebounding and descending types.The coupling relationship between the types of permeability change and the gas permeability,the types and pore-fracture system were revealed.In terms of material composition,the effects of shale organic matter and mineral composition on the types of permeability change.Recognizing the type of permeability change of shale gas facies(CH₄)has a certain indication significance.As for the pore-fracture system,with the decrease of inlet pressure,micropore and fracture will have a significant negative effect on the permeability change,while pore and mesopore will increase the permeability due to gas slippage and matrix shrinkage effect.