In the Bakken, TDS, hardness, and iron tend to be ultra-high, while bacterial activity and pH are typically low. In the Eagle Ford, waters tend to be anaerobic with high bacterial activity, high suspended solids, and iron can exceed 100 mg/L. The Powder River Basin shows moderate TDS, neutral pH’s, and high alkalinity. Bacterial activity can be quite high in this basin as sulfates, a food source for bacteria, also tends to be high. Water quality in the Uinta basin is characterized by a TDS less than seawater, alkalinities greater than 1000 mg/L, and high bacteria. Although sulfates are not naturally high in this area, sulfides tend to be high, and as the water is oxygenated the sulfides are converted back to sulfates providing a potential for a re-cycled food source for bacteria. Produced waters in this basin are highly anaerobic with low to high suspended solids. The Cana Woodford play has similar quality to what is seen in the Uinta except in much of this play we see uncharacteristically high boron concentrations, and suspended solids tend to be lower in the Cana Woodford than in the Uinta. Treatment options related to boron removal and more-boron tolerant frac fluids exist for high boron produced waters. The Permian Basin has moderate to high TDS, and alkalinity is high and pH’s are more basic in this region, perhaps a reflection of the high concentration of carbonate reservoirs in the basin. Sulfates are often found to be high as are suspended solids. Comparatively speaking, hardness is not very high in flowback and formation waters across most of the basins except where ultra-high brines are present. As noted for the Permian, much of this variation is of course due to the geology of the area, but even proximate formations can exhibit striking differences in water quality. The primary reason for choosing one treatment over another is end-use driven.
Table 1 also shows the various end-uses to which these produced waters were successfully treated. The Bakken and Eagle Ford produced waters were treated to be used as make-up water for a zirconium-crosslinked CMHPG frac fluid. The Powder River and Uinta produced waters were treated to be used as make-up water in a borate cross-linked guar frac fluid. Produced waters in the Permian and in Canada have thus far been mostly treated to a slickwater end-use. While Slickwater fluids are more robust, allowing for easier field application, careful attention should still be paid to the water quality to minimize potential damage to the formation and the proppant pack.
Given the wide variability in produced water, it is best that a customized approach with an end- use in mind is utilized to meet the demands of different frac fluid types. . During the design phase of custom water treatment, problem parameters will be evaluated and a plan for reduction or removal will be set. Those parameters that don’t negatively affect the end-use fracturing fluid will not be removed thus making the water more formation compatible and reducing the overall treatment cost.