Coupled chemo-mechanical behavior of CO2 mineral trapping in the reservoir sandstones during CO2–EWR

Abstract

Carbon dioxide (CO2) sequestration in deep saline aquifers is regarded as a potentially useful method of storing CO2 due to their large storage capacity. CO2-trapping mechanisms in such aquifers include solubility trapping, hydrodynamic trapping, structural trapping, and mineral trapping. CO2–water–rock interactions occurring in saline aquifers injected with CO2 are known to play a vital role in these trapping mechanisms. Stress is known to have a significant and positive effect on mineral dissolution, and therefore, pressure solution as a coupled chemo-mechanical behavior could make an important contribution to mineral trapping. Geological storage of CO2 can also be combined with enhanced water recovery (EWR) from deep saline aquifers, a process referred to as CO2–EWR. By exploiting the fluid during CO2–EWR, the pore pressure in the reservoir is altered, which could enhance pressure solution between the mineral grains in the reservoir. In this work, the role played by pore pressure in CO2 mineral trapping from the perspective of pressure solution as a chemo-mechanical coupling process is investigated. To achieve this, seepage–creep tests were performed on sandstone specimens by passing CO2–NaCl solutions through them at different pore pressures. Experimental results show that the lower the pore pressure a specimen is subjected to, the greater the amount of carbon trapped in the sandstone. On the basis of this result, a geometrical model is established for pressure solution in the materials used that quantitatively describes the mechanism responsible for pressure solution. Geometrical model is then used to analyze the effects of the various factors affecting the role played by pressure solution in CO2 mineralization sequestration (mineral type, pore pressure, porosity, and particle size). The results of the analysis are particularly instructive for the evaluation of long-term CO2 storage in terms of pressure solution. As for CO2–EWR, apart from relieving pressure buildup, increasing CO2 injection, regulating CO2 migration, and restricting CO2 leakage, it also enjoys the advantage of enhancing mineral trapping.