Date of Award

2009

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Z.W. Zeng

Abstract

Carbon dioxide (CO2) has been injected into depleted oil reservoirs for enhanced oil recovery for several decades. Injection of CO2 into geologic formations in the Williston Basin is currently under consideration for long-term CO2 storage to reduce anthropogenic CO2 emissions to the atmosphere. The Madison Group in the North Dakota Williston Basin provides the greatest potential for geologic sequestration in either deep saline aquifers or depleted oil reservoirs. Little is known about the geochemical reactions that take place when supercritical carbon dioxide is injected into deep saline aquifers at geologic conditions similar to those found in potential sequestration units of the Madison Group.

Previous studies have shown the injection of carbon dioxide into a saline aquifer makes the formation water slightly acidic, which reacts with the host rock to dissolve carbonate minerals. Dissolution of carbonate minerals may compromise the integrity of the formation, leading to the eventual escape of CO2 to the surface. In order for CO2 sequestration to be effective, CO2 must remain below the surface indefinitely. Studies of the properties of carbon dioxide indicate that CO2 is less soluble with increasing salinity, resulting in less carbonate dissolution. Formation waters in Madison Group aquifers range in salinity from 1,000 ppm to greater than 300,000 ppm total dissolved solids. Sodium chloride (NaCl) is the primary salt of the formation waters of the Madison Group. Water-alternating-gas (WAG) flooding experiments were conducted on limestone rock cores using a core flooding system that simulates the CO2 injection process at subsurface conditions. Deionized (DI) water and three different concentrations of NaCl solutions, 1,000 ppm, 10,000 ppm and 100,000 ppm were used to represent salinities found in the formation waters in the Madison Group in the Williston Basin.

Effluent water was collected for analysis of pH, specific conductance, sodium, calcium, iron, chloride, alkalinity and total dissolved solids. The presence of calcium, and to a lesser extent, alkalinity and decreased pH and in the effluent samples, indicate limestone dissolution took place throughout the flooding experiments at all water flood concentrations. Calcium and alkalinity concentrations were highest during the 100,000 ppm flooding and lowest during the deionized water flooding, indicating CO2 is more soluble with increasing salinities at geologic conditions found in the aquifers of the Madison Group in the North Dakota Williston Basin than was previously reported.

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