Abstract
Fracture propagation in shale under in situ conditions is a critical but poorly understood mechanical pro-cess in hydraulic fracturing for deep shale gas reservoirs.To address this,hydraulic fracturing experi-ments were conducted on hollow double-wing crack specimens of the Longmaxi shale under conditions simulating the ground surface(confining pressure σcp=0,room temperature(Tr))and at depths of 1600 m(σcp=40 MPa,Ti=70 ℃)and 3300 m(σcp=80 MPa,high temperature Ti=110 ℃)in the study area.High in situ stress was found to significantly increase fracture toughness through constrained microcrack-ing and particle frictional bridging mechanisms.Increasing the temperature enhances rather than weak-ens the fracture resistance because it increases the grain debonding length,which dissipates more plastic energy and enlarges grains to close microdefects and generate compressive stress to inhibit microcrack-ing.Interestingly,the fracture toughness anisotropy in the shale was found to be nearly constant across burial depths,despite reported variations with increasing confining pressure.Heated water was not found to be as important as the in situ environment in influencing shale fracture.These findings empha-size the need to test the fracture toughness of deep shales under coupled in situ stress and temperature conditions rather than focusing on either in situ stress or temperature alone.
基金项目
National Natural Science Foundation of China(No.12172240)
NETL
NSTL
万方数据