Digital rock analysis to estimate stress-sensitive rock permeabilities

Abstract

Reservoir deformations due to stress changes may have a substantial impact on the effective flow properties of reservoir rocks, which may potentially impact productivity and injectivity performance. Absolute permeability has been a primary factor in measuring hydraulic conductivity of porous materials. In order to investigate the impact of the granular structure and the extent of intergranular cementation on stress-dependent absolute permeability, we present a workflow that utilizes limited knowledge about the rock sample in the form of micro-Computed Tomography images to simulate stress-induced deformations. This analysis is then followed by Lattice Boltzmann Methods to simulate fluid flow in the deformed medium. While previous works using digital rocks ignored the impact of intergranular cements, we should consider the impact of these cements on the stress sensitivity of absolute permeability of the rock. It is notable that changes in the rock properties like the nature of stress would be a continuous change, so measurements at few stress points may not be enough to describe the overall response in any arbitrary loadings. Despite empirical formulae that are limited to few measurements, we proposed a method based on the equation of states to estimate rock permeability through three independent strain invariants. Digital rock physics, finite element analysis, and Lattice Boltzmann Methods were integrated to obtain a quick estimation of the rock permeability using micro-CT images without the need for repetitive laboratory experiments. Our results show that absolute permeability decreases significantly with increasing first and third strain invariants while it slightly increases with the second strain invariant. The stress sensitivity of absolute permeability was found to be negatively correlated with Young’s modulus of the intergranular cement. We observed that incorporating more intergranular cement will make the absolute permeability more sensitive to stress changes.