Laboratory Assessment and Field Pilot of Near Miscible CO2 Injection for IOR and Storage in a Tight Oil Reservoir of ShengLi Oilfield China

Gas injection into tight oil reservoirs, as a secondary recovery technique, can be favorable and promising in terms of high gas injectivity and good displacement/sweeping efficiency over water injection. Particularly, CO2 injection is the best option due to its superior miscibility effect with oil and in consideration of geological storage of the greenhouse gas. In this study, CO2 injection into a tight oil reservoir for IOR is assessed and a pilot project is underway. The reservoir is located in the G89 Block of Shengli Oilfield East China, which has very poor water injectivity due to very low permeability of less than 5 mD in average, and has been producing via natural depletion since 2005. The original reservoir pressure was over 40 MPa, and the reservoir temperature of 126 . A CO2 injection and storage program has been proposed, and CO2 will be from a coal-fired power plant 30 km away under a Sinopec’s CCS (Carbon Capture and Storage) scheme. Laboratory investigation includes PVT experiments, slim tube test and core flooding/displacement experiments, in order to study the miscibility effect and displacement efficiency via CO2 injection at various conditions. Reservoir simulations were performed to predict the IOR potentials of CO2 injection at different pressures, namely at immiscible, miscible and near-miscible modes. The MMP (Minimum Miscibility Pressure) of the reservoir oil is determined as over 29 MPa, while the reservoir pressure at the beginning of CO2 injection was around 23 MPa after several years’ depletion. Therefore, CO2 flooding at a near miscible mode will prevail. A field pilot of CO2 injection at current reservoir conditions (at near-miscible mode) is designed and its performance is presented in the paper. Introduction Injection of CO2 into oil reservoirs may develop different displacement or flooding processes, namely miscible, immiscible and near-miscible, which mainly depend upon the operating pressure, oil and reservoir characteristics. Miscible displacement is the most preferable mechanism for oil recovery with respect to high displacement and sweeping efficiency (Jeschke et al. 2000). However, miscible displacement is only realizable at pressure greater than MMP; therefore miscibility can be difficult to develop in many cases, such as mature oilfields due to the depletion of reservoir energy, and in low permeability reservoirs where a large pressure drop or difference may develop from injector to producer. The miscibility of oil and CO2 is also affected by compositions of crude oil and purity of CO2, rock heterogeneity and gravity segregation (Johns et al. 2002). Compared with miscible flooding, immiscible flooding is less effective, particularly for reservoirs with serious heterogeneity, in which high permeability channels, inverse mobility ratio and viscous “fingering” can lead to early gas breakthrough (Aziz et al. 2009; Zhang et al. 2010). For those reservoirs with relatively low pressure and less favorable for miscibility, a near-miscible process might be developed to maximize the interactions of oil and CO2 and achieve high oil recovery. In recent years, the concept of near-miscible CO2 flooding has been received more and more attention. The research of near-miscibility effect can be tracked back to early 1980s’, when Orr and colleagues (Orr et al. 1981) published their studies on phase behavior of CO2/crude oil mixture, they indicated that the inflexion point in the slim-tube experimental curve may not necessarily signify the transition from immiscible to miscible displacement, instead it can represent a near miscible state because a full miscibility is not achieved, whereas this “near-miscible” phase behavior can be also good for effective oil displacement. Shyeh-Yung and J-G, J (1991) pointed out that a near-miscible CO2 IOR process has potential to economically recover oil. Hadlow (1992) stated that application of CO2 flooding in reservoirs where a near miscible condition exists can be one of the most significant opportunities for utilization of CO2. Case studies by Thomas et al. (1994) showed that many reservoirs historically designed as miscible flooding would be better described as near miscible. It