Suitability of Brackish Water for Use in Hydraulic Fracturing: Hydrogeochemistry of Dockum Group Groundwater, Midland Basin, Texas
Water scarcity in arid regions can limit hydrocarbon production activities, such as hydraulic fracturing and drilling. The Midland Basin, a major oil-producing area in semi-arid far west Texas, is already impacted by limited availability of freshwater. A typical oil well in the basin requires ∼3.8–11.4 x 10 m of water per fracture treatment (∼5.0-10 m/m; Nicot et al. Texas Bureau of Economic Geology, 2012). Despite being a water-limited environment, freshwater comprises ∼70% of the water used in hydraulic fracturing in the basin. Thus, the Midland Basin represents an ideal location to consider alternative water resources for use in oil and gas operations. The purpose of this study is to characterize brackish groundwaters within the Late Triassic Dockum Group across the Midland Basin and evaluate their suitability for use in local hydraulic fracturing. Potentiometric surface maps indicate that groundwater generally flows southward and eastward across the basin, possibly as a result of basin uplift and eastward tilting in the past 5–10 m.y. Transmissivities range 2–990 m/day (geometric mean = 42 m/day) indicating that water yield from the unit is mixed, but generally productive. Geochemical results suggest two dominant water types of meteoric origin: 1) a high salinity (up to 70 g/L) Na-SO4- to Na-Cl-type water found mainly in the center and western parts of the basin; and 2) a lower salinity (< 7.5 g/L) mixed ion water, with larger proportions of Ca and Mg, found on the southern and eastern basin margins. Both slick water (low viscosity fluid with a friction reducer) and cross-linked gel (high viscosity fluid containing a gelling agent) fluids are used for hydraulic fracturing in the basin. The feasibility of utilizing brackish groundwater for hydraulic fracturing, particularly with cross-linked gels, is limited by a variety of chemical conditions including high concentrations of alkaline earth metals (AEM), SO4, and DOC. Despite having lower salinity, the water in the downgradient southern and eastern margins of the basin is most likely to exceed acceptable limits for AEM and/or SO4. Generally, the majority of the water in the basin is suitable for use with slick water hydraulic fracturing. Findings from this research provide important baseline data on potential use of brackish groundwaters in the oil and gas industry.