Influence of fracturing fluids on shale matrix permeability and micro-mechanical properties after chemical interactions

Geochemical interactions between shale and hydraulic fracture fluids (HFFs), principally mineral dissolution and precipitation, can result in alteration of matrix permeability and tendency of proppant embedment, ultimately impacting hydrocarbon migration through the shale formation. This study aimed to quantify changes of core permeability and micromechanical properties after shale-HFF chemical interactions. Extent of chemical alteration of the solid phase was characterized by synchrotron X-ray fluorescence mapping, micro-computed tomography, and scanning electron microscopy. Post-reaction brine was evaluated for elemental concentrations. Both low-carbonate and high-carbonate shale exhibited carbonate dissolution, an iron oxidation zone, and barite precipitates partially infilling secondary porosity. These reactions were more obvious in the high-carbonate shale but extended further into the low-carbonate shale. Pulse-decay permeability measurements generally revealed increase in permeability after reactions, whereas hardness and modulus reduction were observed only in reaction zones of high carbonate shale. Inevitable heterogeneity of natural shale samples resulted in large deviations in hydromechanical measurements both on the core scale and on the microscale.