Evaluating Risks of Sequestered CO₂ on Groundwater Quality: the Combined and Separate Effects of CH4, As, and Cd

Abstract:

Carbon dioxide (CO₂) sequestration in deep subsurface reservoirs is an attractive strategy for the mitigation and reduction of global CO₂ emissions. Previous studies have identified potential geochemical effects of CO₂ gas leaks on overlying groundwater aquifers. However, in addition to CO₂, other gasses may also be present. For example, methane (CH₄) can be present as an impurity in the CO₂ gas stream, a native gas, or can be produced within the reservoir or aquifer after the introduction of CO₂.

Continuous and stop-flow column experiments and batch studies were conducted to determine changes in pH and aqueous elemental concentrations and the fate of contaminants (As and Cd) when CH₄ is present in addition to CO₂ gas. Two sediment samples were obtained from an aquifer overlying potential CO₂ storage reservoirs (High Plains Aquifer, KS). Column experiments were conducted with synthetic groundwater (SGW) or As/Cd spiked SGW saturated with a mixture of 99% CO₂and 1% CH₄ gases. The spiked SGW contained 114 ppb As and 28 ppb Cd to represent maximum contaminant levels present in CO₂ reservoir brines as shown in previous modeling studies. Batch experiments were also conducted with spiked SGW, with CO₂ or the CO₂/CH₄ mixture.

Column study results showed most elements followed similar concentration trends after injection of CO₂/CH₄ saturated SGW or spiked SGW. Cd concentrations remained below detection limits for both sediments, with or without the spike. As concentrations were similar between SGW and spiked SGW columns until after the 3^rd stop flow. As levels in the spiked SGW column then increased while levels in the SGW column decreased, suggesting the sediments are the source of effluent As until this point. Similar pH values were also observed until the 3^rd stop flow event, where the pH of both SGW columns decreased significantly compared to spiked SGW columns. Batch studies showed As concentrations did not decrease as quickly when CH₄ was present with the CO₂. The concentrations of other elements, including Cd, followed trends similar to the experiments without CH₄. Possible effects of redox changes and differences in mineralogy of the two sediments are under investigation. Results of this study, along with those of our previous efforts, are significant for risk evaluation and site selection for CO₂ sequestration.