Shale Acid Fracturing: Geomechanical Effects and Fracture Propagation

Organic-rich shale reservoirs contain substantial amounts of hydrocarbons. Because of extremely low matrix permeability, they require special treatments such a slick-water hydraulic fracturing. Many shale reservoirs accommodate a significant amount of carbonates, minerals that can be easily dissolved in acid solutions suggesting acid hydraulic fracturing as an enhanced-recovery stimulation technology. Calcite and dolomite in shales can be either embedded into the rock matrix or deposited and localized within natural fractures (so-called veins). In this study we focus on: the effect of carbonate veins dissolution on post-fracturing stresses and the effect of mineral dissolution at the tip on fracture propagation. Reactive transport and mechanics are solved using a FEM-based formulation. Fracture propagation is modeled using the phase-field approach. We propose to link the phase-field variable that usually represents rock damage due to creation of fractures also as an indicator of the damage related to mineral dissolution. Experimental evidence is used to validate developed models. Results indicate that the phase-field variable acts as a diffusive zone and therefore can be assimilated as a mineral dissolution front in predicting fracture propagation. After acid leak-off, the dissolution of mineralized natural fractures can result in stress relaxation and potentially to shear failure within the reservoir. Overall, the results show that combining mineral dissolution and hydraulic fracturing can off er new approaches to hydraulic fracturing.