Geomechanical Coupled Poromechanics Solutions While Drilling in Naturally Fractured Shale Formations with Field Case Applications

Abstract

Abstract Numerous wellbore instability problems have been reported when drilling through chemically active naturally fractured shale. It is usually accepted that wellbore collapses are caused by excessive effective stress concentrations at or near the borehole wall, and excessive mud losses are due to fracturing the rock formation. Since both the pore pressure differential and mud/shale chemical activity modify the effective stresses in the wellbore wall, the cures for such intricate instabilities are therefore in a wellbore solution that accounts for fractures, intact matrix mechanics, and the balance of mud density and salinity. In this work, the effects on collapse and fracture gradient window due to mud chemistry and densities are evaluated through the salinity, membrane efficiency, and the existing natural fracture network. The coupled poromechanics analytical solutions of fracture and matrix dual pressures and stresses predict large variations in the collapse and fracture gradient when compared to wellbore stability in non-fractured shale formations. The physical and mathematical solutions follow the coupled poroelasticity Biot theory through modeling fracture and matrix as dual-porosity and dual-permeability porous media. The solution is helpful in the prediction and visualization of time-dependent alteration of near-wellbore effective stress concentration and the effects on the mud-weight window when compared to non-fractured shale. Specifically, the solution is applied to assess wellbore stability for a simulated downhole drilling condition. Analyses that neglect the naturally fractured nature of the shale fall short in simulating wellbore instability since they predicted a narrower mud-weight window for the drilling operation, while the chemical and mud salinity effects can be utilized as a stabilizing factor.