Pore To Core Scale Influence Mechanism Of Wettability And Heterogeneity On CO₂ Sequestration In Saline Aquifers

CO2 sequestration in saline aquifers is a quick and effective way to achieve the carbon emissions reduction target and mitigate the green house effect,because of its huge storage potential.Micro-scale CO2-brine porous media flow experiment is very important for CO2 storage potential and security evaluation since it can give the most fundamental displacement mechanism.Owing to the complexity of the mineral composition and structure,investigation of wettability and heterogeneity is essential to CO2 sequestration.In this study,we studied the influence of wettability and heterogeneity on CO2-brine flow at pore to core scale.An X-ray CT experimental system was set up and the CT image processing technique was developed.The wettability of porous media was measured in different rock types,temperature,pressure and salinity conditions at pore scale.The migration and morphology of CO2 and brine phase with the effects of wettability and heterogenenity were also investigated.The specific contents are as follows.We designed and set up the three-dimensional X-ray CT and two-dimensional glass micromodel visual platform.A systematic X-ray CT image processing technique specially for porous material in liquid-gas system was developed based on the local threshold segmentation method,watershed method.This method solved the issue of inaccurate segmentation because of the close gray value distribution between water and gas in traditional methods.The important parameters such as pore-throat distribution,tortuosity,and phase surface curvature,were measured in glass,quartz,dolomite and feldspar sand packs using this method.An in-situ pore-scale contact angle measurement method for real porous media was developed using micro-CT.The pore-scale contact angle of the porous media with single mineral composition has a normal distribution.We found the contact angle hysteresis between drainage and imbibition mainly because the different flow pattern in the two process caused CO2-brine interface shrinkage or expansion.The increasement of salinity,temperature and pressure make the rock wettability weak.The influence of wettability and heterogeneity on drainage was studied.The displacing front of CO2 become more and more instable with front development under heterogeneous condition.There is a linear relationship between slice-averaged porosity and CO2 saturation while the 'barrier effect' of porosity distribution weaken this correlation.In upward drainage,the influence of heterogeneity on CO2 phase distribution become stronger in a higher injection rate,while it become weaker in a higher injection rate in downward drainage.The local displacing front become more instable in more hydrophilic porous media under the same experimental condition.The continuity of the wetting phase,brine,become worse when decrease the hydrophily of rock surfaces,which decreses the interface area between brine and CO2 phases.In imbibition,the pore-scale displacing mechanisms,piston-like front and CO2 snap off,were both observed in glass bead and quartz sand packs.CO2 capillary trapping is likely to happen in the porous area with wettability and structure transformation.A new pore-scale displacing mechanism,CO2 droplet fragmentation,was also observed.CO2 doplet fragmentation is prone to happen in the high inject rate and large pore structure with long and narrow throat condition.It forms many global drops in a single pore,increasing the interface between CO2 and brine,which is benefitial to CO2 dissolution trapping.