Simulation-Assisted Diagnosis of GOR Evolution in Carbonate Reservoir, Offshore Abu Dhabi, Undergoing Miscible Gas Injection

Abstract

Abstract A major reservoir, Reservoir A, in an offshore carbonate field in Abu Dhabi underwent a thorough development-concept selection process. The subsurface evaluation recommended miscible gas injection-based development as the most optimal process, and the reservoir is undergoing the miscible gas injection operation. Over the course of production, ‘Reservoir A’ is experiencing a gradual GOR increase. The GOR evolution could be the result of different physical mechanisms, such as production below bubble point, gas coning, or miscible gas breakthrough. In the latter case, it is not clear whether reservoir dispersion has smeared out the miscible displacement front to a point where miscibility is lost. The key objective of this work was therefore to develop a diagnosis methodology using numerical simulation to interpret the GOR response. A 2D numerical, fine-gridded simulation with a producer-injector pair was set up to explore the following scenarios: (a) production below bubble point; (b) gas coning from the gas cap; (3) a perfect miscible displacement; (4) a perfect immiscible displacement; (5) a miscible displacement that turns immiscible due to dispersion; (6) a combination of the above. The key output parameters tracked included GOR and its derivative and the flowing bottom pressure at the producer. The different runs were then compared against each other and with the measured GOR response. A fit-for-purpose surveillance strategy was adopted to help assess the effectiveness of the overall development strategy. Recently, tracer test results confirmed the arrival of gas production down the horizontal wellbore. GOR evolution trends were captured using the inline multiphase flowmeters. The comparison of simulation-based type curves with the evolving GOR trend along with the other subsurface attributes shows an initial indication of the production of swelled oil, followed by the injection gas breakthrough. This paper showcases the effectiveness of the proposed technique and the initial results of the analysis.