Fracture Surface Evolution During Acidized Brine Injection in Calcareous Mudrocks

During hydraulic fracturing, the oxic hydraulic fracturing fluid physically and chemically alters the fracture surface and creates a "reaction-altered zone". Recent work has shown that most of the physicochemical changes occur on the shale fracture surface, and the depth of reaction penetration is small over the course of shut-in time. In this work, we investigate the physicochemical evolution of a calcite-rich fracture surface during acidized brine injection in the presence of applied compressive stress. A calcite-rich Wolfcamp shale sample is selected, and a smooth fracture is generated. An acidized equilibrated brine is then injected for 16 h, and the pressure change is measured. A series of experimental measurements are done before and after the flood to note the change in physicochemical properties of the fracture. High resolution computed tomography scanning is conducted to observe the fracture aperture growth, which shows an increase of ∼8.3 μm during the course of injection. The fracture topography, observed using a surface roughness analyzer, is shown to be smoother after the injection. The calcite dissolution signature, i.e., surface stripping of calcite, is observed by X-ray fluorescence, and mass spectrometry of the timer-series of the effluent also points in the same direction. We conclude that mineral dissolution is the primary mechanism through which the fracture aperture is growing. The weakening of the fracture surface, along with the applied compressive stresses, promotes erosion of the surface generating fines which reduce the fracture conductivity during the course of injection. In this work, we also highlight the importance of rock mineralogy on the fracture evolution mechanism and determine the thickness of the "reaction altered" zone.