Exploring The Hydrogeologic Controls On Brackish Water And Its Suitability For Use In Hydraulic Fracturing: The Dockum Aquifer, Midland Basin, Texas

Water scarcity in arid regions can limit or impede certain hydrocarbon production activities, such as hydraulic fracturing and well drilling. The Midland Basin, a major oil-producing area in semi-arid far west Texas and the eastern sub-basin of the Permian Basin, is already impacted by limited freshwater availability. A typical tight oil well in the Midland Basin requires ~3.8–11.4 × 103 m3 of water per fracture treatment (Nicot et al., 2012). Despite draught conditions and limited resources, data for 2011 indicate freshwater comprises ~70% of the water used in hydraulic fracturing in the Midland 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 the hydrogeochemistry of a brackish groundwater aquifer, the Late Triassic Dockum Group, in the Midland Basin and evaluate its suitability for use in local hydraulic fracturing. Potentiometric surface maps for the Dockum Aquifer indicate that groundwater generally flows south and east across the basin, possibly as a result of basin uplift and eastward tilting in the past 5–10 m.y. (Bein and Dutton, 1993). Transmissivity values from testing of Dockum wells follow a log-normal distribution, ranging from 2 to 990 m2/day (geometric mean = 42 m2/day), indicating that water yield from the unit is highly variable, but generally productive. Geochemical results suggest two dominant water types of meteoric origin within the aquifer: 1) a higher 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 (\u3c 7.5 g/L) mixed ion water, with larger proportions of Ca and Mg, found on the southern and eastern basin margins. Data for δ18O and δ 2H of Dockum Aquifer groundwater samples suggest that the lower salinity waters likely represent recharge from conditions cooler than the present climate. Geochemical data for these waters suggest the composition of the Ca-rich groundwaters is controlled by water-rock interaction with overlying aquifers and the lower section of the Dockum. The high salinity, Na-rich waters are thought to represent meteoric water from eastern New Mexico that experience more complex regional rock-to-water reactions with carbonates, clays, silicates, and evaporite minerals as the water travels eastward through overlying aquifers, and through the upper and lower Dockum. 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 more Ca-rich water found in the down-gradient 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