Hydro-mechanical-chemical effects on permeability evolution of fractures in Longmaxi Shale

Hydraulic fracturing is an essential technology to stimulate shale reservoirs by creating massive fluid-conductive channels to improve access to reservoir. Recently, supercritical carbon dioxide (Sc-CO₂) has been considered as an alternative fracturing fluid to water. Previous Sc-CO₂ fracturing experiments suggest that, in the short-term during stimulation, Sc-CO₂ may create rougher fracture surfaces and more complex fracture networks due to its low viscosity, high diffusivity and low interfacial tension. However, post-stimulation, the remnant CO₂ may react with native water in the shale to create carbonic acid. This in turn may impact long-term permeability evolution in the reservoir. We report experimental observations of permeation of carbonic acid of varied acidic potential (pH) to investigate hydro-mechanical-chemical effects on fracture permeability evolution under various stress states. Surface profilometry and SEM-EDS are employed to quantify the evolution in both roughness on, and chemical constituents within, the fracture surface. Results indicate that, after 12-hours of fluid flow, fracture apertures reduce by ~18.76% with distilled water (pH=7.0), ~9.80% at pH=6.0 and increase by between ~6.30% (pH=5.0) and ~23.64% (pH=4.0) as acidity is further increased. The evolution of roughness and transformation of chemical elements on the fracture surface are in accordance with the evolution of permeability. The experimental observations imply that, CO₂-rich aqueous fluids have significant impact on the evolution of fracture permeability and may influence (and increase) shale gas production. However, the observed response reflects the impact of both the removal of bridging asperities (low acidity) and in enhancing free-face dissolution (high acidity) in a complex and enigmatic fashion.