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N 2 or N 2 /CO 2 mixture is considered as injection gas to enhance oil recovery from Tahe oil field. The reservoir pressure and temperature are around 66 MPa and 414 K. The target light crude oil is produced from S117 well of the Tahe oil field. The two main objectives of this study are: 1) to find an effective fluid characterization procedure to calculate the MMP for the N 2 /CO 2 –S117 oil system accurately. 2) to identify the effect of injection gas composition (i.e. N 2 /CO 2 mixture) on the gas drive mechanisms for the S117 light crude oil. The PVT and multiple-contact experimental data were measured and these data were used to characterize and validate different fluid characterization methods, including the 'conventional methods' and the direct PnA method. We proposed an integrated fluid characterization procedure to accurately calculate MMP. The novelty of the proposed fluid characterization procedure can be summarized as: 1) the procedure regains excellent match in both phase envelopes and critical points before and after lumping; 2) an improved direct PnA regression procedure with only two perturbation parameters was used to calculate pseudo-component properties. The comparison of calculated MMP to the experimental slim-tube data taken from literature shows that the integrated procedure using the direct PnA characterization has the best performance on MMP calculation. For the CO 2 /N 2 –S117 system, the vaporizing-gas drive mechanism controls the miscibility provided the CO 2 mole fraction in the injection gas is less than 70%. The calculated MMP is independent of the CO 2 mole fraction within the range approximately from 0 to 70% in the injection gas. At a higher CO 2 mole fraction (ranging from 71% to 100%), the complex condensing/vaporizing (C/V)-gas drive mechanism dominates the miscibility and thus the MMP of the fluid system decreases dramatically as CO 2 mole fraction increases. • The PVT and HPHT composition experimental data were reported for nitrogen and Tahe S117 light crude oil system. • We proposed an integrated fluid characterization procedure to accurately calcualte the MMP for gas flooding. • CO 2 has no effect on the MMP of the gas-oil system given that the CO 2 content is less than 70% in N 2 +CO 2 injection gas. [ABSTRACT FROM AUTHOR]

Spontaneous imbibition has been proved to be a promising enhanced oil recovery method for tight reservoirs, but the mechanisms of spontaneous imbibition oil recovery (SIOR) are still unclear. Here, we use the self-prepared novel lower-phase nano-emulsion (LWPNE) to reveal the mechanisms. In LWPNE solution, nano-scale oil drops (NODPs) in 6 nm diameter are formed and dispersed with co-presence of micelle solubilizing N-hexane. The effects of LWPNE on both interfacial tension (IFT) and contact angle were studied at 0–10,000 mg/L salinity condition, and spontaneous imbibition efficiency was determined from imbibition experiments in Amott cells at 60 °C. Electron Microscope and Transmission Electron Microscope were used to analyze the emulsification and solubilization mechanisms of LWPNE. Experimental results show that the original 135° (oil-wet) contact angle can be reduced to 32.7° (water-wet); the IFT is decreased to 0.0038 mN/m, and electrolyte has no contribution to either IFT reduction or wettability alteration. The average oil recovery from spontaneous imbibition testing using 0.3 wt% LWPNE is 44.1%, which is 20.5% higher than that from systems at similar conditions but using brine only. The LWPNE can solubilize oil by increasing NODPs' sizes to average 36 nm in diameter, which is the dominant mechanism to improve SIOR for LWPNE. The solubility of LWPNE is evaluated by the diameter growth rate of NODPs, and once it reaches 350%, a self-driving force drives oil drops to move forward. Moreover, the transformation mechanisms of spontaneous imbibition modes from imbibition to drainage are revealed by using the imbibition discriminant parameter (N B m − 1 ). • The nano-scale oil drops (NODPs) with a size of 6 nm in diameter are formed and evenly dispersed with co-presence of micelle solubilizing N-hexane. • The emulsification and solubilization effects of NODPs are discovered to be the dominant mechanisms for spontaneous imbibition oil recovery (SIOR). • A self-driving force (pressure difference at the curved interface) is proposed to drive oil drops moving forward during the solubilization processes. [ABSTRACT FROM AUTHOR]