Research On The Weakening Mechanism Of The Soaked Shale And Corresponding Permeability Evolution

The detection and production of shale gas in China are increasing year by year,but the recovery is still low and obviously lower than that in North America.Improving horizontal drilling technology and hydraulic fracturing technology which are used in exploiting shale gas,ensuring the stability of shale gas well wall,enhancing the permeability of shale reservoir,and realizing the steady increase of shale gas production are key issues to optimize the energy structure and alleviate the contradiction between energy supply and demand in China.In the process of drilling and hydraulic fracturing,the surrounding rock of shaft wall is immersed in drilling fluid for a long time,and in the process of repeated pumping of drilling fluid,the surrounding rock of shaft wall will undergo cyclic loading and unloading,which cause nonlinea change of the physical parameters and mechanical characteristic of shale,menwhile the seepage characteristics of shale changes.It affects the wellbore stability and subsequent fracturing effect in shale gas development.These paper taken the reservoir shale as the research object and analyzed the physical properties of the shale,then through the shale triaxial mechanics test,and the cyclic loading and unloading seepage test to analyze the mechanics weakening characteristic and the permeability characteristic of shale which is hydrated for different time.Based on the case study,the hydraulic evolution mechanism of hydrated shale is revealed.The main research of this paper is as follows:(1)The physical properties of Longmaxi formation shale collected in the field are analyzed,and the contents of brittle minerals and viscous minerals are obtained based on XRD diffraction pattern.The mechanical tests of bedding shale with different hydration days were carried out,and the mechanical properties of shale under different stresses were analyzed.The results show that the stress-strain curves of different bedding shale are similar and show hard brittleness.The peak stress decreases significantly with the increase of hydration days,and the slope of stress-strain curve decreases.The compressive strength of vertical bedding shale is greater than that of parallel bedding.Considering different hydration days,the compressive strength of bedding shale decreases,and its physical and mechanical parameters,such as elastic mold,the internal friction angle and cohesion decreased,but the Poisson's ratio increased slightly.With the increase of hydration days,the failure form of vertical bedding shale is basically unchanged,while the fracture network formed by parallel bedding shale failure is becoming more and more complex.(2)The permeability evolution curves of hydrated shale with different hydration days were obtained by carrying out the permeability test of hydrated shale during one loading process.The results show that the initial permeability of shale increases gradually with the increase of hydration time,mainly due to the change of micropores and cracks in rock samples by hydration,and the increase of seepage channels.Before the yield stage,the permeability change is not obvious,but after the yield stage,the permeability increases gradually.Based on the seepage-damage theory,the variation of shale permeability characteristics in the process of full stress-strain is analyzed,the evolution model of shale permeability expressed by damage variables is established,and the permeability evolution machine of load-induced hydrated shale damage is revealed.Reason.(3)Based on the analysis of permeability characteristics of hydrated shale under single loading,the permeability test of hydrated shale during cyclic loading and unloading is carried out,and the permeability damage and recovery characteristics of hydrated shale during loading and unloading are analyzed.The results show that the initial loading permeability decreases exponential function with the increase of confining pressure,and the permeability of shale under reloading shows a power function relationship with the confining pressure,while the unloading permeability increases with the decrease of confining pressure.The more hydration days,the greater the damage rate of shale permeability in the loading stage,and the smaller the recovery rate in the unloading stage.In the process of cyclic loading and unloading,the permeability decreases gradually with the increase of loading and unloading times.The loading permeability of shale under load is greater than that under unloading.(4)Based on the above test and theoretical analysis,the wellbore stability evaluation in the drilling process of bedding shale reservoir is carried out.Under the action of the initial stress field,the stress of the surrounding rock of the shaft wall is redistributed,which leads to the change of bedding aperture.The aperture of bedding which situated on the upper and lower sides of the shaft wall increased,while aperture of bedding which situated on the middle of the shaft wall decreased.Hydration reduces the mechanical strength and increases the permeability of the surrounding rock,and the rock near the bedding surface on the upper and lower sides of the shaft wall is the most prone to deformation and failure.With the increase of hydration time,plastic failure will occur to the surrounding rock of the left and right sides of the shaft wall,and shear failure will occur to the bedding surface which located the upper and lower sides of the shaft wall,resulting in the instability of the shaft wall.