Lei Jin

Depletion-induced faulting has been documented in a number of hydrocarbon reservoirs. This type of faulting has mostly been attributed to poroelastic effects: in-situ horizontal stresses are coupled with a pore pressure change according to a certain coupling coefficient (known as the stress path), which is generally less than 1. For faults with certain orientations, if the stress path is sufficiently high, the shear stress and effective normal stress resolved on the fault increase in such a manner that the fault is brought towards the shear failure line. An underlying assumption associated with this mechanism is that homogeneous pore pressure depletion occurs on both sides of the fault.
This study addresses an additional mechanism for depletion-induced faulting in cases where the pore pressure reduction is bounded by a hydraulically impermeable fault. Unbalanced pore pressure changes on the two sides of the fault, in conjunction with the poroelastic response, cause redistribution of the stress state. Two key assumptions are made: (1) pore pressure depletion is homogeneous within the reservoir on one side of the impermeable fault, and (2) the overburden stress and shear stresses are decoupled from pore pressure, while the two horizontal principal stresses are coupled with pore pressure by their respective stress paths (we show that the poroelastic coupling effect is anisotropic). Given a fault that is arbitrarily oriented with respect to the original stress field, we derive a generalized 3D analytical solution for the new state of stress after depletion. We then quantify the change in magnitude and rotation of the three principal stresses. Finally, we compare the corresponding Coulomb Failure Functions and Mohr Circles before and after depletion. For demonstration purposes, we determine the stress path tensor using poroelastic plane strain solutions in conjunction with frictional equilibrium for three different faulting regimes. Our hypothetical case studies show that, for bounded reservoirs, depletion-induced principal stress rotation and magnitude changes have a significant impact on fault stability, and are a complex function of fault orientation, the original in-situ stress state and pore pressure, the degree of depletion, and the degree of poroelastic coupling.