Pressure Transient Analysis to Determine Anisotropic Fault Leakage Characteristics

Abstract

Deep saline aquifers have the large required capacity to dispose of or store fluids in the subsurface. The consequent overpressure may cause undesirable brine leakage through possible interlayer pathways. Faults can accommodate fluid leakage to shallower formations. Pressure transient analysis can be used for early detection of leakage through faults. This study developed an analytical model for pressure transient behavior in response to leakage through a fault to an overlying formation. A fault structure characteristically includes a core zone surrounded by damaged zones. In a departure from previous models and in order to honor the general fault configuration, we considered the anisotropic flow in three dimensions inside the fault zone. The solution was derived by a system of diffusivity equations for the reservoir layers as well as the fault zone in the Laplace–Fourier domain. The analytical solution was validated against the numerical simulation results. Applying the pressure derivative, we showed that the solution can differentiate between up-, across-, and along-fault fluid migration. The fault characterization method was provided based on the pressure derivative dependency on the fault conductivities, which illustrates the ability to identify up-fault leakage while differentiating it from partial communication across the fault. Pressure derivative curves were grouped based on the values of fault conductivities for fault leakage characterization. In addition, the effect of altered regions’ properties was investigated for the other side of the fault and the overlying formation. An example problem was presented to show the potential of the analytical approach and its application for fault leakage characterization.