INVESTIGATING CONTROLS ON MINERAL PRECIPITATION IN HYDRAULICALLY FRACTURED WELLS

During hydraulic fracturing operations, mineral precipitation in tight shale gas wells may reduce late-stage productivity. This is because mineral growth can partially or fully occlude pore throats and micro fractures. In turn, this may reduce the rate at which gas can disperse from the formation. Sulfate and iron oxide minerals have both been shown to precipitate and block flow-paths in shale hydraulic fracturing fluid (HFF) reactions, conducted under proxy reservoir conditions. The system is chemically dynamic, and mineral precipitation is influenced by many factors including the chemistry of the fracturing fluid, mineral composition of the rock, and microbial reactions. The use of ammonium persulfate as an additive in hydraulic fracturing fluid and the amount of carbonate minerals present in the shale formation have been shown to play a key role in controlling mineral precipitation reactions.

Ammonium persulfate is a strong oxidizing agent that is often included in fracturing fluid as a breaker. At reservoir conditions, persulfate breaker can lead to oxidation of pyrite and secondary precipitation of iron oxides and barite. Carbonate minerals act as a buffer for the system’s pH, and their dissolution can increase Ba in solution both directly or through cation exchange with clays. This could promote precipitation of barite and the paired dissolution of pyrite and precipitation of iron oxides, which is more favorable at higher pH’s. Previous studies conducted in our lab and elsewhere have revealed interesting relationships between persulfate breakers, carbonates and shale-HFF reactions. However, the true impact of these two variables on mineral precipitation reactions is still unknown. This is because several factors such as clay content, adsorption of ions in organic matter, and formation of organo-metallic complexes can influence these precipitation reactions. In this study, we investigate the specific effects of ammonium persulfate breakers and carbonate minerals on mineral precipitation by conducting a series of experiments with varying concentrations of these two variables while keeping all other factors identical. This study will help operators design better HFF composition to minimize the impact of mineral precipitation on gas productivity.