The reuse of produced water for hydraulic fracturing operations has both environmental and economic benefits. A demonstration scale study using a high-rate clarification process was conducted in the Delaware Basin to evaluate technical feasibility and cost-effectiveness of treating produced water to remove dissolved iron and hardness. Pilot studies were conducted in following three phases to prepare designer's water to match with multiple types of fracturing fluid.
Phase 1: Iron removal - near neutral pH treatment (oxidation using peroxide)
Phase 2: Iron removal and partial hardness removal - pH 9.5 treatment (partial softening using caustic hydroxide)
Phase 3: Complete removal of hardness ions - pH 11.5 treatment (complete softening using caustic soda and soda ash)
The demonstration study showed that a high-rate clarifier technology, capable of handling elevated levels of suspended solids, is suitable for reuse applications in the operating conditions of the Permian hydraulic fracturing. For iron removal during Phase 1 treatment, oxidation with peroxide is holistically more cost effective than the High pH method. This is primarily due to lower chemical and sludge handling costs. On-site dewatering of sludge allowed the solids to pass paint filter test, a requirement for sludge hauling to a certified landfill. Phase 2 treatment (at pH 9.5) removed 95% of magnesium in addition to near complete removal of dissolved iron. Therefore, Phase 2 treatment is expected to have much lower scaling tendency when completing well with pH 9.5 or higher pH crosslink fluid. No significant differences between Phase 1 and Phase 2 are expected for Slick Water completion fluids. While Phase 3 treatment can remove more than 99% of hardness ions, it is not practical to implement for Delaware produced water given the high costs of chemical usage and sludge disposal.
Damage testing was conducted with the raw produced water, Phase 1 water and Phase 2 water with various fracturing fluid systems. A produced water slickwater system was identified as causing the least amount of proppant pack damage in tests described in this study when used with Phase 1 water. Additionally, produced water minimizes rock softening as compared to fresh water systems. Maintaining formation hardness would lead to less proppant embedment. Less proppant embedment would enhance fracture conductivity in the proppant pack.