The Mechanism Of The Loading-Unloading Paths On Shale Fracture Conductivity

Shale fracture conductivity directly determines the effectiveness of the fracture network in shale reservoirs and the shale gas production.The gas well opening–shutting process causes stress changes and the horizontal-well staged fracturing process causes stress shadows to be formed.These stress loading–unloading paths substantially alter the fracture gap and the gas flow state in a fracture,impacting shale fracture conductivity.There have been many studies on fracture conductivity of sandstone and granite;however,research on fracture conductivity of shale is very limited,especially the mechanisms involved in the loading–unloading paths and their effects on fracture conductivity.In the present paper,shale specimens from the Chongqing Fuling area,were investigated with respect to the surface roughness of shale fractures;aperture change regulation with stress variation of shale supporting and non-supporting fractures under loading and unloading;flow patterns of gas and friction coefficients in shale nonsupporting fractures.The research achievements are as follows:(1)ased on elastic–plastic theory and accounting for loading–unloading paths,a hydraulic aperture model of shale non-supporting fractures was proposed.The results show that,as shale fracture surface roughness increases,more damage is caused to the hydraulic aperture by the loading–unloading paths.When the apparent effective stress exceeds the maximum effective stress in the loading history,the fracture undergoes plastic deformation;when the apparent effective stress is less than or equal to the maximum effective stress in the loading history,the fracture undergoes only elastic deformation.The proposed model can calculate the fracture hydraulic aperture under different loading–unloading paths.The calculated results were in good agreement with experimental data.(2)Based on the Reynolds number and relative roughness,a new model of friction coefficient was proposed,to reveal the nonlinear flow characteristics of methane gas in shale non-supporting fractures.With increasing effective stress and relative roughness,fluid pressure head loss increases and the critical Reynolds number between transition and turbulent flows decreases simultaneously.Compared with the loading process,the unloading process occurs at a lower critical Reynolds number.Compared with the commonly used Lomize friction model,the friction coefficient for Lomize model is overestimated under laminar flow and underestimated under turbulent flow conditions.The proposed friction model is similar to the pipe flow friction model and can describe laminar flow,transitional flow and turbulent flow,reflecting the real flow characteristics of methane gas in shale non-supporting fractures.(3)The variation rule that the opening of supporting fracture,the permeability vs stress under the loading and unloading path was revealed.Based on elastic–plastic theory and the non-associated flow rule,mechanical constitutive and permeability models were proposed for shale supporting fractures.These models consider loading–unloading paths and assume the proppant filling layer is a continuous medium.The supporting fracture permeability can be calculated under different loading–unloading paths using the proposed model.(4)The shale fracture flow model was imported into TOUGH-FLAC3 D simulation software.The influence of complex loading–unloading paths and nonlinear flow on shale fracture conductivity in shale gas reservoirs was studied.The results show that the total flow of shale gas decreases by 14% when only loading–unloading paths are considered;by 28% when only nonlinear flow is considered;and by 39% when the loading–unloading paths and nonlinear flow are both considered.