Open-file Report - 566
Hydrologic assessment of oil and gas resource development of the Mancos Shale in the San Juan Basin, New Mexico

Here, we summarize our assessment of the impact of unconventional oil and gas exploration and development on groundwater supply sustainability in the San Juan Basin (SJB). The measurement of actual water use in the SJB is difficult, so we tackle this problem using three indirect approaches. First, we evaluate the amount of groundwater that could be used by the petroleum industry in the basin by tabulating the water rights/permits 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 mining (coal and uranium, 31.1 %), domestic users and municipalities (28.2%), and food production (24.7%). The petroleum industry owns 6.3% of the groundwater rights, totalling ca. 6674 acre-ft/year (afy). Second, using data from the Oil Conservation Division, 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 Group, Gallup Sandstone, and the Dakota Sandstone account for 83% of hydraulically fractured completions since 2005. Mesaverde Group (Cliff House Sandstone, Menefee Formation, Point Lookout Sandstone) vertical wells averaged 150,000 gallons/well (0.46 acre-ft (af)), vertical Gallup wells averaged 207,000 gallons/well (0.63 af) and vertical Dakota wells used 105,000 gallons/well (0.33 af). The water usage for horizontal wells in the SJB averages 3.13 af/well. Operators in the SJB are using produced water, foam, and nitrogen as hydraulic fracturing agents to reduce water use. Third, we used formation top data from scout cards and well logs to create structure contour and isopach maps of the ten major aquifers in the San Juan Basin. The volume of material in each aquifer, including rock, fluids, and gas, is estimated from the structure contour and isopach maps in ArcGIS using two methods. We then calculate the volume of material above a depth of 2,500 ft below the ground surface (bgs) in the each unit, which is in the accessible part of each aquifer that tends to hold fresh water (<1,000 mg/L TDS). Finally, we estimate the amount of groundwater in storage in the shallow part of each aquifer. For estimated specific storage values of 1.40 to 1.96 x 10-6 /m, the maximum volume of pre-development water in the shallow portions of confined aquifers <2500 bgs was ~3.25 million acre-ft; this estimate does not include Quaternary aquifers. The maximum amount of water in the San Jose and Nacimiento formations is 83 million acre-ft assuming a specific yield of 0.05 and unconfined conditions, and was 1.21 million acre-ft (pre-development) if the aquifer is assumed to be confined. We calculate that at least 4.5 million acre-ft of groundwater was stored in the accessible parts of the major aquifers prior to the development of groundwater resources in the San Juan Basin. These calculations are approximations due to the inherent stratigraphic complexity of the aquifers and must be used with care. 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. Furthermore, the amount of water that can be realistically extracted is limited by the depth of the screened interval and the spacing of water wells. The calculated volumes are coupled with water chemistry data to document the fact that fresh groundwater is located only 3 to 20 miles basinward of the outcrop belt for each aquifer. Brackish to saline waters are dominant in the center of the basin.

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