Evaluation of Miscible Water Alternating Gas WAG Injection by Comparing with Lesson Learnt from an Immiscible Gas Injection Pilot

Abstract To sustain the long term production from a giant offshore reservoir, a simulation study evaluating and comparing miscible hydrocarbon (HC) Water-Alternating-Gas (WAG) with a prior pilot test implemented in the field evaluating immiscible HC Simultaneous Water-And-Gas (SWAG) has been carried out. This paper describes the evaluation of recovery uplift for the miscible HC WAG by comparing study findings with actual field results for the immiscible HC SWAG pilot. In order to evaluate miscible HC WAG potential, sector model simulation studies were carried out and it was concluded that a 11% recovery uplift over base water injection could be achieved by miscible HC WAG. On the other hand, an analysis of the immiscible HC SWAG pilot test including history matching simulation was conducted and it was concluded that immiscible HC SWAG had little impact on recovery uplift. In order to evaluate the differences between the planned miscible HC WAG implementation and the immiscible field pilot test results, the comparative investigation was conducted using sector model. The area where the immiscible SWAG pilot was conducted was geologically similar to the target area for the miscible WAG. To confirm the geological similarity of the two areas, simulations for immiscible injection using a 5-spot configuration consistent with the immiscible SWAG pilot were carried out for each area. Results indicated gas distribution was similar in the two areas and was in agreement with actual pilot results; hence, it was confirmed that the impact of geologic differences of the two areas was limited. The other differences between the planned miscible HC WAG and the immiscible SWAG pilot were (i) injection scheme (WAG or SWAG), (ii) well configuration (line drive or 5-spot pattern), (iii) well spacing (500 m or 1 km) and (iv) injectant (miscible HC gas or immiscible HC gas). Simulation models capturing each scheme were developed. Through the injection scheme sensitivity simulation studies, it was concluded that (i) WAG, (ii) line drive, (iii) 500 m spacing and (iv) miscible HC gas were preferential to (i) SWAG, (ii) 5-spot pattern, (iii) 1km spacing and (iv) immiscible HC gas, respectively. The combination of these factors resulted in a 21% recovery uplift beyond the prior immiscible HC pilot. Hence, through this work we demonstrate that a reliable evaluation for recovery uplift with miscible HC WAG was achieved through the comparative investigation of the immiscible HC SWAG field pilot test results and the planned miscible HC WAG implementation, including the analysis of the impact of geology, injection scheme and well configurations.