Case Study of Gas Flaring Mitigation Through Optimized Gas Re-Injection While Improving the Recovery in High-Temperature Offshore Reservoirs

Abstract

Associated gas needs to be handled properly especially if the gas is deemed to be uneconomical as flaring emerges as being the first ad-hoc" solution" and for many cases prolongs for a long time. Such flaring practices are exercised even more for the gas streams with undesirable compositions such as high CO2, as the separation costs could be significant. When the daily amount reaches significant numbers such as 60 MMSCF/D, the impact in terms of emission of greenhouse gases, air pollution and waste of resources become very significant. In this study, new approaches of zero gas flaring coupled with supplemental CO2/gas injection, for not only mitigating the flaring but also for recovery improvement, were introduced. The feasibility of gas re-injection was assessed using real field data. Input data was assessed using experimental information as well as numerical, and custom-made machine learning models. Reservoir fluid characterized using the available PVT reports from two oil production wells and one shut-in gas injection well were used to develop the EOS model in use. Moreover, the combination of produced gases and supplemental CO2 are evaluated in terms of solubility/swelling and miscibility and as well as compatibility with the in-situ reservoir fluids. Performance of the gas re-injection was evaluated and optimized using the final calibrated reservoir simulation model for the gas injection process. This study quantifies the amount of incremental oil recovery and improving field performance from the re-injection of produced gas. Produced gas is intended to increase reservoir energy through re-pressurizing the reservoir and as well as displacing the remaining oil with the optimized injection gas compositions, and thus, reducing the oil viscosity due to swelling, reduction in interfacial tension leading to increase in local displacement efficiency. Furthermore, the gas re-injection process leads to relaxing the GOR constraints from 2000 scf/stb up to 4000 scf/stb, as the excess gas can be utilized in the injection process. The placement of new gas injectors is optimized considering the existing gas injector location and by injecting into the gas-cap close to GOC while targeting the best rock types locally aiming to ensure that sustained good injectivity is achieved. The outcome of this coupled process demonstrated enhancement in the reservoir energy management by mitigation of the gas coning due to the movement of the secondary gas cap towards the producers and optimum utilization of the natural reservoir energy in combination with the injection process. Furthermore, injection of the gas in up-dip of formation and in the gas cap with optimum gas injection rate yield stable gas sweep efficiency with the help of the gravity forces. In this study, re-injection of the produced gas considering the reservoir heterogeneity, optimum injection gas compositions (including CO2 enhancement options) and limiting injection pressure of 5500 psi (below fracturing pressure) results in improved oil recovery while reducing the gas emission with the optimum utilization of reservoir energy. This paper presents guidelines for mitigation of gas flaring by re-injection of the produced gas along with the supplemental gas (where available)/water and hence leading to reduced emissions.