The Effective Prediction Approach for Dynamic Geological Storage Quantity of CO2 in CO2-EOR Reservoir

Abstract

In the CO2-EOR reservoirs, the complex physical and chemical actions between CO2 and minerals, formation water and crude oil lead to the geological storage of CO2, which can be respectively sealed in four ways: structural trapping, residual trapping, solubility trapping and mineral trapping. Due to the implementation of CO2's injection process, pressure, time and CO2-water alternating flooding, the amount of CO2 in four ways has dynamic change. Structural and residual trapping are the dominant forms in the early period. The amount of CO2 in the form of solubility and mineral trapping will increase after a long period. Therefore, it is necessary to analyze the main factors that affect the amount of CO2 in each way, and to explore the technical methods for predicting the amount of CO2. The related research shows that the core simulation experiments will help to predict the mineral composition in reservoir conditions. The core simulation experiments, time-lapse well logging and numerical simulation analysis will help to predict the amount of CO2 in the form of structural and residual trapping at different periods corresponding to the conditions of temperature and pressure in CO2-EOR reservoirs. The amount of CO2 in structural trapping is mainly determined by effective porosity, CO2 saturation, effective thickness and trap area. The amount of CO2 in residual trapping depends on the rock specific surface and micro-pore structure, such as the maximum throat radius, the average throat radius and so on. At the same time, according to the temperature and pressure of reservoir in different periods, the amount of mineralized CO2 in reservoir with different mineral components, that is, solubility trapping, will be predicted by core simulation experiment, time-lapse well logging and computational chemistry analysis. The amount of mineralized CO2 is mainly controlled by the concentration of CO2, the composition of the reservoir minerals, temperature and pressure condition in reservoir. Furthermore, the amount of CO2 dissolved in water and remaining oil will be predicted by core simulation experiment, field measurement and numerical simulation analysis according to the corresponding hydrochemistry, water salinity and remaining oil characteristics in different periods in the CO2-EOR reservoirs. Reservoir temperature, pressure, hydrochemical characteristics, water salinity, residual oil properties and CO2 concentration control the amount of CO2 in solubility trapping. Through the above steps, the dynamic storage quantity of CO2 in different periods will be predicted. CO2-EOR can not only improve oil recovery, but also promote low carbon environmental protection [1,2,3,4]. In the process of CO2-EOR, complex physical and chemical processes occur between CO2 and rock minerals, pores, crude oil and formation water in reservoirs. CO2 was sequestered respectively in the form of structural trapping (unbound state), residual trapping (bound state), solubility trapping (dissolved in crude oil and formation water) and mineral trapping [5,6,7,8,9]. In view of CO2 being sealed in any one of those forms, the amount of CO2 storage in different forms and its influencing factors will change in different periods, which will lead to the dynamic variation of CO2 geological storage amount. Some scholars believe that the physical capture process such as structural trapping, residual trapping and solubility trapping will gradually decrease with time, while the role of geochemical capture processes, such as solubility trapping and mineral trapping, will gradually increase. In the meantime, the safety of storage will be gradually improved. Therefore, on basis of analyzing the influencing factors of CO2 sequestration in different periods and exploring the corresponding prediction methods, it will be beneficial to form an effective way to predict the dynamic geological sequestration of CO2.