New Mexico Geological Society Annual Spring Meeting — Abstracts

Here, we summarize progress in our assessment of the impact of unconventional oil and gas development in the Mancos Shale on groundwater supply sustainability in the San Juan Basin (SJB). The measurement of actual water use in the SJB is a formidable task, so we tackle this tough problem using three indirect approaches. First, we evaluate the amount of groundwater that could be used in the basin by tabulating the water rights that have been allocated to a variety of stakeholders by the Office of the State Engineer. The largest allocations in the SJB are assigned to domestic users and municipalities (28.2%), mining (coal and uranium, 31.1 %), and food production (24.7%). The petroleum industry owns 6.3% of the groundwater rights, totalling ca. 6674 acre-feet/year (afy). Second, using data from the Oil Conservation Division and FracFocus.org, we tracked the amount of water reportedly used in hydraulic fracturing of both vertical and horizontal oil and gas wells since 2005. Vertical wells drilled into the Mesaverde, Dakota, and Mancos/Gallup account for approximately 75% of hydraulically fractured completions since 2005. Mesaverde vertical wells averaged 150,000 gals/well (0.46 af), Mancos/Gallup vertical wells averaged 125,000 gals/well (0.38 af) and Dakota vertical wells used 105,000 gals/well (0.33 af). The water usage for horizontal Mancos/Gallup wells in the SJB averages 3.0 af/well. This average derived from data for 46 of the 51 (90%) producing horizontal wells as of November, 2013. Operators in the SJB are using produced water and nitrogen foam as a means to reduce water use. Third, we are using the formation top data from scout cards and well logs to create isopach maps of the eleven major aquifers in the San Juan Basin. The volume of groundwater in storage in each aquifer is estimated from the isopach maps using GIS software and stochastic analysis of hydraulic properties. We begin by calculating the volume of material between formation tops, then estimate volume of fluids in the pore spaces and the amount of fluid retrievable, and finally estimate the amount of potable (< 1000 mg/L TDS) versus brackish water. The complex stratigraphy of the San Juan Basin caused by multiple northeast-southwest transgressions and regressions of a northwest-trending, irregular shoreline across the area complicates these calculations. Complications include discontinuity of units, mixtures of rock types, variable porosity and permeability laterally and with depth, the presence of oil and gas in pores, and the presence of natural fractures.