Geomechanical implications of dissolution of mineralized natural fractures in shale formations

Abstract

Organic shale reservoirs accommodate large amounts of hydrocarbons in extremely tight rock matrix. Slick water hydraulic fracturing and proppant placement is the most spread stimulating technology for this type of reservoirs. Some shale formations contain a fair amount of easily-dissolvable carbonates, which prompts shale acidizing as a potential method to increase reservoir permeability. Carbonates in shale formations can be present as a part of rock matrix or can be localized in natural fractures — so-called mineral veins. Previous studies investigated the effects of chemical dissolution within the matrix, implying that acidizing can etch the rock, open new conduits for flow, and increase fracture permeability after closure. This study rather focuses on the reservoir-scale geomechanical implications of chemical dissolution of mineral filling in natural fractures. We use a combination of elasto-plastic analytical methods, analogous thermo-elastic numerical solutions, and experimental results to elucidate the impact of veins dissolution on in-situ stresses. Analytical, numerical, and experimental results indicate that mineral dissolution of localized carbonate fractures leads to changes of local in-situ stresses. The direction of stress relaxation strongly depends on the orientation of mineralized fractures. Dissolution of near-vertical mineralized fractures contributes the most to decreases of effective horizontal stress and reduce stress shadow effects in multistage hydraulic fracture completions. Mineral dissolution, similar to pore pressure depletion or thermal cooling/heating, can increase stress anisotropy, which can reactivate critically-oriented natural fractures. In-situ stress chemical manipulation can be used advantageously to enlarge the stimulated reservoir volume.