Pore-Scale Modeling of Fluid-Rock Chemical Interactions in Shale during Hydraulic Fracturing

During the hydraulic fracturing process in unconventional shale gas reservoirs, chemical interactions between the hydraulic fracturing fluid (HFF) and the shale rock could result in mineral precipitation and dissolution reactions, potentially influencing the gas transport by dissolving or clogging the fractures. The pore-scale distribution of the minerals, especially the highly reactive minerals, such as calcite, in the shale matrix can impact the structural evolution of the shale rocks. In the present study, a pore-scale reactive transport model is built to investigate the impact of the pore-scale distribution of calcite on the structural alteration of the shales. The alteration of the shales is caused by the barite precipitation and the dissolution of calcite and pyrite. The simulation results show that the calcite dissolution leads to a permeability enhancement. The permeability enhancement for the shales with coarser calcite grains is more pronounced than that for the shales with finer grains of calcite. The results also indicate that the extent of the permeability enhancement is even more noticeable if the HFF is injected with a higher velocity. The fluid chemistry analysis indicates that the fluid pH for the shale with the fine grains of calcite is higher than that of the shale with the coarse calcite grains and that the injection of the HFF with a higher flow rate leads to the lower pH values. The calcite dissolution observed in the simulations mainly occurs near the inlet. For the shale with the finer calcite grains, barite precipitation occurs mostly close to the inlet, but for the shale with coarser calcite grains, barite precipitation extends more into the domain. This penetration depth increases when the HFF is injected with a higher velocity. In addition to the effect of the calcite distribution, we also used the pore-scale model to study the effect of the calcite content on the structural evolution of the shales. The results from these simulations showed that a higher calcite content can result in higher pH values, higher permeabilities, and more barite precipitation in the domain.