Evaluation Of Shale Compressibility And Geological Control Of Fracturing Fracture Propagation In The Longmaxi Formation,South Sichuan Basin

Shale reservoirs have the characteristics of low porosity and low permeability,and effective productivity can be obtained only with the help of fracturing technology.The stimulation effect largely depends on the fracture shape after fracturing,and the generation and expansion of fracturing fractures are affected by reservoir geological factors.Based on this,this thesis takes the shale of Longmaxi formation of Lower Silurian in Luzhou block as the research object,carries out experiments such as mineral composition,porosity and permeability characteristics and rock mechanical parameters,selects analytic hierarchy process to evaluate the compressibility of shale,and uses ABAQUS numerical simulation software to study the propagation law and geological control of fracturing fractures,and obtains the following understanding:（1）The shale of Longmaxi formation contains quartz,feldspar,calcite,dolomite,clay minerals and pyrite.Shale types are mainly mixed shale and silicon/mud mixed shale.The internal pores of the rock are relatively developed,mainly including organic pores,intragranular pores,intergranular pores and microfractures.The porosity is between 1.85%～3.02%and the permeability is between 12.80%×10^-3m D～17.65×10^-3m D.（2）The tensile strength of shale is 0.95～5.05 MPa,the elastic modulus is19.903～21.279 GPa,the Poisson’s ratio is 0.21～0.28,the uniaxial compressive strength is 90.83 MPa,the shear strength is 26.57 MPa,the cohesion is 12.00 MPa and the internal friction angle is 29.34°.（3）The content of brittle minerals is 54%～69%,the brittleness index is0.339～0.484,the horizontal stress difference coefficient is 0.21～0.25,the fracture development is medium,the value is 0.7,and the compressibility index is0.425～0.539,belonging to medium compressibility.（4）Combined with experiments and simulations,it is found that there are three splitting modes:tensile failure mode,comprehensive shear tensile failure mode of matrix and bedding plane and matrix tensile failure mode.Hydraulic crack propagation can be divided into four stages:initial fracture occurrence,rapid crack propagation,stable crack propagation and pump shutdown and pressure relief.（5）There is a negative correlation between elastic modulus and hydraulic fracture width,and the shale fracture pressure fluctuates without obvious law;Poisson’s ratio has a weak negative correlation with hydraulic fracture width,and the fracture pressure first decreases and then increases;The maximum horizontal principal stress has a weak positive correlation with the crack width and a weak negative correlation with the fracture pressure.（6）In the case of natural fracture development,when the horizontal stress difference is large,the hydraulic fracture penetrates the natural fracture and the direction remains unchanged,and the fracture width increases.On the contrary,the hydraulic fracture and natural fracture communicate and expand together,and the included angle increases,and the opening degree of natural fracture increases.