Numerical investigation of potential cernent failure along the wellbore and gas leak during hydraulic fracturing of shale gas reservoirs
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Copyright 2016 ARMA, American Rock Mechanics Association. We investigated wellbore stability during hydraulic fracturing using the Mohr-Coulomb failure model and an intrinsic permeability step function for failure status. In order to assess the risk of failure, we varied the values of bottom-hole pressure, permeability multipliers for our permeability step function, and the cohesion values between the well casing and the surrounding cement to represent different quality levels of the cementing operation. Additionally, we studied shear failure for a homogeneous and a heterogeneous three shale layer reservoir. From numerical results, higher bottom-hole pressures increased the fracture propagation speed because it implied higher geomechanical loading. Furthermore, a higher permeability multiplier leads to a larger failed zone because higher permeabilities pressurize the failed zone faster. Also, simulations showed that there is very little fracturing when the cement is of high quality. On the other hand, incomplete cementing and/or weak cement can cause significant shear failure and the evolution of long fractures along the vertical well, where they become a factor for potential contamination of aquifers. Moreover, simulations of heterogeneous shale reservoir showed how mechanical properties of the reservoir rock can help resist shear failure of the wellbore. Finally, a coupling strength sensitivity analysis of the steel-cement interface layer demonstrated that higher coupling strengths induce shear failure. Thus, high-quality cement, complete cementing, strong formation rock properties along the vertical well, and low poromechanical effects between fluid and geomechanics appears to be the strongest protection against shear failure of the wellbore cement and contamination hazards to water aquifers during hydraulic fracturing.
