  CO2 injection into a saline aquifer crossed by a tectonic fault is studied
with coupled fluid mechanics - geomechanics modeling. The simulation approach
is based on coupling of the MUFITS reservoir simulator and the FLAC3D
mechanical simulator via an in-house API (i.e., an algorithm for data transfer
between simulators). MUFITS simulates the non-isothermal multiphase flow of CO2
and brine in rock formation accounting for phase transitions and thermal
effects. The modeling workflow is sequential, so that hydrodynamical
simulations are carried out at a certain time interval, after which pressure,
temperature, and density distributions are passed to FLAC3D, which calculates
the equilibrium mechanical state. Computed deformations and stresses are
utilized to update the porosity and permeability fields for the subsequent
hydrodynamic modeling. In particular, we focus on the tectonic fault and its
behavior during CO2 injection. We distinguish the damage zone and core inside
the fault and derive the closure relations for their permeability alteration
analytically. The coupled approach developed here is applied to simulate CO2
injection into synthetic and realistic reservoir models. For the former one, we
study the effect of formation depth and presence of the tectonic stresses at
the initial mechanical state, while for the latter, we consider different
injection modes (bottomhole pressure). In each numerical experiment, we
describe the evolution of the fault permeability due to the slip along its
plane and the development of plastic deformations leading to the loss of
reservoir integrity and CO2 leakage. Sensitivity analysis of the coupled model
to realistic values of input parameters to assess the fault stability is
carried out.
