The injection of hydraulic fracturing fluids (HFF's) into shales can result in a variety of chemical reactions that affect hydrocarbon recovery during production. Previous work by our group indicated the presence of precipitates, despite use of anti-scaling agents. In addition, we found variability in experiments, with the dependent variables being the base carrier fluid composition and the time between simulated fluid mixing and injection. Additional experiments have been conducted on Marcellus shale core from the Marcellus Shale Energy and Environmental Laboratory (MSEEL) site in Morgantown, WV and outcrop samples using simulated HFF's formulated from regional averages. The rock specimens were exposed to control fluids, such as deionized water and HFF's for up to 5 days, with geochemical samples taken at regular intervals. Volumetric imaging of the rock specimens was completed using Computed Tomography (CT) scanning before and after exposure and reaction, with additional surface imaging via scanning electron microscopy (SEM). In this study, the objectives are to evaluate how mixing-to-injection times, HFF components, and rock composition affect the geochemical transport out of the system and fracture to matrix reactions.
The use of additives in hydraulic fracturing is nearly ubiquitous among all major unconventional basins. In general, the purpose of the additives is to enhance favorable reactions with the host rock, tailor HFF properties, keep rock fractures open, and promote gas migration while reducing unfavorable chemical reactions that cause scaling and fracture closure. Dissolution reactions can cause mass to export out of the system, which in turn stimulates further gas production by opening new pathways for migration out of the tight matrix. In contrast, mineral precipitation can cause fracture closer and wellbore clogging which reduces the overall system permeability to gas. However, some precipitation reactions can potentially cause positive stress in the fractures which enhances fracture migration and can also act as a proppant. Additives are chosen to address the aforementioned effects, but in general the formations are highly heterogeneous and the reactions not well-constrained due to this heterogeneity.