Using geomechanical models to simulate induced seismicity: Application to CO₂ storage at In Salah, Algeria

We demonstrate a workflow to simulate seismicity generated by CO₂ injection into the In Salah field, Algeria. Seismic activity in hydrocarbon reservoirs is caused by stress changes on pre-existing fractures that lead to their reactivation. As inputs to our workflow, a history-matched reservoir flow simulation is used to model changes in pressure caused by injection; while a geomechanical model gives the stress state at each node of the flow model. The locations, lengths, and orientations of pre-existing fractures in the reservoir are modeled via a mass-spring solver, which restores the faulted, folded reservoir to its initial, undeformed conditions. This algorithm predicts the intensity and orientation of strain through the model, from which fracture sets can be generated. To simulate seismicity during CO₂ injection, we compute changes in effective stress caused by pore pressure changes, and map these stress changes into shear and normal stresses acting on the fractures. Where stresses exceed Mohr-Coulomb failure criteria, seismic events are predicted. We compare our modeled events with observed seismicity at In Salah, finding excellent agreement between model and observation in terms of event timing, event location, and event magnitude.