Self-sealing of caprocks during CO2 geological sequestration

Abstract

The self-sealing of caprocks plays a vital role in CO2 geological sequestration. However, the mechanisms by which distinctive mineral combinations and types of caprock bring about self-sealing are unclear. Studies of the self-sealing of caprocks with different mineral combinations and rock types promise to effectively identify the self-sealing capabilities of cap rocks and evaluate the long-term safety of CO2 geological sequestration. This study focuses mainly on the mechanisms of self-sealing in three caprocks, specifically the shale of the Qingshankou Formation in the Songliao Basin, the calcareous mudstone of the Da'anzhai Formation in the Sichuan Basin, and the shale of the Yanchang Formation in the Ordos Basin. This study applies the key technology of CO2–fluid–rock numerical simulation and petrological and analytical methods to systematically investigate the self-sealing capability of these caprocks, which have different mineral compositions, and identify the key minerals that trigger self-sealing. The results show that the key minerals that can trigger self-sealing after CO2 injection are carbonates such as calcite, siderite, and dolomite and clays such as kaolinite. The CO2–fluid–rock interactions in feldspar- and illite-rich shale are relatively mild, which is conducive to the safe geological sequestration of CO2. In contrast, the CO2–fluid–rock interactions in carbonate-rich shale are violent and fluctuate greatly, which is not conducive to the short-term safe sequestration of CO2, but they are favorable for the safe sequestration of CO2 in the medium to long term. After CO2 injection, the main minerals precipitated in a carbonate-rich shale are K-feldspar and small amounts of ankerite and dawsonite. The main minerals precipitated in illite-rich shale are quartz and small amounts of ankerite and dawsonite. The results of this study show that it is possible to effectively evaluate the sealing capability of caprocks, thus providing a basis for the safe sequestration of CO2.