Precipitation of scale minerals such as barite and calcite may clog fractures and pores, leading to reduction in gas production.  How, when, and where scaling occurs, particularly barite, during hydraulic fracturing is not well understood.  In this study, fractured Marcellus shale cores were reacted with hydraulic fracturing fluid (HFF) in a flow-through system for 4 days with fast flow rate to mimic the injection period; and then for 21 days with slow or no flow to mimic the shut-in period. Hydraulic fracturing chemicals used in the study were consistent with those in previous work (Vankeuren et al, 2017); with the exception that Monongahela River water was used in this study as the base fluid. Salts were added to represent species in produced water.  SEM and CT scans were conducted on the cores before and after reaction to study precipitate morphology and locations. Barite colloids formed when mixing the high-sulfate-containing river water and Ba-containing produced water.  After injection of the fluid, barite settled and continued to grow inside the shale fracture. More and larger barite precipitates formed in the fracture during injection period. In contrast to the shut-in period, most proppant were flushed away with the fast flow during injection period even with confining pressure on the cores. Aqueous analysis of Fe and sulfate content in influent and effluent is indicative of pyrite oxidative dissolution from the shale cores; with more Fe and sulfate being released during shut-in period. Cross area thin sections underwent micro-XRF and micro-XANES analysis at SSRL for Fe and S species to investigate pyrite oxidation dissolution beneath the fracture surface. Barite formed 10~50 µm coating on regional fracture surface, beneath which Fe(II) and pyrite species remained almost intact; whereas some of the matrix about 100 µm beneath the fracture surface without barite coating contains Fe(III) species, indicating fluid imbibition into shale matrix and pyrite oxidative dissolution. Results from this study will provide insights the impacts of scaling on fracture-to-core geochemical reactions during hydraulic fracturing in shale.

Experimental study of barite scaling in Marcellus shale during a simulated injection and shut-in period of hydraulic fracturing

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.