Optimizing Metal-Removal Processes for Produced Water With Electrocoagulation

Abstract

gas basin. For example, recycled produced water is less than 10% of the total water used to drill and fracture in the Barnett, Fayetteville, and Haynesville shale plays. However, the fraction of water recycled is significantly higher in the Marcellus play-greater than 90% of the total water used (Mantell 2011). Although treatment methods have been developed to recycle produced water for subsequent fracturing operations, widespread adoption of these methods is often limited by costs. Important treatment aspects for fracturing-water reuse include particle removal, reduction of scale-forming metals, and disinfection. Removal of total dissolved solids (TDS) is expensive and therefore avoided, if possible. Fracturing fluids have been developed that are compatible with high TDS concentrations, but the other objectives (solids reduction, scale control, and bactericide) almost always need to be satisfied. The focus of this study was to examine the metal-removal processes associated with reducing scaling potential by use of laboratory-scale data and chemical-equilibrium modeling, with the goal of optimizing chemical use and minimizing cost. Softening. Inorganic elements such as calcium, barium, magnesium, and strontium can cause hardness/scaling issues or failure in fracturing-fluid development. While there are proven methods for the removal of some of these contaminants, data on precipitate removal at high pH for produced water are lacking (Parks and Edwards 2006). Precipitative softening, one of the oldest and most common methods of divalent-cation removal (Lane and Duff 1954; Harden and Hull 1957; Liang et al. 1980; Owen and Humenick 1985), is often less expensive than membrane processes, and it can achieve multiple objectives, including more than 90% hardness removal. Calcium and magnesium are removed from water as calcium carbonate and magnesium hydroxide; strontium and barium are removed as the carbonate and sulfate, respectively. Electrocoagulation. Electrocoagulation (EC) is an electrochemical method of treating polluted water whereby an electrical current induces sacrificial anode corrosion, releasing trivalent iron and aluminum ions. As in chemical coagulation, the iron and aluminum ions hydrolyze to form metal hydroxides that can charge, neutralize, and aggregate with solution particles to form flocculants that can be removed with a range of solid/liquid-separation processes. Accompanying electrolytic reactions evolve gas (usually as hydrogen bubbles) at the cathode (Matteson et al. 1995). EC has been applied successfully for more than 3 decades as a water-treatment technology to remove an extensive range of pollutants (