Oil-Field Brine Plumes in Shallow Ground Water, Sheridan County, Montana: Sixteen Years Later

Reiten, Jon C. ¹ (1) Montana Bureau of Mines and Geology, Billings, MT

Oil-field development in eastern Sheridan County, Montana has resulted in the local transfer of highly concentrated sodium chloride brines from deep, oil producing horizons into shallow aquifers and surface-water bodies. In this part of the Williston Basin, the total dissolved solids of produced water ranges from about 100,000 to more than 300,000 mg/L. Releases of such concentrated brines can have serious effects on the usability of fresh water resources. Most of the releases occurred prior to 1980 when environmental concerns and regulations were less stringent than at the present. Dozens of oil-field sites were assessed in the late 1980’s to determine the effect of brine releases on shallow ground-water and surface-water resources. Electromagnetic induction surveys were used to identify brine plumes in the subsurface. The extent of these plumes was confirmed by data collected from monitoring wells. Sixteen years later, several of the original wells have been reestablished as monitoring points. Preliminary sampling indicates a very dynamic ground-water system where many wells show improved water quality while at other wells water quality has been degraded. A chloride index calculated by determining the ratio of the field chloride concentration to the field specific conductance allows the magnitude of brine impacts to be compared over a wide range of background water quality. Results of recently collected samples are being compared to older samples in order to develop a better understanding of the characteristics of these brine plumes and to help predict future impacts.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana