Geochemical modeling of Iron (Hydr)oxide scale formation during hydraulic fracturing operations

Laboratory experiments have shown that hydraulic fracturing fluids (HFF) can chemically interact with iron(Fe)-bearing minerals in shale, releasing Fe(II) which is then oxidized to form Fe(III)(hydr)oxide scale. The Fe(III)-(hydr)oxide scale can occlude pore space and reduce oil and gas production in wells. Our previous experimental studies show that Fe(III)-(hydr)oxides can precipitate even under acidic conditions where Fe(II) oxidation is unexpected. This is due to bitumen that is extracted from shale by chemical additives in HFF, which can aid in Fe(II) oxidation and lead to the precipitation of Fe(III)(hydr)oxides. In this numerical modeling study, we built two geochemical models to simulate our experimental observations. One model was developed to construct the rate law of Fe(II) oxidation in the presence of bitumen in a shale-free system, while the other model was used to understand the chemical reaction network and quantify the impact of bitumen on iron oxidation/precipitation during shale-HFF interactions. Our modeling results have shown that in both high- and low-carbonate shale systems, the presence of bitumen can increase the rate of iron oxidation/precipitation by more than an order of magnitude compared to bitumen-free systems. In addition, the availability of dissolved oxygen to pyrite grains is critical for Fe dynamics during shale-HFF interactions. The chemical reaction network obtained from this study, along with the bitumen-aided Fe(II) oxidation rate law, pave the way for future modeling studies on chemical reactions during hydraulic fracturing and their influence on formation damage and lost hydrocarbon production.