Mechanism Investigation On The Interactions Of Supercritical CO₂-Brine-Rock Of Continental Tight Reservoir

The tight oil is a conventional oil found within reservoirs with extremely low permeability,which provides an alternative to mitigate the conflicts between rapid energy supply.China shows large reserves of tight oil,which is widely distributed but difficult to develop.Among the proposed EOR methods,gas injection especially CO2 injection has been recognized as a promising method to improve oil recovery and CCS especially for tight reservoirs.The injection of CO2 into reservoirs will interfere the initial equilibrium of the subterranean environment and trigger chemical interactions between CO2,formation brine and reservoir rocks.These interactions would consequently change the geochemical and physicochemical properties of the target reservoirs,and may also affect the performance of CO2-EOR and sequestration.Hence,knowledge of the interactions triggered by the injected CO2 as well as the resultant alterations of the reservoirs is highly crucial for the success of these activities.This work targeted a representative tight sandstone reservoir in China,locating in the Lucaogou formation of Jimsar sag,Junggar Basin.The solubility of CO2 in the formation brine was first determined in a high temperature and high pressure cell followed by CO2-exposure experiments using reservoir rocks.Then the dynamic corrosion and multicycle corrosion experiments were tested under the reservoir pressure and temperature.CO2 adsorption behavior in tight sandstone rock under different pressure and temperature was also conducted.The solubility of supercritical CO2 in the formation brine increased with equilibrium pressure before finally attaining saturation at 12.7 MPa.The maximum solubility of supercritical CO2 in this brine was 0.94 mol/kg.The results showed no signs of precipitants and the solution remained optically transparent at the end of the experiments.The interactions between CO2,brine and rock led to the dissolution of surface minerals into the brine(0.3 wt%of mass loss in our work).The analysis of the brine composition after the experiments showed an increase in K+,Na+and the precipitants in the brine after the experiments were mainly composed of iron-bearing and clay minerals.The contact angle measurements indicated that the wettability of the rock surface was altered to be more water-wet.The improved affinity of the reservoir rock for water can be ascribed to the dissolution of surface minerals,kaolinite formation and surface corrosion.The results of dynamic corrosion and multicycle corrosion showed that the permeability and porosity increased with the pressure and corrosion time.The clay minerals and solid particles generated in the dissolution process migrated in porous media would block the pores and channels,which caused reduction of core permeability temporarily.The improvement of permeability for dynamic corrosion performed better than multicycle corrosion and corrosion reaction mainly occurred in big pores according to the NMR tests.Isothermal adsorption method was used to measure the adsorption capacity of CO2 under different pressure and temperature.The results indicated that CO2 adsorption capacity was increased with pressure before finally reaching saturation at 11.0 MPa.As temperature increased,CO2 adsorption capacity increased first and then decreased.Extended-Langmuir model and Langmuir-Freundlic model performed better fitting precision than Langmuir model and Freundlich model.The fitting accuracy of four models under low temperature performed better than high temperature according to our study.The results provide convincing evidence of the geochemistry changes in this tight reservoir caused by CO2,brine and rock interactions,which may be of significant importance to CO2-EOR and CO2 sequestration activities in this site and other similar reservoirs.