Numerical Simulation Study Of CO₂and Water Two-phase Flow In Porous Media

Investigations of the possibilities to reduce CO2emissions are receiving a significant amount of attention as a consequence of the significant effects of global warming due to the accumulation of carbon dioxide gas (and other green house gases) in the atmosphere. Sequestration of CO2in saline aquifers is a means with great potential for reducing atmospheric emissions of CO2. Core-scale research may help to improve our understanding of multi-phase flow process and CO2trapping mechanisms in CO2-brine system, and hence provide theoretical bases for more accurately predicting the long-term fate of the stored CO2and ensuring the storage security, which hence contains important theoretical research value.Firstly, in order to improve the accurate of digital cores used for simulations, we study the applicability of different porosity-permeability relationships. Results show that great difference exists among the applicability of different porosity-permeability relationships for porous media. The relationships with the high power form, such as the high-order K-C equation, the simplified fractal model and the power-law model are more suitable for glass bead packs. In addition, based on CT imaging technique, we modify the pore-scale imaging and analysis method by taking the geometry structure of porous media into account, and finally make a more exact prediction of the permeability distribution of the porous media.Based on a combination of core-scale numerical simulation and laboratory experimental methods, we then investigate the effects of the permeability and the capillary pressure of the porous media on the CO2-water two-phase flow. Our results indicate:(1) The heterogeneity of permeability can slow down the movement of CO2and enhance the CO2displacement efficiency. Besides, the CO2plume tends to preferentially pass through the region with higher permeability, thus leading to the irregular CO2-water displacement front;(2) The core-scale permeability anisotropy does not have significant influence on the CO2-water displacement.(3) The capillary pressure is one of the main factors in trapping porous water. The displacement efficiency and the time needed for achieving system stability both decline with increasing capillary pressure. Further, with the increase of capillary pressure, its influence on the displacement efficiency and the steady time also decrease;(4) the capillary heterogeneity may lead to the anomalous distribution of CO2.Besides. the capillary heterogeneity trapping mechanism is also verified and further investigated using both numerical and experimental methods. Results shows:(1) The capillary heterogeneity trapping is an important CO2trapping mechanism. The local heterogeneity may cause the immobilization of CO2plume beneath the capillary barriers with higher capillary entry pressure. With the increasing capillary entry pressure of the capillary barriers, the trapping effect may become more obvious:(2) The low-permeability barriers may give rise to the change of the flow path of a portion of CO2, but it does not have the ability to trap the CO2for long. Besides, the low-permeability barriers are able to enhance the CO2displacement efficiency and help CO2plume to break through the capillary barriers.