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Visualizing Depletion and Depletion-Induced Stress Changes in Unconventional Reservoirs

Abstract

Numerous questions surround stimulation and depletion in unconventional reservoirs with many important implications for optimizing production. In addition, depletion-induced stress changes have important implications for in-fill drilling and avoiding phenomenon such as hydraulic fracture growth into depleted areas and hydraulic fractures from in fill wells affecting pre-existing wells (the frac-hit or parent well/child well phenomenon). In this paper, we utilize a fully-coupled poroelastic model described by Jin and Zoback (Jour. Geophys. Res., 2017) to evaluate the pressure and stress changes associated with pore-scale flow from the low permeability matrix into a much more permeable discrete fracture network. These fractures represent pre-existing fractures stimulated in shear during multi-stage hydraulic fracturing as well as the hydraulic fractures themselves. Because of the marked permeability contrast, depletion is quite heterogeneous and can be visualized as halos adjacent to the more permeable shear fractures and hydraulic fractures. To examine the importance of depletion-induced stress changes and its effect on infill wells, we consider three cases representing different initial pore pressures and corresponding different amounts of horizontal stress anisotropy. For the base case we consider (moderate initial stress anisotropy, initial overpressure and depletion), the area in which depletion-related stress magnitudes change is limited to that surrounding the hydraulic fractures and there is essentially no change in the orientation of stress in the depleted region. In marked contrast to this, in a case in which there is significant initial overpressure, little horizontal stress anisotropy and significant depletion, the effect on the stress field is quite considerable. Two factors contribute to this – the superposition of relatively large poroelastic stress changes on a relatively isotropic stress field and the flipping of the directions of the directions of maximum and minimum horizontal principal stress because the poroelastic stress change is larger in the direction of initial minimum horizontal stress (that is, the component of stress acting perpendicular to the hydraulic fractures). Taken together, these calculations illustrate the importance of understanding the importance of pre-existing fractures and fracture networks on both initial stimulation and production, as well as depletion-induced stress changes affect infill drilling.