From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

Bureau Seminar, December 2, 2011

Seasonal Flush-Recovery Cycles in Groundwater Flow, Ogallala-Santa Rosa Aquifer System, SACROC Area, Texas

Link to streaming video: available 12.02.2011 at 8:55am

Dr. Seay Nance, Research Associate
Bureau of Economic Geology, UT Austin

Dr. Seay Nance

Over 175 million metric tons [3.3 trillion standard cubic feet (TCF)] of CO2 has been injected at the ~233-km2 (90-mi2) SACROC oilfield in Scurry County, TX since 1972 at depths of 1,800 to 2,100 m (6,000 to 7,000 ft) for enhanced oil recovery. To characterize regional hydrochemical variability prior to assessing potential impacts to groundwater from CO2 injection, groundwater samples were analyzed from 60 water wells in a ~3,000-km2 (1,160-mi2) area centered on SACROC between June 2006 and July 2008. Historical data from the Texas Water Development Board were included in the investigation.

Most water wells in Scurry County are dually completed in the Ogallala and underlying Dockum Santa Rosa aquifers to maximize groundwater availability in spite of water-quality differences between the aquifers. Generally, water levels decline during summer to autumn due to pumping for irrigation. Controls of hydrochemical variability include well-specific differences in completion depths, water-rock interactions, bacterially-produced CO2, seasonal rainfall and irrigation activities, potential for brine expulsion from over-pressured deep aquifers, and oil-field activities.

This presentation focuses on interpretation of the shallow flow system revealed by Br- and Cl-based conservative mixing models supported by variations in δ13C and ratios of sulfate to total dissolved solids (SO4/TDS). The data suggest that systematic seasonal variations in water quality reflect contributions from three hydrochemical end-members: fresh meteoric groundwater from shallow aquifers and two distinctly different brines from deep aquifers. Each of the brines appears to be concentrated in separate aquifers (Santa Rosa or Ogallala) in the shallow aquifer system. End-member brines have Br/Cl values (X 105) of 25 and 180, respectively. The highly diluted lower Br/Cl brine is characteristic of the Ogallala aquifer, whereas the higher Br/Cl brine is characteristic of the Santa Rosa member of the Dockum Group. The ultimate sources of the brines are unclear. Brine composes a small fraction (<1%) of specific water-well samples but profoundly affects the Br/Cl value of brine-freshwater mixtures and enables detection of aquifer-specific influences on water quality. Seasonal hydrochemical variations reflect a system most highly influenced by influx of Ogallala water during late spring to fall when precipitation and irrigation (the “flush cycle”) are most active. Influx of Santa Rosa groundwater takes place over the winter and early spring (the “recovery cycle”). Systematic early spring to autumn average depletion of 4.8 ‰ in δ13C and lower Br/Cl values reflect flushing through the Ogallala by irrigation and rain water. Elevated δ13C, SO4/TDS, and Br/Cl values found in early springtime samples reflect greater proportions of Santa Rosa water during the recovery phase.