Study On Mechanism Of Increasing Oil Recovery By Gas Injection In Kekeya

After years of natural depletion in X52volatile oil reservoir, the crude oil is significantly degassing, the volume of volatile oil obviously shrinks, producing gas-oil ratio increases sharply. The stratum energy is low currently (formation pressure has been at or near abandonment pressure), productivity declined sharply (annual natural decline is25%). The recovery of the reservoir is low, the remaining reserves are large, and the adjustment of stabilized production is difficult. The study of EOR mechanism of volatile reservoir in mid-to late development period is very important, especially by dry gas injection.The content of this paper is from PetroChina Tarim Oilfield Company reservoir development research, aims to explore the EOR mechanism in mid-to late development period of depleted volatile oil reservoir, and to provide technical support for plan programming and implementation of Kekeya volatile reservoir secondary development. The conclusions obtained from related work are as follows:1. The components of X52volatile reservoir are lighter fluid with high GOR, formation fluid P-T phase diagrams show that it belongs to light crude oil. Dynamic phases of injecting hydrocarbon gas and CO2show the near-critical volatile oil system changes into near-critical condensate oil system by injecting hydrocarbon gas and presents the the characteristics of volatile oil by injecting CO2with increasing proportion of gas injection.2. The rising range of crude oil bubble point pressure of X52volatile reservoir gradually increases, but the overall increase is small. The compatibility of formation oil is good with injecting hydrocarbon gas. When injecting the hydrocarbon gas, the bubble point pressure of crude oil rises, the corresponding volume coefficient of crude oil increases, injected gas is conducive to solubilization flooding.3. The visual characteristics of phase behavior change of X52volatile oil reservoir with injecting gas have been obtained, the mechanisms of EOR by recovering the original formation pressure and keeping pressure with gas injection are mutually soluble extraction, solubilization capacity, viscosity reduction, density reduction and multi-contact miscible flooding. The phase simulations of multiple contact flooding show that the oil flooding is multi-contact immiscible under the current reservoir pressure (close to11.4MPa). When the injection pressure increased to49.4MPa, it is multi-contact near miscible and miscible flooding. Near miscible flooding can be achieved under reservoir condition by increasing gas injection pressure properly.4. The multi-contact miscible pressures of remaining oil by injecting hydrocarbon gas and CO2from experimental tests and phase simulations have been obtained, miscible flooding can be achieved by recovering reservoir pressure to22.33MPa by injecting CO2for X52volatile reservoir, but it is44.83MPa(exceed original reservoir pressure) by injecting hydrocarbon gas. Therefore, near miscible flooding can be achieved by injecting hydrocarbon gas under original formation pressure for X52volatile reservoir.5. Long-core flooding experiments show that development effectiveness of injecting water is better than CO2and hydrocarbon gas when the reservoir pressure depletes to present reservoir pressure11.4MPa, and the effectiveness of natural depletion is the worst. As the X52reservoir with strong water sensitivity, water flooding should not be used in the development of it. However, the recovery factors of increasing pressure to20MPa by injecting CO2and hydrocarbon gas and soaking for12hours increase19.02%and7.8%respectively. Thus, the recovery of increasing the pressure by injecting CO2and hydrocarbon gas can be greatly improved.Therefore, for Kekeya X5volatile reservoir, according to the reservoir conditions and the actual situation, miscible flooding by injecting CO2and near miscible flooding by hydrocarbon should be considered, the recovery of injecting gas and soaking for a period of time can be greatly improved. We recommend the development method of using CO2injection and soaking.