Date of Award

1986

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

A.E. Kehew

Abstract

A common practice during oil and gas well-site reclamation in North Dakota is to bury the drilling muds in shallow trenches near the borehole. These muds are saltwater based (between 100,000 and 300,000 mg/L of NaCl) and can contain high concentrations of chromium, lead, and other. toxic trace metals.

Two reclaimed oil and gas well sites were chosen for study in north-central North Dakota: the Winderl site in southeastern Renville County, and the Fossum site in west-central Bottineau County. The Winderl oil well was drilled in 1959, and the drilling fluids were disposed of in a shallow pit excavated in Pleistocene glaciofluvial deposits. The Fossum oil well was drilled in 1978 and the drilling fluids were disposed of in trenches excavated in Pleistocene till.

A total of 41 shallow piezometers (maximum depth is 62 feet {18.9 m)) and 13 pressure-vacuum lysimeters were installed in and around the two disposal sites to obtain groundwater and pore-water samples. Vertical electrode sounding resistivity profiles were conducted at both sites utilizing 14 electrode spacings down to a depth of 100 feet (30.5 m). Sediment samples were obtained with Shelby tubes for x-ray fluorescence and x-ray diffraction analyses. Additional chemical analyses were performed on saturated-paste extracts from the Shelby-tube samples.

The results of chemical analyses of pore water, groundwater, saturated-paste extracts, and the earth resistivity surveys indicate that leachate is being generated from buried drilling fluid at both study sites. At the Winderl site, contaminants have migrated beyond 400 feet (122 m), the extent of the monitored area, which has resulted in degradation of the Spring Coulee Creek Aquifer. A one-dimensional analytical solute transport equation was utilized to illustrate the potential for contaminant migration at the site. The equation predicts high concentration of contaminants over 3300 feet (1000 m) from the source area.

Contaminant migration within the till at the Fossum site is believed to occur along fractures directly below the water table. The estimated groundwater velocity through these fractures is 3.8 m/day {12.8 ft/day) compared to 7.2 x 10-7 (2.4 x 10-6 ft/day) estimated for the till matrix. However, it has been reported that molecular diffusion is an important retardation mechanism that reduces the concentration of contaminants along these fractures with distance from the source. Also, the fractures constitute a small volume of pore space; ·therefore, the quantity {or flux) of water flowing along the fractures is small.

Disposal of drilling fluids in glaciofluvial sediments is not recommended. The study at the Winderl site is evidence of the adverse environmental impact such disposal can lead to. The impact of drilling fluid disposal in till is dependent upon the geologic setting. Migration of the drilling fluid constituents will occur along fractures in the till; widespread contamination could result if these contaminants intersect penneable lenses. A subsurface investigation is necessary at the disposal sites in till to identify these permeable lenses and to determine if any nearby aquifers exist.

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