Why Mohr-circle analyses may underestimate the risk of fault reactivation in depleting reservoirs

Abstract

Fault reactivation may lead to induced seismicity, damage to wells and loss of containment of potentially harmful and economically valuable fluids or gas in the subsurface. For this reason, fault reactivation is assessed in hydrocarbon development projects, in particular when injecting water, gas, or steam, but also when disposing of (contaminated) water or CO2 in depleted oil or gas reservoirs. In this paper, we compare the results obtained by a Mohr-circle evaluation and two-dimensional Finite-Element (FE) analysis that are commonly used to assess the likelihood of fault reactivation. It is found that Mohr-circle evaluations provide a non-conservative estimate of the allowable reservoir pressure in cases where the reservoir is offset by a fault. In practice, this may comprise virtually all imageable faults, as the limited seismic resolution does not allow interpretation of faults with no or small throws. Furthermore, we demonstrate that the maximum (vertical) element size for capturing the onset of fault slip with sufficient accuracy is about 2% of the reservoir thickness. Finite-element models with larger vertical element size increasingly over-estimate the depletion pressure required to cause fault reactivation or won't predict fault reactivation at all, thereby under-estimating the hazard associated with fault reactivation. This paper concludes with recommendations for evaluating onset of fault slip using Mohr-circles, and two- and three-dimensional finite-element analysis.