EXPERIMENTAL INVESTIGATIONS OF THE EFFECT OF CARBON DIOXIDE ON MINERAL-WATER INTERACTIONS

Dissolution of CO₂ in formation waters may enhance the safety and stability of storage reservoirs, but the interactions between fluid and surrounding rocks, and their influence on total CO₂ solubility, are not well constrained by experiment. We will present results from two complementary series of experiments: measurements of CO₂ solubility in rock-buffered fluids, and kinetic experiments on mineral reactions with pore water. Experiments have been carried out at at both supercritical and subcritical CO₂ pressures and at both room temperature and 70^oC.

CO₂ solubility measurements have been conducted with calcite and clay minerals singly or together as mineral phases. Mineral interactions result in significantly higher pH values than for simple CO₂-water interaction at the same conditions.

Exchange experiments between Na-exchanged SWy-2 montmorillonite and CaCl₂ solutions show that the Ca:Na ratio of the fluid adjusts in days, with extensive reaction in less than 1 day. The fluid evolves from Ca-dominated to a Ca-Na fluid dependent on the starting bulk composition. Where CO₂ is added to NaCl solutions equilibrated with both calcite and montmorillonite, there is a rapid rise in Ca due to calcite dissolution, but over a few days, Ca begins to drop. This reflects both a drop in the solubility of calcite as the clay further neutralises the fluid, and the effects of cation exchange with the clay. The end product is an Na-Ca bicarbonate fluid, similar to those found naturally in some continental basins.

Our database complements modelling studies and provides direct information on reaction rates. These indicate that where gas, water and minerals interact, solution trapping will be rapid and will lead to enhanced levels of dissolved CO₂. If it can be shown that the overall consequences are beneficial for the safety case, injection strategies should be designed to maximise gas-water interaction and avoid the development of dry gas pockets in the reservoir.