Effects of Flow and Geomechanics Coupling in Faulted Reservoirs for CO2 Storage

Abstract

Abstract Accurately reproducing the coupling of fluid flow in porous medium and rock mechanics is crucial for the modelling of CO2 geological storage in order to properly evaluate and prevent the risk of inducing fault instability during injection operations. As an alternative to using monolithic flow/mechanical suites, the process can be modelled by linking individual codes, i.e., a reservoir simulator for flow and a geomechanical package to account for fluid induced stress change, which tackle the two problems sequentially. We developed a flexible numerical framework where different coupling logics can be selected, i.e., one-way coupling, two-way coupling, and explicit coupling, which are characterized by different levels of accuracy and computational costs. A multirate two-way coupling algorithm, which allows the flow and mechanical simulators to exchange information periodically rather than at every time step, is also available to reduce the computational cost of two-way coupled simulations. In this work, we employ this coupling infrastructure to perform numerical experiments aimed at defining whether sequential iterative coupling is strictly needed or not, and which less expensive logic can be used in case to attain a similar solution accuracy. First, a synthetic test case is used to illustrate the onset of fault instability during CO2 injection operations for different sets of coupling parameters (type and frequency), rock properties and fault permeability. It is thus possible to evaluate, for a reasonable range of coupling strength, which depends on fluids and rock properties, the optimal level of coupling. Results are strongly influenced by the coupling strength and two-way iterative coupling should be selected for tightly coupled systems to accurately reproduce the fault behavior. For loosely coupled system instead, the one-way approach should be the preferred choice due its lower computational cost. Later, we consider CO2 injection into a realistic formation, and we analyze the impact of the coupling frequency on the computational performance. We show that for complex cases there is no one-to-one correspondence between the reduction in the number of coupling iterations and the reduction in computational time for increasing coupling period.