Evaluation of Compositional Effects on Gas-Oil Miscibility and Gas-Assisted Gravity Drainage (GAGD) EOR Process

Abstract

The Vanishing interface technique (VIT), offers fast and accurate determination of minimum miscibility pressure (MMP) between the reservoir crude oil and the various gases injected for enhancing oil recovery (EOR). The technique is based on miscibility being the condition where an interface between the injected fluid and the reservoir crude oil vanishes. This research has evaluated the effect of the gas-oil ratio on zero-IFT pressure. VIT experiments were simulated using an equation of state (EOS) and the Parachor model. This work proves the robustness of VIT in estimating miscibility pressures for gas injection EOR projects. Also this work presents a simple method for estimating near-miscibility pressures (NMP) that are based on a gas-oil interfacial tension (IFT) calculation procedure. The method evaluates near miscibility conditions at low (as against zero) IFT and relies only on using EOS and the Parachor model. The results show that NMP estimation method is reasonably accurate, MMPs agreed with slim tube MMPs within 7%. In recent years CO2-based EOR projects have been steadily gaining ground over all other EOR processes. However, due to limited supply of pure CO2 availability, an alternative option is to utilize emitted flue gas. Miscibility pressures were successfully determined using VIT of different gas mixtures with n-decane (nC10), namely CO2, N2, CO2- N2 (85%-15%) and CO2- N2 (15%- 85%). This proves the ease of applicability of VIT to evaluate the effect of injected gas type and composition on gas-oil miscibility. Earlier reported Gas-Assisted Gravity Drainage (GAGD) work using CO2 for gas injection resulted in good oil recoveries. To expand the applicability of using GAGD in areas where there is limited CO2, an experimental study was conducted to examine the effect of using different gases on GAGD performance. CO2, N2 and flue gas were used with nC10 as the oil phase, in conducting high-pressure corefloods. The floods resulted in oil recoveries in the range of 30-49 % (immiscible mode) of remaining oil after water floods. This work has demonstrated the superiority of GAGD over the other gas injection methods currently practiced, such as WAG and CGI, using CO2 or any other gas.