CO2 Sequestration in Deep Aquifers

Disposal and long-term sequestration of anthropogenic "greenhouse gases" such as CO2 is a proposed approach to reducing global warming. Deep, regional-scale aquifers in sedimentary basins are possible sites for sequestration, given their ubiquitous nature. We used a mathematical sedimentary basin model, including multiphase flow of CO2, groundwater, and brine, to evaluate residence times in possible aquifer storage sites and migration patterns and rates away from such sites in the Powder River Basin of Wyoming. We also used the model to simulate CO2 flow through fractures, to evaluate partitioning between fracture and rock matrix. These simulations provide insight regarding the ultimate propensity of permeability reductions versus permeability increases in the fracture zone associated with carbonate reactions. Regional-scale hydrologic properties, including the presence of fracture zones, were calibrated using surface heat flow data. Our initial results suggest that, in general, long-term (~1000 years or more) sequestration in deep aquifers is possible, if subsurface structure and permeability are well characterized. However, additional risks are involved. In addition to CO2 escape from sequestration aquifers into other aquifers or to the land surface, another environmental threat posed by subsurface sequestration is contamination by brines. We evaluated the potential for such unintended aquifer contamination by displacement of brines out of adjacent sealing layers such as marine shales. Results suggest that sustained injection of CO2 may incur wide-scale brine displacement out of adjacent sealing layers, depending on the injection history, initial brine composition, and hydrologic properties of both aquifers and seals.