Study On The Interaction Between CO 2 Sequestration And Enhanced Oil Recovery In Condensate Gas Reservoirs

CO2 injection into gas condensate reservoir to enhance condensate oil recovery and implementing CO2 sequestration is a win-win initiative to achieve emission reduction and utilization of greenhouse gas, which is petroleum industry and environmental protection's research hotspot. Research on the effection of multiphase flow, reservoir characteristics, injection and production parameters on CO2 storage capacity and sequestration mechanism in the process of CO2 EOR are very scare, what's more, previous study on CO2 injection into gas condensate reservoir usually ignored the formation water, there is a certain deviation from the actual situation. Through phase behavior simulation, two-dimensional cross-sectional and three-dimensional numerical simulation, phase behavior of CO2 injection into gas condensate reservoir; effect of formation water on the phase behavior of CO2 injection into gas condensate reservoir; impact of injection pressure, relative permeability hysteresis and reservoir heterogeneity on condensate oil recovery and CO2 storage efficiency were performed on a actual near-critical gas condensate reservoir, the paper had gain insight into EOR mechanism by CO2 injection and CO2 sequestration mechanism migration rule as well as the coupling relationship between them. The results achieved and recognized are as follows:(1) During the pressure depletion process, the heavy components occupy an increasing proportion of formation fluid, and the condensate becoming heavier, when the pressure is reduced to a certain pressure point, formation fluid undergoes a phase transition from condensate gas to volatile oil.(2) CO2 injection can effectively reduce the retrograde condensate volume, and the greater the amount of CO2 injection, the more condensate oil saturation decreased, the lower the condensate gas compression factor, thus increasing the CO2 storage capacity, namely, large amount of CO2 injection conducive to CO2 sequestration. Moreover, with the increase of CO2 injection volume, the fluid is transformed from wet phase to dry phase.(3) The reservoir pressurized due to CO2 injection, so that part of the condensate re-evaporation, and through extraction of middle and portion of heavy hydrocarbon of the condensate oil into the gas phase by CO2, re-evaporation of condensate achieved.(4) The mount of retrograde condensation with the presence of formation water was higher than that without the presence of formation water at same depletion pressures, and the fluid tends to be heavy qualitative, for this particular near-critical gas condensate reservoir fluid, the phase transition from gas condensate to volatile oil and back to gas occurs earlier in the isothermal decompression process. During the CO2 injection period, the presence of formation water decreases the evaporation of condensate oil by CO2, which cause 10%lower condensate oil recovery, however, with the presence of formation water, CO2 breakthrough time delay, leading to an increasing total volume of CO2 sequestration and a reducing amount of CO2 exist in free state (supercritical state) at the same time, so that CO2 can be stored safer.(5) When considering hysteresis, CO2 injection for increasing the degree of condensate oil recovery decreased, but increased the total amount of CO2 sequestration. The volume of trapped CO2 increases, the amount of dissolved CO2 in the water is more, which enhanced dissolution sequestration mechanism.(6) Reverse rhythm reservoir has latest CO2 breakthrough time, highest condensate oil recovery, and the largest total amount of CO2 sequestration. Based on this, reverse rhythm reservoir is the most appropriate option to implement CO2 injection EOR while perform CO2 sequestration at the same time.(7) The higher injection pressure will result in higher condensate oil recovery and greater CO2 gas deposit rate, however, the higher the pressure, the sooner CO2 breakthrough, the lower the oil change rate; as to gas injection rate, under same CO2 gas deposit rate, the higher the gas injection rate, the lower condensate oil recovery, when under the same condensate oil recovery, CO2 gas deposit rate increases with high gas injection rate, but the amplification become smaller and smaller.(8) The density difference between CO2 and gas condensate is the main factor causing the flow of CO2, CO2 will tend to migration towards the bottom of the reservoir and low construction site, thus CO2 can achieving secure and stable sequestration in the bottom of the reservoir.