Analytical Model to Detect Fault Permeability Alteration Induced by Fault Reactivation in Compartmentalized Reservoirs

Underground fluid injection induces changes in in situ stress condition of the target formation and local faults that can potentially lead to fault reactivation, which may result in the leakage of injected and/or native fluids into neighboring formations. In this paper, we introduce an analytical method to detect fault reactivation caused by fluid injection into deep faulted aquifers considering across‐fault leakage. The fundamental assumption made in our model is that fault permeability will be altered upon fault slip. Therefore, we model fault reactivation as a sudden change in fault permeability at the onset of fault slip. The fault is modeled as a linear interface between two permeable formations with equal rock and fluid properties. The governing equations are coupled through the fault interface and are solved using the Laplace‐Fourier integral transform technique. Based on the analytical solution, we find the characteristic bottomhole pressure and pressure derivative responses that enable detecting fault reactivation using diagnostic plots. We observe that pressure derivative undergoes a rapid change at the onset of fault slip followed by a late‐time trend to attain a new equilibrium governed by the altered fault permeability. Furthermore, we discuss the evolution of the across‐fault leakage rate upon and after fault slippage. Results from this study are presented in the form diagnostic plots and type curves that may be used for reservoir and fault characterization purposes. Plain Language Summary The purpose of this study is to derive an analytical model that uses pressure data from an injector well to determine whether a fault has been reactivated due to fluid injection, even if no seismicity has been felt at all. It is well documented that nonconductive/slightly conductive faults suddenly allow fluids to migrate upon fault slippage; hence, we propose a mathematical model in which fault permeability suddenly changes at the onset of fault reactivation. We consider that fluid can only migrate across the fault. We solve the mathematical problem using an integral transformation technique. The general solution allows us to infer how pressure at the well would respond to a sudden change in fault permeability; we examine this behavior with pressure derivative too. We replicate this procedure for various values of altered fault permeability, and onset times and results are compiled in the form of diagnostic plots (to diagnose whether fault reactivation had