鄂尔多斯盆地长7致密油储层二氧化碳驱油实验

鄂尔多斯盆地长7致密油储层具有致密和低压的特征,采用常规注水开发存在采收率低的问题,从而制约了致密油的开发效果。针对鄂尔多斯盆地长7致密油储层注水开发采收率低的问题,基于CO_2驱油细管实验、原油流变性测试实验、CO_2浸泡岩心实验以及岩心驱替实验,并结合润湿接触角测试方法和核磁共振成像技术,研究了长7致密油储层CO_2驱油的增产机理。研究结果表明:长7致密油最小混相压力为23.9 MPa,在长7致密油储层CO_2驱过程中,注采井间CO_2非混相驱占主导,在注入井附近局部区域可能出现混相驱;在地层温度压力(75℃,18 MPa)条件下,未溶解CO_2原油的黏度为8.87 mPa·s,溶解CO_2的原油黏度为7.99 mPa·s,其黏度降低幅度为9.9%;CO_2水溶液浸泡24 h后,长7致密砂岩的润湿接触角从66.1°降低到54.0°,亲水性增强;水驱致密砂岩岩心的驱油效率为47.2%,CO_2的驱油效率为71.5%,较水驱提高驱油效率24.3%,且致密砂岩渗透率越高CO_2驱油效果越好。实验证明CO_2驱可以显著提高长7致密油储层的驱油效率,是长7致密油高效开发的重要技术。

Experimental investigation on CO2 flooding in the Chang-7 tight oil reservoir of Ordos Basin

Chang-7 tight oil reservoir in Ordos Basin is characterized by low permeability and low formation pressure. Conventional water injection development method has a low recovery rate problem, thus restricting the development effect of tight oil. In order to address the problem, the production stimulation mechanism of CO2 flooding in Chang 7 tight oil reservoir was studied based on the slim tube experiment, crude oil rheology test experiment, CO2 soaking core experiment and core displacement experiment, combined with wetting contact angle test, and magnetic resonance imaging (MRI) technology. Experimental results show that the minimum miscibility pressure of Chang-7 tight oil is 23.9 MPa. When applying CO2 flooding to the development of Chang-7 tight oil reservoir, CO2 immiscible flooding is dominant, and miscible flooding may occur in the limited area near the injection well. Under the temperature and pressure conditions of Chang-7 formation (75 °C and 18 MPa), the viscosity of crude oil is 8.87 mPa.s, and that of CO2-saturated crude oil is 7.99 mPa.s, with a viscosity reduction of 9.9%. After soaking in CO2-saturated brine for 24 h, the wetting contact angle of Chang-7 tight sandstone decreases from 66.1° to 54.0°, and its hydrophilicity increases. The oil displacement efficiency of water flooding for Chang-7 tight sandstone cores is 47.2%, and that of CO2 flooding is 71.5%, which is 24.3% higher than that of water flooding. The higher the permeability of tight sandstone, the better the oil displacement efficiency of CO2 flooding. The obtained results indicate that CO2 flooding can significantly improve oil displacement efficiency and is an important technology for efficient development of Chang-7 tight oil resources.