CO₂ Flooding And Geological Storage Mechanism In Oil Reservoir

CO2 storage and EOR are the most economical and effective measures to reduce CO2 emission and to respond to the environmental issues. CO2-EOR technology has been utilized in China’s Jilin Oilfield, with initial success of higher recovery. However, depth research has not been carried out on CO2 storage mechanism, storage capacity and migration regularity in oil reservoir. Therefore, the purposes of this study are to reveal the CO2 storage mechanism, storage capacity and migration regularity by combining laboratory with numerical simulation methods, which would result in providing a theoretical basis for program design and capacity evaluation of CO2 sequestration in oil reservoir. In addition, the study is part of a major national S&T project, "CO2 flooding and sequestration key technologies" (2011ZX05016)",Some works have been carried out as follow.(1) An experimental evaluation method about CO2 storage mechanism has been established. And experiments on CO2 dissolution in oil and water have been launched by using high-temperature and high-pressure reaction kettle and PVT device. Then based on the experimental results, the PR-HV Eos model’s paraments have been modified, and the solubility charts of CO2 haver been drawn at different pressure, temperature and different salinity of formation water. The experiments on CO2-water-rock mineralization storage mechanism has been achieved, using X-ray diffraction instrument, SEM instrument and ionic chromatograph. The storage mechanism experiments of supercritical CO2 and irreducible CO2 have been developed by using the long core experimental apparatus. Therefore, the effects of formation slope angle, permeability, injection pressure, CO2 purity and injection process have been analyzed by contrast for CO2 flooding and storage mechanism.(2) Based on the composition model, a mechanism model of CO2 displacement and storage is established. Some factors on CO2 flooding and sequestration have been analyzed, such as slope angle of formation, size of water, permeability, heterogeneity, dissolution, mineralization reaction, the most residual gas saturation, shape index of relative permeability curve, purity of gas, injection pressure and injection method and so on.(3) CO2 migration regularity and storage capacity of H59-12-6 well group model, which is a part of Jilin oilfield model, have been studied.Some results and acknowleges have been obtained as follow:(1) Based on experimental results of CO2 solubility in formation water, PR-HV model parameter has been modified to obtain CO2 solubility chart in formation water considering temperature, pressure and salinity, (chapter 2)(2) It is revealed that the solubility of CO2 in crude oil and formation water is affected by water saturation when considering both of oil and water through expansion experiment on CO2-water-oil dissolved with each other. The larger the water saturation is, the smaller the solubility, (chapter 3)(3) The CO2-rock mineralizing experiments results have shown that calcite could be dissolved by the carbonic acid solution which is generated by chemistry reaction of CO2 and formation water, affecting pore size and pore structure and resulting in bigger pores. CO2-water-rock chemical reaction results have showed that the ion contents in water would increase, especially for the contents of Na+, K+ and HCO3-. It is indicated that mineral composition of rock has been dissolved by carbonic acid, (chapter 4)(4) Based on the long core experiments results, the bigger the slope angle is, the greater the CO2 storage capacity and the oil displacement efficiency. The greater the permeability, the higher the oil displacement efficiency. As is known, the breakthrough time of CO2 can be effectively delayed after WAG injection. Therefore, CO2 could been effectively stored in the oil reservoir and CO2 storage capacity and the oil recovery could be increased. The earlier CO2 flooding begins, the higher storage capacity and oil recovery for flooding reservoir, (chapter 5)(5) The mechanism model numerical simulation results showed that a bigger slope angle, a smaller the reservoir water, a greater permeability, a weaker chemical and mineralization reaction, a higher purity of CO2, and a larger injection pressure cause a larger storage capacity and oil recovery for flooding reservoir, (chapter 6)(6) The storage capacity and CO2 distribution/migration regularity results of H59-12-6 well group showed that a largest storage is obtained with a gas injection rate of 25000 m3/d, and CO2 storage rate increases while the gas injection rate decreases. The effect of different gas injection rates on the final oil recovery is small with a medium value of 24.6%.It’s easier for those CO2 injected into the oil-storey part of the reservoir to migrate up-dip. And it’s easier for CO2 injected into lower part of the reservoir to migrate down-dip. Faster CO2 spread rate during the production of crude oil, lower molar content of CO2 is. After shut-in of the production wells, the molar concentration of CO2 increased rapidly, but spread rate decreased. Ph of the reservoir overall would decrease from 6.5 to 4.6 after 150 years of sequestration, (chapter 7).