Cross-Scale Geomechanics of Wellbores in Depleted Reservoirs

Uniaxial models are commonly used to describe the change in magnitude of horizontal principal stresses in depleted reservoirs and consequently to analyze the stability of infill wellbores. The accuracy of these models is assessed in this paper through a three-dimensional axisymmetric stress solution for a depleted disk-shaped reservoir embedded within an infinite and homogenous model of the subsurface rock. The solution is used to obtain the limits of mud weight window that would not cause wellbore breakout or fracturing, as well as the safest trajectory of wellbores that are drilled in general vicinity of depleted reservoirs. Inside and outside of a reservoir subjected to different faulting regimes are considered for wellbore stress analysis. Results indicate that despite their common use, the uniaxial models may return significantly erroneous predictions of the change in the in-situ stress state around and about a depleted reservoir. In particular, the stress change is found to strongly depend on the geological azimuth and radial distance from the reservoir center point. Consequently, the uniaxial models may fall short in accurate prediction of the tensile or shear failure gradients of infill wells. These errors would be more substantial for wells that are closer to the reservoir boundaries.