Abstract

Previous studies conducted by the United States Geological Survey (USGS) and the Sioux and Assinniboine Tribes, Fort Peck Indian Reservation; Office of Environmental Protection indicated that the shallow Quaternary aquifer in this region has been heavily impacted by historical oil and gas operations. In 1999 Pioneer Natural Resources was notified that a plugged and abandoned well, the Mesa Biere #1-22, operated by Mesa Petroleum and plugged in 1986, appeared to have been improperly plugged and had released chloride-rich oilfield brine into the shallow aquifer. Parker and Parsley Oil and Gas Company merged with Mesa Petroleum in August 1997 to form Pioneer Natural Resources USA, Inc (PNR).

An investigation conducted by Pioneer in late 1999 and early 2000, revealed that the Biere #1-22 wellbore was still leaking outside of the casing in the Cretaceous, Judith River Formation at approximately 1,000 feet below ground and that brine was channeling upwards into the shallow aquifer approximately 40 feet below the ground surface. Chloride concentrations in nearby shallow monitor wells were as high as 44,700 mg/L and groundwater temperature was as high as 202 degrees Fahrenheit. Typical background chloride concentrations are in the order of 70 mg/L and normal ambient groundwater temperatures are around 45 degrees Fahrenheit. In addition, shallow monitor wells in the immediate vicinity of the Biere #1-22 well measured as much as 30+ feet of crude oil on top of the ground-water. In response to this investigation Pioneer drilled three relief wells adjacent to the old wellbore and successfully sealed the wellbore.

From 2001-2005 Pioneer, along with other operators in the area, conducted further delineation drilling and aqueous geochemical analyses of the regional contamination and constructed a public water system to supply drinking water to affected landowners.

Beginning in mid-2006, Pioneer initiated an integrated approach to characterize both the regional setting of the oilfield contamination including a detailed study of the geological, hydrological, aqueous-geochemical and geophysical setting the Biere #1-22 contaminant plume area. The results of the detailed study of the Biere #1-22 plume area revealed that the contaminant plume was located within an isolated, well-defined channel of fine to coarse-grained gravels of glacio-fluvial origin. The gravel channel exhibits a very high hydraulic conductivity and overlies impermeable shale of the Cretaceous Bearpaw formation and is separated both geologically and hydraulically from other contaminant plumes in the area by very dense, compact clays and silts of the Sprole Till(?) which exhibit very low corresponding hydraulic conductivities. Based on the recognition of favorable geological conditions, Pioneer initiated a more detailed analysis of the hydrogeology of the gravel aquifer and surrounding geological units by conducting numerous pump and slug tests. This data was then input into a 3-D groundwater model and a number of contaminant plume containment and capture scenarios were run utilizing different recovery well placements and pumping rates to evaluate the feasibility of undertaking remedial actions of the Biere #1-22 plume.

Based upon the positive results of the groundwater modeling Pioneer began an aggressive remediation approach and has installed five crude oil recovery wells, ten groundwater removal wells and permitted and drilled a deep 7,800 injection well capable of disposing of 10,000 bbl/day of contaminated groundwater into the Mississippian, Mission Canyon and Devonian, Nisku Formations.

To evaluate the effective of the remediation system there is a monitor well network of over 44 wells with six Schlumberger down-hole dataloggers strategically placed in selected wells to continuously provide data on the pressure, temperature and conductivity of the groundwater as the aquifer undergoes brine removal and remediation. Also the spatial and temporal variations of the highly conductive brine plume will be monitored throughout the remediation process via scheduled, surface geophysical surveys utilizing the EM-34, Electromagnetic method. A pre-remediation baseline ground EM-34 survey conducted by Pioneer showed remarkable correlations with a 2004 Helicopter Electromagnetic (HEM) survey flown by the USGS and also with the brine plume geometry as mapped using groundwater geochemistry data. The use of EM34 geophysical surveys will hopefully provide Pioneer with an inexpensive and quick tool to evaluate the effectiveness of the system.