Influence of inherent mineral matrix on the geochemical reactions and pore structure evolution from shale oxidative dissolution

Abstract

Oxidant stimulation presents a promising avenue for enhancing fluid mobility in shale reservoirs with ultra-low permeability and porosity. However, the effect of diverse minerals, major components of shale, on geochemical reactions and associated microstructures still warrants further exploration. In this study, we employed acid demineralization followed by oxidant stimulation to explore the impact of carbonate and silicate minerals on shale oxidative dissolution, considering both geochemical reactions and pore structures. Combined with artificial neural network (ANN) modeling, the dominance of shale composition on pore structures variation was further elucidated. Results demonstrated effective removal of carbonate and silicates minerals following treatment with HCl and HF acids. Interestingly, while silicate minerals exhibited greater mass loss in response to all oxidant stimulations, carbonate minerals only contributed to mass loss induced by Na2S2O8. Integration of X-ray diffraction, Fourier transform infrared spectroscopy, water chemistry and total organic content analysis revealed mineral- and oxidant-dependent geochemical reactions within the stimulation process. These included preferential dissolution of K-feldspar, dedolomitization accompanied by different precipitates, and transformation from quartz to albite, among other conventional reactions. The alteration in mineral and organic matter composition also induced variation in shale nanopores structures, resulting in increased specific surface area, nanopore volume, and average pore diameter induced by acid treatment, while the effects of oxidants varied. To better understand the relationship between shale composition and micropore structure, an ANN model was established to quantify this underlying connection, emphasizing the significant role of carbonate, chlorite, pyrite, and organic matter in shale oxidative dissolution. Our study provides further evidence for the influence of minerals on shale oxidative dissolution, offering insights for the engineering application of oxidant stimulation in shale gas extraction.