Numerical analysis of mixed-mode rupture propagation of faults in reservoir-caprock system in CO2 storage

Abstract

Injection-induced seismicity and caprock integrity are among the most important concerns in CO2 storage operations. Understanding and minimizing fault/fracture reactivation in the first place, and rupture growth/propagation beyond its surface afterwards, are fundamental to achieve a successful deployment of geologic carbon storage projects. Rock fracture mechanics provides useful concepts to study the propagation of discontinuities such as pre-existing faults and fractures. In this paper, we aim at developing a methodology to investigate the rupture propagation likelihood of faults/fractures inside a reservoir and its surrounding (including the caprock) as a result of reservoir pressurization. In this methodology, mode I (tensile) and mode II (shear) stress intensity factors of a given fault/fracture are calculated based on Linear Elastic Fracture Mechanics. A fault/fracture can propagate either in mode I, mode II or a combination of both (also called mixed-mode) based on the comparison of the stress intensity factors and fracture toughness. The proposed methodology, which has been embedded into a hybrid Finite Element Method-Discrete Element Method in-house code called MDEM, has the capability to obtain the direction of mode I and mode II rupture in front of a fault/fracture tip. Two coefficients are defined as stress intensity paths (κ) for a fault/fracture, as the change of stress intensity factors for the two failure modes of a given discontinuity per unit pore pressure change of the reservoir after injection. Based on these stress intensity path coefficients, a relationship is given to calculate the threshold pressure buildup above which the two propagation modes may occur. We use the proposed methodology to investigate the rupture growth likelihood of faults in and around a closed reservoir due to its pressurization. Simulation results indicate that mode I failure is likely to occur inside the reservoir for faults with low dip angle in compressional stress regimes. However, the initiated mode I failure may not have the chance to grow upwards because the minimum principal is in the vertical direction and thus, the initiated rupture tends to rotate and grow horizontally. In contrast, mode I rupture is likely to occur in the caprock for faults with high dip angle in extensional stress regimes. The initiated rupture may grow upwards if the newly created fracture becomes hydraulically connected with the reservoir. We find that mode II rupture is not likely to occur in any of the investigated scenarios. Simulation results show that the coefficients of the stress intensity factors depend on the faults location, dipping angle, fault length, presence of other faults, reservoir aspect ratio and reservoir and caprock stiffness.