FATE AND TRANSPORT OF OIL-FIELD BRINE AT THE OSPER SITES, OKLAHOMA

The USGS has been investigating environmental impacts of oil production activities at the Osage-Skiatook Petroleum Environmental Research (OSPER) Sites “A” and “B,” which are located near Skiatook Lake, Osage County, Oklahoma. The “B” site is an active oil production area, whereas the “A” site is an oil field that was largely abandoned 68 years ago. Contamination of soil, ground water, and surface water from crude oil and brine production is extensive at both sites. We drilled about 50 wells up to 36 meters deep at each site for ground-water sampling and hydraulic testing. Water samples indicate there are extensive subsurface plumes of salt water (2,000-30,000 mg/L TDS) at both sites. Borehole cores identified many thin and poorly permeable sandstone layers (<1 m thick) separated by thicker shale and mudstone confining units. Well tests indicate the hydraulic conductivity of the permeable sandstone units is about 1 cm/day. Continuous monitoring of water levels in wells indicates the ground-water recharge rate is low. During the oil-production era at the “A” site, brine from oil wells (TDS up to 150,000 mg/L) was evidently discharged directly into pits and an ephemeral creek bed. Although the rate of recharge was low, enough brine seeped from the pits and into the creek bottom over the 25-year life of the field to form a substantial subsurface plume in the sandstone. Since the “A” site was abandoned, natural dilution of the plume has been very slow because the natural ground-water flow rate is low. Also, the down-slope migration of the plume likely slowed when Skiatook Lake reservoir was filled in 1987, raising the water table elevation at the toe of the plume. Mixing of fresh rain water with the deeper salt water may have been limited by density stratification. At the “B” site, brine holding pits have leaked and created salt scars at the surface. We believe that over the 40-year life of the “B” site, brine has flowed from the pits through a 7-meter thick shale layer near the surface into the deeper sandstone units. Results indicate the brine plumes at both sites will exist for many years to come with continued deleterious environmental impact at ground-water discharge points.