Study On The Process Model Of CO₂ Migration And Phase Transformation In Enhanced Oil Recovery System

Carbon dioxide enhanced oil recovery（CO₂-EOR）is considered to be one of the most important engineering tools to reach the strategic aim of"carbon neutrality".On the one hand,CO₂ will be sequestered in four phases,including supercritical,liquid,oil and carbonate solid phase,to achieve a certain scale of geological sequestration;on the other hand,the pressure gradient formed by the injection and the special physicochemical properties of CO₂ can improve the recovery of crude oil,making it an important tertiary oil recovery method.However,most of the present studies have ignored the characteristics of CO₂ acidic gas,and not well characterized the CO₂-water-rock geochemical reactions and carbon migration and transformation processes after CO₂ injection,so it is important to study the migration,transformation and phase fate of CO₂ during the oil flooding process to quantitatively evaluate the carbon sequestration potential and oil recovery efficiency of CO₂-EOR technology.In this paper,the interaction mechanism between CO₂ and multi-component oil phase,reactive solute transport mechanism and water-rock-gas geochemical reactions were further clarified by using theoretical analysis.Based on the existing international simulation program TOUGHREACT,we developed the multi-phase multi-component THC（Thermal-Hydrogeological-Chemical）coupled program TOUGHREACT-EOR that can systematically consider the reaction kinetics of mineral dissolution and precipitation in CO₂-EOR process by adding the mass conservation equation of oil phase components and phase equilibrium flash calculation,and based on the more comprehensive mineral dissolution and precipitation modules and the database of thermodynamic and reaction kinetic parameters included in the original program.TOUGHREACT-EOR was validated using experimental data of CO₂ flooding and compared with CMG（Computer Modelling Group）software.The validated program was applied to the miscible flooding system in the Daqingzi well block and the immiscible system in the Xinli block of Jilin Oilfield to analyze the migration,transformation and phase fate of the injected CO₂ in the miscible and immiscible systems,respectively,and to clarify the effect of gas-water-rock chemistry on the carbon sequestration and recovery efficiency of CO₂-EOR immiscible flooding.Considering that most of the actual processes are immiscible or partially miscible,a sensitivity analysis on the effects of carbon sequestration and oil recovery efficiency in immiscible systems was carried out.From the above work,the main innovative achievements of the paper are summarized as follows:（1）The interaction mechanisms between CO₂ and multi-component oil phase,multi-component reaction solute transport,and water-rock-CO₂ chemical reaction process were clarified,and the mathematical model of CO₂ migration and transformation between supercritical phase-water phase-oil phase-carbonate mineral phase was established.（2）The verification indicate that the program TOUGHREACT-EOR can well characterize the CO₂ oil flooding and geochemical processes,and the applicability of the program at the site scale is verified by comparing with the actual measured data of site.The advantage of TOUGHREACT-EOR over other component simulators is that1)the program takes into account the reaction kinetics of mineral dissolution/precipitation rates as a function of mineral content,pH,liquid phase physical characteristics,specific surface area,and pressure,and provides a more accurate depiction of the carbon migration and phase transformation processes during CO₂-EOR;2)it invokes the well-developed mineral dissolution and precipitation module of the original TOUGHREACT program,which can comprehensively consider the possible secondary minerals in the CO₂-EOR process;3)it inherits the rich database of mineral reaction kinetics and thermodynamic parameters of the original program,and users can directly build the initial geochemical system of the model by adding the ions and minerals,and correct the initial geochemical system through the program’s water-rock equilibrium calculations to reduce the errors caused by sample testing.Overall,the program TOUGHREACT-EOR is a good choice for studying the CO₂ geological storage in in oil-bearing multiphase flow systems.（3）By applying the program TOUGHREACT-EOR to Jilin oilfield,the necessity of chemical field consideration in the application of CO₂-EOR technology was clarified,and the influence of geochemical reactions on oil recovery efficiency,CO₂ migration and transformation and phase distribution was quantitatively evaluated for the first time.In addition,the sensitivity analysis of CO₂-EOR project was carried out considering both carbon sequestration efficiency and recovery efficiency.The effects of engineering conditions and formation conditions on CO₂ migration and transformation,phase distribution and recovery efficiency were quantitatively evaluated.This work provided the theoretical basis and technical support for the optimization of CO₂-EOR as a CCUS technology to realize considering both sequestration efficiency and oil production efficiency.