Water. desalination + reuse

TECHNOLOGY Table 5: Characterization of Frac Water, Flowback & Produced Water for the Carbonates (Slave Point) Resource Play with the remaining 1.15% fluid volume consisting of additional frac fluid chemistries. The additional fracturing fluid chemistries may include any combination of the following: gelling agents (guar), crosslinkers (boron, zirconium, iron or titanium), surfactants, scale inhibitors, pH buffers, breakers (oxidative or enzyme), iron control agents, corrosion inhibitors and/or biocides8. A summary of the Tight-oil Development Water-Based Crosslinked Hydraulic Fracturing Fluid Composition is provided in Figure 1. Although the overall percentages of water-based crosslinked hydraulic fluid chemistries are quite low, the impact of these residual chemistries within flowback waters must be considered. Determining which water constituents within flowback and/or produced water should be targeted for treatment relies on two factors. Firstly, any treated water source must remain compatible with the desired fluid system for the respective tight-oil development area. Secondly, removal of water constituents may be needed to mitigate surface and subsurface fouling, scaling and corrosion. Presently, where hydraulic fracturing operations rely solely on fresh water sources, minimal pre-treatment is required to ensure hydraulic fracturing fluid compatibility. In most cases, pre- treatment is limited to the application of biocide and filtration to eliminate bacteria and remove suspended solids that may be present within the fresh water. During the stimulation process, the hydraulic fracturing fluids comingle with the formation water to create a flowback water containing elevated concentrations of contaminants. The potential flowback water contaminants include residual hydraulic fracturing fluid chemistries, iron, total hardness, alkalinity, silica, NORMs, bacteria and solids. The increase in concentration of these species is known to have detrimental impacts on the fluid compatibility of water-based crosslinked hydraulic fracturing fluid systems. In most cases, the desired viscosity and thermal stability of the waterbased crosslinked fluid is compromised either through chemical, mechanical or biological degradation resulting in hindered proppant carrying capacity of the hydraulic fracturing stimulation fluid 9 . An inability to achieve the desired fluid viscosity and the required proppant carrying capacity of the water-based crosslinked fluid will hinder the effectiveness of the stimulation. Without the appropriate distribution of proppant into the opened fractures, the newly formed fractures may close once the fracturing pressures are released 10. In addition to preserving fluid viscosity, another important consideration to keep in mind is that in most cases the gelling agent, crosslinker and proppant are added to the hydraulic fracturing base fluid on the fly. To achieve the desired downhole rates, the gelling agent must hydrate and crosslink within only a few minutes. Any constituents present within the flowback or produced waters that delay the hydration or crosslinking reaction times may also require treatment. To mitigate water-based crosslinked fluid compatibility concerns, Table 4 outlines the identified water quality guidelines to prevent undesired hydration rate reactions, overcrosslinking, delayed crosslinking, thermal destabilization, viscosity inhibition, gelling agent precipitation as well as chemical and biological degradation. In addition to mitigating potential water-based crosslinked fluid compatibility concerns, other constituents present in the fresh, flowback and produced waters may contribute to surface and subsurface fouling, scaling and corrosion. The most likely constituents known to contribute to fouling, scaling, and corrosion related concerns include: total dissolved solids (TDS), precipitated solids, total suspended solids (TSS), emulsions, dissolved oil, dissolved gases, and bacteria. * August-September 2013 | Desalination & Water Reuse | 29 |

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