Investigation of possible wellbore cement failures during hydraulic fracturing operations
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2016. We model and assess the possibility of shear failure along the vertical well by using the Mohr-Coulomb failure model and employing a rigorous coupled flow-geomechanic analysis. To this end, we take various values of cohesion between the well casing and the surrounding cement to represent different quality levels of cementing operation (low cohesion corresponds to low-quality cement and/or incomplete cementing). The simulation results show that there is very little fracturing when the cement is of high quality. Conversely, incomplete cementing and/or weak cement can cause significant shear failure and evolution of long fractures/cracks along the vertical well. Specifically, low cohesion between the well and cemented areas can cause significant shear failure along the well, while high cohesion does not cause shear failure. The Biot and thermal dilation coefficients strongly affect shear failure along the well casing, and low Young's modulus causes fast failure propagation. Still, for the high quality of the cementing job, failure propagates very little.When the hydraulic fracturing pressure is high or when permeability increases significantly, low cohesion of the cement can cause fast propagation of shear failure and of the resulting fracture/crack, but a high-quality cement with no weak zones exhibits limited shear failure that is only concentrated near the bottom of the vertical part of the well. Thus, high-quality cement and complete cementing along the vertical well appears to be the strongest protection against shear failure of the wellbore cement and, consequently, against contamination hazards to drinking water aquifers during hydraulic fracturing operations.
