In Situ Characterization of Wettability in CO2-Brine-Rock System Based on X-Ray Microtomography

It is of great significance to evaluate wettability accurately because it controls the fluid distribution in the pores and throats. And the wettability is directly related to the safety of CO2 sequestration in aquifers. In this paper, in situ characterization of reservoir wettability based on micro-CT is investigated due to its repeatability and convenience. At first, a CT scanner is used to obtain the fluid distribution. Then, we use the artificial intelligence wizard segmentation method (AIWSM) to segment the images. The AIWSM combines threshold segmentation method with deep learning segmentation method to improve segmentation accuracy. After that, we measure CO2 contact angles and contact surface area between CO2 phase and rock phase to evaluate reservoir wettability, respectively. The measurements consider the influence of the roughness of the reservoir rock surface, which can better reflect the real situation of the reservoir. At last, we also study the influence of wettability on CO2 residual trapping. The results indicate that the images segmented by AIWSM is more accurate for each phase in CO2-brine-rock system. Subsequently, we consider the influence of salinity and surfactant concentration on CO2 contact angles and contact surface area between CO2 phase and rock phase. It shows that when salinity increases from 10,000 mg/L to 100,000 mg/L, CO2 contact angles decrease from 124.55° to 116.32° and contact surface area between CO2 phase and rock phase increases from 7.44×107 μm2 to 12.03×107 μm2 gradually. In addition, when surfactant concentration increases from 0 wt% to 0.5 wt%, CO2 contact angles increase from 124.55° to 137.88° and contact surface area between CO2 phase and rock phase decreases form 7.44×107 μm2 to 3.03×107 μm2 gradually. The results obtained by two wettability characterization methods are consistent: the reservoir hydrophilicity is weakened with the increase of salinity and enhanced with the increase of surfactant concentration. After we accurately characterize the reservoir wettability in situ, the influence of wettability on CO2 residual trapping can also be obtained. It shows that the more hydrophilic the rock is, the higher the CO2 residual trapping efficiency is. Accurate characterization of wettability in CO2-brine-rock system is essential for the long-term safety of CO2 sequestration. This can help better select the suitable CO2 sequestration locations for aquifers and provide reliable experimental data for CO2 sequestration simulation.