Vadose Zone Remediation of Carbon Dioxide Leakage from Geologic Carbon Dioxide Sequestration Sites

In the unlikely event that CO 2 leakage from deep geologic CO 2 sequestration sites reaches the vadose zone, remediation measures for removing the CO 2 gas plume may have to be undertaken. Carbon dioxide leakage plumes are similar in many ways to volatile organic compound (VOC) vapor plumes, and the same remediation approaches are applicable. We present here numerical simulation results of passive and active remediation strategies for CO 2 leakage plumes in the vadose zone. The starting time for the remediation scenarios is assumed to be after a steady‐state CO 2 leakage plume is established in the vadose zone, and the source of this plume has been cut off. We consider first passive remediation, both with and without barometric pumping. Next, we consider active methods involving extraction wells in both vertical and horizontal configurations. To compare the effectiveness of the various remediation strategies, we define a half‐life of the CO 2 plume as a convenient measure of the CO 2 removal rate. For CO 2 removal by passive remediation approaches such as barometric pumping, thicker vadose zones generally require longer remediation times. However, for the case of a thin vadose zone where a significant fraction of the CO 2 plume mass resides within the high liquid saturation region near the water table, the half‐life of the CO 2 plume without barometric pumping is longer than for somewhat thicker vadose zones. As for active strategies, results show that a combination of horizontal and vertical wells is the most effective among the strategies investigated, as the performance of commonly used multiple vertical wells was not investigated.