EXPERIMENTAL CO2 SATURATED BRINE-ROCK INTERACTIONS AT ELEVATED TEMPERATURE AND PRESSURE: IMPLICATIONS FOR CO2 SEQUESTRATION IN DEEP-SALINE AQUIFERS

Carbon dioxide (CO₂) storage is increasingly being viewed as a tool for managing anthropogenic CO₂ emissions to the atmosphere. Disposal of this excess CO₂ into deep aquifers is one of several proposed repositories, but the details of the geochemical reactions between supercritical CO₂ and potential host fluids and formation rocks are largely unknown. Our approach to understanding the processes of geologic sequestration of CO₂ is to explore experimentally the geochemical reactions, involving mineral dissolution and precipitation that might provide the irreversible fixation of CO₂. These results provide fundamental experimental data for thermodynamic models of CO₂ sequestration in saline aquifers.

We carried out a systematic experimental study to evaluate the potential for CO₂ storage in deep saline brines by solubility and ionic trapping mechanisms and for CO₂ sequestration by mineral trapping from 25° to 125° and from 100 to 600 bars, both in the presence and absence of reactive formation rocks ranging from limestones to arkosic sandstones.

The solubility of liquid CO₂ is reduced threefold in nearly-saturated aquifer brines relative to pure water but is enhanced 6 to 9% relative to the brine alone in the presence of limestone rocks due to the dissolution of calcite. In addition, calcite dissolution increases dissolved Ca, alkalinity, and formation porosity by ~ 6%. Dolomitization of the limestone occurs in the presence of sulfate-bearing brines in which anhydrite precipitation elevates the dissolved Mg/Ca ratio. We observed also significant desiccation of the brine. These relations are dependent on temperature, pressure and in some cases the ratio of liquid CO₂/brine. The results compare favorably to theoretical equilibrium calculations and earlier experiments carried out at higher temperatures.

Long term CO₂ brine-rock experiments are underway to evaluate the effects of multiphase H₂O-CO₂ fluids on mineral equilibria and the potential for CO₂ sequestration in mineral phases within deep-saline aquifers.