不同油藏压力下CO2驱最小混相压力实验研究

CO2-原油体系的最小混相压力是影响CO2驱开发效果的关键因素。随油藏开发阶段的不断深入,当油藏压力低于原始饱和压力后,溶解在原油中的溶解气会部分脱出。油藏流体组分及其高压物性也会发生变化,影响CO2-原油体系的最小混相压力,利用原始地层流体样品测试得到的最小混相压力不再适用。为此,以中国西部某油田8个典型区块为例,进行细管实验测试和多组分数值模拟,对不同油藏压力下的最小混相压力进行系统研究。与其他油田相比,研究区各油藏油样的C1摩尔含量较高,为31.12%~51.69%,平均为43.25%;C2-C6摩尔含量较低,为8.0%~18.48%,平均仅为11.3%。细管实验和数值模拟结果表明,在原始地层压力下,CO2均与8个典型区块地层原油样品发生混相驱替,但不同区块CO2驱最小混相压力差异很大,其值为17.60~41.18 MPa。当油藏压力低于原始饱和压力后,CO2驱最小混相压力主要呈微小幅度下降的趋势。随脱气压力进一步降低,油相组分构成中,C1N2摩尔含量呈递减趋势、C7+和C24+组分呈递增趋势,而中间组分(C2和C3+)摩尔含量变化较小。在各级脱气压力下,脱出气体以C1为主,中间组分摩尔含量仅在最后一级脱气压力下急剧升高。CO2-原油混相带出现在注入CO2波及前缘靠近注入端的位置,混相带随着驱替的进行而逐渐变宽。

Study on minimum miscibility pressure of CO2 flooding at different reservoir pressures

The minimum miscibility pressure(MMP)of the CO2-crude oil system is a key factor affecting the development effect of CO2 flooding. When the reservoir pressure is lower than the original saturation pressure,the part solution gas dissolved in the oil will be separated from the oil with the development of oil reservoir. Thus,the composition of oil phase and its high-pressure physical properties will also change,which will affect the MMP of CO2-crude oil system. The MMP obtained from the original formation oil sample is no longer applicable. Based on the slim tube experiment and multi-component numerical simulation,the MMPs of crude oil samples at different reservoir pressures in eight typical blocks of an oilfield in western China are systematically studied. Compared with other oilfields in China,the C1 molar content of oil samples from various reservoirs in the study area is higher,ranging from 31.12% to 51.69%,with an average of 43.25%;while the molar content of C2-C6 is lower,ranging from 8.0% to 18.48%,with an average of only 11.3%. The experimental and simulation results show that CO2 can be miscible with the oil samples from eight typical blocks under at original reservoir pressures,but the MMPs of CO2 flooding in various blocks have significant differences,ranging from 17.60 MPa to 41.18 MPa. After the reservoir pressure is lower than the original saturation pressure,the MMPs in CO2 flooding of typical reservoir oil samples tend to decrease slightly. With further decrease of degassed pressure,the molar content of C1 N2 in the oil phase decreases,the molar contents of C7+and C24+increase,and the changes of C2 and C3+are minor. C1 is the main component of the separated gas at different pressures. The intermediate component increases sharply only at the last degassing pressure. CO2-crude oil miscible zone occurs at the swept front of the injected CO2 close to the injection side. The miscible zone becomes wider with the displacement.