POROELASTIC MODELS FOR FAULT REACTIVATION IN RESPONSE TO WASTEWATER INJECTION AND HYDROCARBON PRODUCTION (Invited Presentation)

Concurrent hydrocarbon production and injection of produced wastewater in stacked reservoirs as commonly conducted in unconventional resources exploitation potentially leads to seismic reactivation of nearby faults. Using three-dimensional, fully-coupled poroelastic finite-element simulations, we assessed the potential for fault reactivation for generic and site-specific injection-production scenarios to test how fault reactivation is controlled by the spatial distribution of production and injection wells relative to the faults, by the stacking order of production and injection layers, and by the permeability structure of the faulted reservoirs. Site-generic simulations display significant variation in Coulomb failure stress (CFS) with depth along the fault plane for these scenarios, reflecting differences in pore pressure distribution and associated poroelastic changes in normal and shear stress across the fault. Based on CFS trends with depth we find that 1.) concurrent production and injection reduces or increases the fault reactivation potential in the injection reservoir depending on the lateral position of the wellbores relative to the fault plane; 2.) the fault is most prone to reactivation with both wellbores in the hanging wall and injection into the upper reservoir, and least prone to reactivation for injection into the lower reservoir; 3.) among scenarios with production from the upper reservoir, production from the hanging wall favors fault stability; 4.) injection in the upper reservoir makes the fault more stable in the formation underlying the lower production reservoir; 5.) injection and production within the footwall instead of the hanging wall, with injection deeper than production, make the fault more stable within the upper basement. These simulations demonstrate that the three-dimensional arrangement of injection and production relative to mapped faults and basement provide significantly control on fault reactivation potential and associated seismic hazard. Ongoing site-specific simulations for the Dallas-Fort Worth area demonstrate significant limitations in the predictive quality of model simulations due to large uncertainties in hydromechanical properties of poorly characterized injection reservoirs.