Effects Of CO₂ Phase State And Purity In Various Rock Cores On Residual Water In CO₂ Geological Storage

Geological storage of CO₂ in deep saline aquifers is currently one of the effective solutions to reduce the greenhouse effect and cope with global climate change.The implementation of this program is achieved by injecting CO₂ into deep reservoirs to drive away salt water.In the process of drainage,the residual water formed in the core pores has an important impact on the storage potential and safety of the formation structure.Therefore,in the process of CO₂ geological storage,it is very important to explore the influencing factors and mechanism of residual water in the CO₂-water-rock system,and to find a method to reduce the residual water saturation in the core.CO₂ drainage experiments is carried out through physical experiment methods,18 groups of experiments were carried out for different CO₂ phases（gas phase,supercritical phase and liquid phase）,different CO₂ purity（99.999%,75%and 50%CO₂）and four different sandstone cores（The core numbers are SH2018-19,FK2018-2,FK6-10 and FK2018-X）to study the CO₂ phase state,purity and the mineral composition,porosity,permeability and pore structure of the CO₂-water-rock system on the formation and mechanism of residual water.The experimental results show that:（1）In the displacement experiment based on different CO₂ phases,the residual water saturation in descending order is gas phase,supercritical phase,and liquid phase;The analysis shows that when the CO₂ phase changes from the gas phase to the supercritical phase and then to the liquid phase,the reduction of interfacial tension and cosine of contact angle in the CO₂-water-rock system leads to the reduction of residual water saturation;the increased gas-liquid two-phase viscosity ratio reduces the viscous fingering phenomenon and improves the displacement efficiency.In addition,the higher viscosity of liquid-phase CO₂will reduce its diffusion capacity,which will increase the breakthrough time and the end of displacement time;but the storage of CO₂ in the form of liquid phase can increase the storage capacity.（2）In core displacement experiments based on different CO₂ purity,the residual water saturation in descending order is 50%CO₂+50%N₂ mixture,75%CO₂+25%N₂ mixture,and 99.999%CO₂.The analysis shows that when the CO₂ content in the CO₂+N₂ mixed gas gradually decreases,the residual water saturation in the displacement system increases with the increase of interfacial tension and cosine of contact angle;The serious viscous fingering phenomenon will lead to the reduction of displacement efficiency.In addition,the breakthrough time and the end time of displacement showed a decreasing trend with the increase of CO₂ purity;and the higher the CO₂ purity during the displacement process,the more CO₂ stored in the core.（3）In core displacement experiments based on different cores,the residual water saturation increases with the increase of the content of quartz and feldspar minerals in the core.The strong adsorption capacity of clay minerals will affect the formation of residual water.The relationship between permeability and residual water saturation is negatively correlated.CO₂ will preferentially displace the deionized water in the macropores during the displacement process.During the displacement process,CO₂ will preferentially displace the deionized water in the macropores.When the core pore throats are unevenly distributed and the connectivity is poor,the CO₂ displacement will be slower（breakthrough time and displacement end time of the SH2018-19 core is the longest）,CO₂ will enter the small pores of the core and drive out the deionized water in it,making the residual water saturation in the core smaller.In addition,the core with the smallest porosity measured the lowest residual water saturation after the displacement experiment,and the amount of CO₂ stored in the core was positively correlated with the porosity.Although the residual water saturation in the SH2018-19 core is the smallest,its smaller porosity leads to poor CO₂ storage capacity of the core.Therefore,it is necessary to comprehensively consider the above factors to achieve a large amount of CO₂ storage.