| Global warming caused by anthropogenic CO2 emissions into the atmosphere from using vast amount of fossil fuel energy is becoming most serious energy and environmental problem nowadays.Countries in the world now are trying best to conserve energy and reduce emissions,at the meantime,efficient actions are taken to tackle with global warming.Emission reduction of main greenhouse gas CO2 can be achieved efficiently via CO2 geological storage and CO2 enhanced oil recovery methods.The scientific and technical issues involved in these methods have aroused wide interest among researchers.During CO2 saline aquifer storage process,the gas-liquid-solid interactions such as interfacial tension and wettability of caprock determine the fluid distribution on the surface of reservoir rocks as well as the magnitude and direction of the capillary pressure,that in return determines the injectability,sealing capacity and safety of this scheme.In order to better predict the storage capacity to evaluate the storage safety,this work aims at carrying out the laboratory experiments and modelling work on the interfacial tension of CO2-brine/oil system,wettability of caprocks,solubility of gas-liquid binary systems and pH of CO2 saturated brine and as well as CO2-oil minimum miscibility pressure under the reservoir temperatures and pressures condition.Based on experimental data,the seepage characteristics of CO2 and brine in porous media were investigated via pore network modelling under the CO2 geological storage condition.The influence of the wettability of caprock and the interfacial tension on relative permeability and capillary pressure curves were investigated.The interfacial tension and contact angles measurement system based on the drop shape analyse method was developped.The axisymmetric drop shape analysis technique was used to measure the CO2-brine/oil interfacial tension.According to the experimental results,the CO2-brine interfacial tensions increased with the temperature and molality and decreased with the pressure until reaching a plateau when pressure larger than a threshold pressure,where interfacial tensions are independent from the pressure.A linear relationship between the increase in average interfacial tension and molality was observed and it is a function of the ionic type.Finally,modified empirical correlations based on experimental data,using only few regression coefficients with a relatively low error for most of the experimental data,were presented to estimate the CO2-brine interfacial tensions under wide range of temperatures,pressures,and the ionic strength.Then,the sessile drop method also based on the drop shape analyse method was used to measure the contact angles of Berea sandstone,limestone and quartz samples covering CO2 in the gaseous,liquid,and supercritical phases.The wettability alteration of the rock surfaces with the temperature,pressure,molality and in the presence of supercritical CO2 was systematically investigated.The contact angles of the different rock samples varied with the temperature and pressure due to the difference in the substrate,and roughness.However,all the three rock samples became less water-wet when the CO2 phase state changed from subcritical to supercritical,thus the wettability alternation at the CO2 phase change region was observed.Based on the experimental data in this work and literatures,the original Spycher&Pruess solubility model has been modified using Peng&Robinson Equation of State,then new solubility and interfacial tension models for CO2-water/brine binary system were developed.The average absolute relative deviation AAD%between solubilty model and experiment was reduced from 5%to 2%with the modified version.Then,the pH of CO2 saturated KCl solutions obtained from the geochemistry model PHREEQC based on Pitzer modelling were compared with the measured pH values obtained via electrometric method using the high pressure and temperature electrode probes.The experimental results indicate that the pH of the CO2 saturated KCl solutions decreased with pressure and the molality of KCl solutions while they increased with the temperature.The pH of CO2 saturated KCl solutions is larger than the pH of the CO2 saturated NaCl solutions under same conditions.That indicates the saline acquifer containing high molality of K+ions will reduce the degree of salt water acidification after CO2 injection,that helps to improve the sealing efficiency of the caprock.The drop shape analyse method was also used to measure the interfacial tension during CO2-oil mixing process,and the minimum miscibility pressure was estimated based on vanishing of the interfacial tension.The experimental results indicate that the CO2-oil interfacial tensions decreased sharply with pressure until gas-oil interface disappeared where CO2-oil interfacial tension is zero and it is considered as the CO2-oil minimum miscibility pressure.CO2-oil interfacial tensions increased with temperature at higher pressure but decreased with temperature at lower pressure in the experimental pressure range.They also increased with carbon number of the alkane at high temperatures.The CO2-alkane interfacial tension exhibited the weakest dependence on the temperature at the high pressure(4-6 MPa)and the high temperature.Moreover,the linear increase of the CO2-oil minimum miscibility pressure with the temperature was observed and slops increased with the increasing of carbon number.Both linear regression and vanishing interface methods can be used for the estimation of the CO2-oil minimum miscibility pressure.An improved maximal ball method was used to extract pore networks of CT images of quartz,feldspar,dolomite,and Berea sandstone samples.CO2-brine-rock system interfacial tension and contact angle parameters obtained from above experimental results and modelling work were used as input parameters for gas-liquid two phase flow simulation based on the pore network model,then relative permeability and capillary pressure curves of different rock samples under reservoir conditions were predicted.It was observed that the hydrophilic reservoir is the prior sealing site for long and safe storage of a large amount of supercritical CO2 duo to its better gas-liquid seepage characteristics and more conductive to CO2 capillarity capture.Overall,in this study a large amount of experimental and predicted thermodynamic properties,such as interfacial tension,contact angle,pH and solubility of CO2-brine-rock system were obtained,that will provide data base and theoretical support for the feasibility of CO2 long&safe storage capacity assessment and the storage path prediction scheme for the reservoir. |
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