Study On The Formation Mechanisms Of Residual Water In CO₂-brine-rock Systems

CO₂ geo-sequestration in deep saline aquifers is achieved by injecting CO₂ into deep saline aquifers and the brine is then drainaged. However, residual water forming during the drainage process has negative influences on CO₂ storage process. Therefore,it is necessary to fully understand the formation mechanisms of residual water in CO₂-brine-rock systems to minimize the residual water saturation in rock pore. Firstly,three different groups of super-critical CO₂ drainage experiments were carried out.Then, the experimental results were combined with other studies on the storage of CO₂ in deep saline aquifers as a comprehensive review to determine the main influence factors of residual water formation/residual water saturation and their function laws.The experiments were carried out at aquifer temperature of 40 ℃ and pressure of8 MPa. We studied the effects of brine concentration（0, 17.5 and 35 g/L）, brine species（deionized water, 35 g/L NaCl and CaCl2 brine） and quartz content（58, 31 and 98%） in rocks on the formation and saturation of residual water. The results show that,（1） the amount of residual water（and the irreducible water saturation） of high concentration was larger than that of the lower concentration in all the stages,（2）deionized water drains more completely than either of the two brines, and NaCl brine drains more completely than CaCl2 brine, and（3） the irreducible water saturation was lower with higher quartz content in the rock core when other effective factors were stable. We divided the drainage process into three stages according to the curve of water flow rate, the Pushing Drainage, Portable Drainage and Dissolving Drainage Stages. The paper also presents a capillary model to analyze and interpret the mechanism of the drainage by CO₂ in these three stages.There are number of influencing factors on this residual water formation process.Among them, CO₂-brine interfacial tension（IFT） related factors are dominant, where increasing of IFT has positive influence on residual water formation depending on the formation pore fluid concentration and type and reservoir conditions. Contact anglesin brine-CO₂-rock systems are also significantly influence the residual water trapping process due to its direct correlation with capillary pressure, where residual water saturation decreases with increasing contact angles. Furthermore, aquifer’s rock mass heterogeneity create positive influence on residual water trapping with offering more locus for residual water and permeability and porosity of the aquifer also affects its residual water formation. Importantly, injecting of CO₂ at a low rate and long time offers less opportunity for residual water formation. However, it should be noted that these factors mutually contribute to the formation of residual water and therefore, the overall influence decides the final residual water formation efficiency in the aquifer.