Wellbore stability in naturally fractured formations featuring dual-porosity/single-permeability and finite radial fluid discharge

This work presents an analytical solution for an inclined wellbore drilled in a fractured formation displaying a high contrast between the diffusion rates in the fractures and in the porous matrix, with the latter treated as a storage phase and thus not participating in the fluid transport. This scenario may result from different mud cake building rates, and/or adverse wettability of the rock matrix with respect to the drilling fluid being used. Different from the models available in the literature, the fractures are characterized by a more realistic finite radial fluid discharge (namely, the fractures network hydraulically connected to the wellbore is of limited extent). The models output in terms of pore pressures, total stresses and effective stresses response has been compared with the results of a widely accepted dual-porosity/dual-permeability model. Major differences and asymptotic behaviors have been analyzed. Subsequently, the results of the model have been used to perform a comprehensive wellbore stability analysis. The effect of time dependency, fractures network length and rock properties on the instability of the borehole have been investigated, and upper and lower mud weight bounds have been determined using several failure criteria. This investigation shows that under the specified conditions where dual-porosity/single-permeability conditions arise, the proposed model provides with theoretically more conservative mud weights predictions.