Depletion-Induced Poroelastic Stress Changes in Naturally Fractured Unconventional Reservoirs and Implications for Hydraulic Fracture Propagation

Abstract

Numerous questions surround stimulation and depletion in unconventional reservoirs with many important implications. Understanding depletion-induced stress changes is critical for designing 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 fracture-poro-mechanical computational model described by Jin & Zoback (https://doi.org/10.1002/2017JB014892) 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 natural fractures stimulated in shear during hydraulic fracturing as well as the hydraulic fractures themselves. Because of the marked permeability contrast, depletion is rather heterogeneous and can be visualized as halos adjacent to the more permeable shear fractures and hydraulic fractures. While this might be expected, the calculations are helpful in understanding the extent of depletion and limitations of utilizing the concept of a stimulated reservoir volume to predicting production. To examine the evolutions and characteristics of depletion-induced stress changes, we consider three cases representing different initial pore pressure, horizontal stress anisotropy and pressure drawdown. We show that while the total normal stress decreases overall due to depletion as expected, the changes are anisotropic due to the pressure gradient oriented predominantly perpendicular to the well; the changes are also highly heterogeneous due the presence of fractures and unexpected local increases among fractures can occur. Additionally, newly induced shear stress also develops with heterogeneous distributions. These changes jointly produce complex magnitude variations and rotational patterns of the horizontal principal stresses. For the base case we consider (high initial stress anisotropy, low initial overpressure and insignificant depletion), rotations occur mostly surrounding the fractures and the degree of rotation is mild. In marked contrast to this, in a case in which there is high initial overpressure, little horizontal stress anisotropy and significant depletion, rotations become less dependent on fractures and is prominent throughout the domain to a degree such that the directions of the two horizontal principal stresses are essentially flipped. Taken together, these calculations help illustrate how depletion-induced stress changes can affect problems like in-fill drilling and re-stimulation and therefore provide insights into better drilling and completion designs.