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Summary Recycling oilfield wastewater for hydraulic fracturing requires a good understanding of the water chemical characteristics and how these interact with the fracturing fluid. The viscosity and rheological properties of fracturing fluids affect proppant placement, length and width of fractures, fracture conductivity, and, consequently, the success of the treatment. The objective of the research described here was to understand if dissolved organic matter (DOM) at high concentrations influences subsequent fracturing with a gelled fluid. Experimental studies were conducted on four types of water: (1) model water with low DOM, (2) recycled water from an industrial-treatment facility (medium DOM), (3) untreated early-time flowback (ETFB) water (high DOM), and (4) untreated produced water (high DOM). A low-pH, zirconium-crosslinker-gelled fluid at 200°F was examined in the study. All three water samples that had significant levels of organic matter [total organic content (TOC) > 1000 mg/L] exhibited lower peak viscosities and more-rapid viscosity decay than the model water without organic matter. The destabilizing influence of organic matter on carboxyl methyl cellulose (CMC) gelled fracturing-fluid viscosity is thought to be caused by secondary crosslinking of the short-chain polymer residuals in the flowback, resulting in lower initial viscosity and stability.

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Summary It has been common practice to use a freshwater source (either ground water or surface water) as the base fluid for hydraulic fracturing with crosslinked-gel fluids. Currently, oil and gas operators are beginning to reuse and recycle the main byproduct of oil extraction, which is produced water. However, because of the high variability of produced-water quality (temporal and spatial), and the high content of total dissolved solids (TDS), the viscosity targets for the fluid can be difficult to achieve. The research described in this paper examines the sensitivity of higher-salinity waters to several variables related to the gel-formation process. The polymer used for the gel fluid was a carboxymethyl cellulose (CMC) derivative, and zirconium (Zr) was used as the crosslinking metal. Rheology experiments were conducted at different pH values, polymer loading, and crosslinker concentrations. The outcome of this research is presented in 3D contour-peak-viscosity maps that can be used by oil and gas operators and service companies to optimize the chemicals that are applied, thus reducing costs.

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 (