Semi-Analytical Model of Pressure Perturbations Induced by Fault Leakage in Multilayer System

Abstract

Lateral and vertical migration capacity of faults has been studied for applications related to fluid extraction and injection from and into the subsurface. Fluid exchange between formations connected by a fault can cause pressure perturbations in nonoperating formations. An analytical model was previously developed to evaluate the leakage rates through a fault in a multilayer system. However, the effect of the fault’s lateral resistance to flow was neglected and therefore the model was not applicable in evaluating the pressure changes caused by fault leakage. Pressure perturbations induced by fault leakage are important to determine the safety of injection operations near faults and/or in characterizing and remediating fault leakage. Here, the previous model on fault leakage to evaluate the pressure changes is extended by accounting for the lateral resistance to flow in all modeling components. As a result, the leakage rates at the two sides of the fault are no longer identical. Also, the model presented here allows for well perforation in any layer compared with the previous model where perforation must be in the bottom layer of the system. This modification is important to enable estimation of pressure changes in multilayer operation [e.g., injection in one or more aquifers while producing from other aquifer(s)]. Hence, the model can be used to determine basin-scale pressure changes considering multiple injection or production wells in multiple layers. The semianalytical solution presented here involves obtaining a set of coefficients to determine the pressure changes in Laplace and Fourier domains followed by numerical Fourier-Laplace transforms inversion. The model is applied to two example problems. In the first example, the model is verified against results of a two-layer system and compared to pressure perturbations in a three-layer system where the solution behavior is investigated. In the second example, the model is applied to investigate pressure management close to a critically stressed fault zone using relief wells, which is of interest in CO2 sequestration and/or waste disposal applications.