LONG TERM PREDICTIONS OF CO_2 STORAGE BY MINERAL AND SOLUBILITY TRAPPING IN THE WEYBURN MIDALE RESERVOIR

摘要

The IEA Weyburn CO_2 Monitoring and Storage Project is studying the potential for CO_2 storage in the Weyburn Midale field, where Encana is currently undertaking a CO_2 EOR program. One of the major research objectives is to predict the potential amount of CO_2 storage in the reservoir through geochemical processes. Both closed and open system geochemical models have been used to calculate the geochemical reactions that occur between the injected supercritical CO_2, the injected water, the formation fluid and the mineralogy of the seven major flow units in the Weyburn Midale field over a period of 5-10,000 years. The geochemical trapping potential of the overlying and underlying units has also been assessed in case any of the CO_2 migrates out of the reservoir. Although there are differences between the various flow units within the reservoir, the net long-term reactions are similar in all of them. Up to approximately 10 years, the precipitation of calcite and kaolinite, and the dissolution of anhydrite and various silicate minerals are predicted. At approximately 10 years, a major change occurs with the precipitation of dawsonite (Na-Al carbonate) and minor amounts of anhydrite, and the dissolution of calcite. This change has occurred because of the increased significance of silicate mineral dissolution. Long term one dimensional reactive transport modeling has shown that calcite dissolution and anhydrite precipitation occurs preferentially at the inlet while all other changes occur along the entire flow path. Integrating the results over each of the flow units results in a maximum potential solubility, ionic and mineral trapping for CO_2 in the Weyburn Midale reservoir of 45 million tons. Planned injection CO_2 injection is on the order of 20-25 million tons. Thus the Weyburn Midale reservoir has the potential to store all of the injected CO_2 through solubility and mineral trapping mechanisms. The reactions in the overlying and underlying formations are quite varied due to the amount and nature of reactive silicate minerals in each of the formations. In summary, many of these formations will act as a very strong 'geochemical' barrier to CO_2 migration through the formation of carbonate minerals.