SIMULATION OF CO2 INJECTION NEAR THE HUNTER POWER STATION, CENTRAL UTAH

Injection of CO2 into deep saline aquifers is an attractive option for the long-term sequestration of the gas. The injection technology required is well proven in enhanced oil recovery by CO2 injection projects. Modeling studies show that in some situations the gas is likely to be contained for very long periods of time and the existence of natural CO2 reservoirs supports this conclusion.

Permanent sequestration of CO2 can be achieved when the CO2-rich brine reacts with reservoir rocks to form minerals. However there is evidence of leakage from the natural CO2 reservoirs on the Colorado Plateau (Allis et al. 1991) and it is likely that artificial reservoirs created by sequestration projects may also leak through seal faults or by exceeding seal containment pressures. Mineral forming reactions are slow.

This paper investigates the injection of CO2 into geological structures that are not dome shaped and thus do not provide the geology required for the development of an artificial CO2 reservoir. Such structures may, however, provide very long flow paths between the injection point and the surface, allowing the permanent sequestration of the injected CO2 as a mineral or dissolved in the groundwater. The geology beneath Hunter Power Plant, located in central Utah, is one example of such a structure. The sedimentary sequence contains potential reservoir and seal formations at over 1 km depth beneath the power plant, but the regional dip exposes some of these formations at the surface some 40 - 50 km away.

A two-dimensional numerical model of the groundwater in this area has been developed and used to investigate the long-term behavior of CO2 injected beneath the power plant. The model represents the major physical and chemical processes induced by injection of CO2 into the reservoirs including transport in the liquid and gas phases, the effect of dissolved CO2 on brine density, and the reaction between the CO2 plume and reservoir rocks.

We will discuss transit times to the surface for gas injected at different depths, the effectiveness of mineral and liquid phase sequestration, the effect of fault rupture creating high-permeability flow paths to surface, and the possibility of increased mineral deposition in such areas of focused flow providing a self sealing mechanism.