Experimental Characterization and Pore-Scale Modeling of Iron Precipitation in Shale Reservoirs by Interacting with Hydraulic Fracturing Fluid

It has been reported that â 60% of total U.S. hydrocarbon production comes from shale reservoirs. Understanding of reactive transport is of fundamental importance to the application in subsurface systems of natural shales that have rich compositions of carbonate, clay, and sulfide, which have high reactivity with water. In this study, we focus on the interaction between pyrite (sulfide) and hydraulic fracturing fluid in shale to investigate the potential impact of iron precipitation on fluid transport. We first conducted the experiments with pyrite samples to calibrate the reaction rate constants for pyrite oxidation (at the pyrite surface) and Fe²⁺oxidation (in solution). The obtained reaction rate constants were utilized to establish the pore-scale numerical model to track these oxidation reactions. In other words, the reaction rate constants of pyrite surface oxidation and Fe²⁺oxidation were calibrated by matching the results of numerical simulations with the experimental measurements of ion concentrations. By doing so, we could also obtain confidence in our developed numerical simulator. In numerical simulation case 1, where the reactions of pyrite oxidation and Fe²⁺oxidation mainly occurred, the transport patterns in the systems were investigated based on the digital rock image model. In numerical simulation case 2, the level-set method was coupled with the reactive transport model to simulate iron(III) hydroxide precipitation on the pyrite surface. The precipitation patterns in the digital rock image model were investigated under different Damköhler numbers (Da_II). Under the larger Da_II, the precipitated iron(III) hydroxides had a longer dendritic shape, and the precipitation pattern was highly random. The quantified pore-scale parameters obtained from this study are expected to improve continuum-scale models to accurately predict the potential impact of the interaction between pyrite in shale and hydraulic fracturing fluid.