Casing Load And Strength Analysis Of Horizontal Shale Gas Wells Under Fracturing Condition

With the development and application of horizontal wells and fracturing technology,the problem of casing failure caused by fracturing during shale gas exploitation is also appearing.The main causes of casing failure during fracturing in shale gas horizontal wells are the casing load change,cementing quality problem and fracture-induced formation slippage.Starting from the above three aspects,this paper studies the casing load and strength of shale gas horizontal wells under fracturing conditions.The main research contents are as follows:Considering the variation of casing pressure and temperature under the fracturing condition,the calculation method of casing load and strength theory applicable to this condition are studied,and the influence of temperature change on the axial stress and compressive strength of casing is analyzed.Considering the actual cement sheath configuration,using finite element analysis software to establish two-dimensional model,simulated the casing stress distribution in the incomplete cementing section in horizontal wells,analyzed the influence of the location,thickness and angle of the cement sheath gaps on the casing stress.Using the theory of material mechanics and fluid thermodynamics,researched the impact of the pressure reduction of bound water on the safety of casing strength in the fracturing process.Considering the physical and mechanical properties of shale reservoir,analyzed the mechanism of the shale formation slippage due to the fracturing operation.Using finite element analysis software to establish the three-dimensional model,the stress and deformation of casing under different slip distance,slip surface angle and elastic modulus of the cement sheath were simulated and analyzed.Taking mud shale as sample,the compression strength of the mud shale with different water contents under different confining pressures is tested by using the electro-hydraulic servo triaxial test system,and the variation rule of the strength of the mud shale under different water contents is analyzed.Through the above research and analysis,it is found that: increased casing pressure and lower wellbore temperature during fracturing will reduce the casing safety.The incomplete cement sheath affects the strength and safety of the casing,and the position,thickness and angle of the gaps have different effects on the stress of the casing.Fracturing construction is closely related to shale formation slippage,which seriously threatens the safety of casing.Under certain conditions,a small amount of formation slip will lead to casing deformation and damage.The change of water content affects the physical and mechanical properties of shale,the increase of water content will lead to the decrease of the mud shale's brittleness and strength.The research results are of certain reference value to the future research on casing failure in horizontal shale gas wells,casing design and wellbore integrity control during fracturing.