Research On Wellbore Integrity Failure Mechanism During Multistage Fracturing In Shale Gas Horizontal Wells

Shale gas development is of great significance to alleviate the burden on natural gas supply in China and adjust the energy structure.With the accelerating exploration and development process,shale gas is becoming a major growth point in China's oil industry.However,in the process of muti-stage hydraulic fracturing completion of shale gas horizontal wells,serious wellbore integrity problems have appeared,which has restricted the process of efficient shale gas development in China.It is mainly embodied in two aspects : Firstly,the problem of casing deformation is outstanding,espeacially during the muti-stage hydraulic fracturing,some of the blocks even reached 40%;Secondly,the sustained casing pressure issues become prominent,that is,the sealing failure of cement sheath,and some blocks have exceeded 75%.Wellbore integrity failure has seriously affected the fracturing effect,reduced single well production,threatened wellhead safety,and require urgent solution.Therefore,it is of great significance for clarifying the cause of complete failure of casing and cement sheath,and for proposing scientific and reasonable improvement measures to study the failure mechanism of shale gas horizontal well integrity during multi-stage fracturing.This paper mainly carried out the following research:Based on the anisotropy experiment of shale rock core mechanical parameters,the anisotropic characteristics of shale mechanics are revealed,a numerical model of shale reservoir casing-cement-formation combination has been established,the influence of anisotropy of the mechanical parameters on casing stress and cement stress is analyzed.Under the anisotropy/isomorphism condition,a comparative study about the influence of the inhomogeneity of the ground stress,the mechanical properties and the shape of the cement sheath on casing stress and cement stress is conducted,and the initial stress state of the casing and cement sheath before the multi-stage fracturing of the anisotropic shale reservoir is clearly revealed,which is the foundation of the mechanical analysis of the casing and cement sheath in the subsequent multi-stage fracturing process.Considering the influence of fracturing fluid displacement on the heat transfer coefficient of liquid-solid convection,the characteristics of wellbore temperature field during fracturing are analyzed.Considering the heat-fluid-solid coupling effect,a numerical model of the wellbore combination is established and the characteristics of casing transient stress is studied.At the same time,the research is conducted under the existence of annular bound water,lithological change interface,and multi-stage cyclic loading and other complex conditions.Based on the measured data of multi-finger caliper tool and microseism,the possibility of fault slipping was anylized and the characteristics of the shear deformation occurred during multistage fracturing were defined.The model of relationship between slip distance and casing deformation degree was eatablished,the influence of casing inner pressure,slip distance,casing thickness and mechanical properties of the cement sheath on casing deformation degree was studied.The new method of stage cementing technology was proposed to maintain the casing integrity in shale gas wells.Aiming at the characteristics of multiple increase and decrease of casing internal pressure in shale gas horizontal wells,the stress-strain experiment of cement stone under triaxial cyclic loading is carried out.The stress-strain parameters of cement stone damage under cyclic pressure are obtained.Based on the damage theory,the calculation model of the casing-cement interface plastic deformation is established,the cumulative plastic deformation at the casing-cement interface is analyzed,and the effects of elastic modulus,Poisson's ratio,cohesion and internal friction angle on gap generation and development is studied.Based on the cement mechanics experiments,the the compressive strength and tensile strength are acquired.Considering the dynamic changes wellbore temperature,a numerical model of wellbore combination under transient temperature-pressure coupling is established,and the dynamic changes of radial and tangential stress of the cement sheath are analyzed.The cement sheath integrity during the fracturing process are evaluated based on the Mohr-Coulomb criterion.And the influnces of wellbore internal pressure,fracturing fluid displacement,initial temperature,elastic modulus and Poisson's ratio on the crack of cement sheath body are studied.Studies have shown that reducing the elastic modulus can significantly improve the cement sheath integrity.Through the research,the main controlling factors of the wellbore integrity during the muti-stage fracturing completion of shale gas horizontal wells are clarified,and corresponding calculation and evaluation models are established.The research results have improved the wellbore integrity theory system of oil and gas wells,and could be a reference for the design and control of casing and cement sheath integrity in the mutistage fracturing process of shale gas wells.And it has great theoretical and engineering significance.