Exploring the Delicate Balance in the Optimal Execution of Concurrent Gas Cap Blowdown and Oil Rim Production in a Saturated Reservoir

Abstract As the global drive for providing cleaner energy for current and future generations intensifies, there is an increasing need to develop natural gas resources which are cleaner when compared with other liquid hydrocarbon sources. This is because hydrocarbon gases generally have lower carbon content compared to crude oils. Hydrocarbon gases can come from solution gas, associated gas (primary or secondary gas cap) or non-associated gas depending on the initial reservoir pressure, temperature, composition, and development phase of the hydrocarbon resource. Imperatively, we have studied a field in which the reservoir sedimentation occurred in a wave-dominated fluvio-marine environment. These sands are usually capped by well-defined marine shales and the sands are predominantly rich in gas. Natural gas production is traditionally limited to gas from non-associated gas reservoirs. However, a lot of oil accumulations around the globe have associated gas caps and these gas accumulations, often referred to as associated gas have historically been reserved to expand and provide pressure support for production from the oil rim while gas production from the gas cap is deferred to later in the reservoir life after the oil reserves have been produced. Forces related to economics and the global energy trilemma have seen an increasing interest in developing these associated gas resources together with the underlying oil volumes. Maximizing recoveries from oil rim reservoirs and their gas cap requires an understanding and optimization of the delicate balance that exists between the dynamics of production from both the oil and gas legs, associated aquifer support and secondary pressure maintenance scheme. This study worked with a history-matched and calibrated three-dimensional reservoir model from a producing brown field in the Niger Delta to test different scenarios as follows; Assess the impact of oil rim production followed by gas cap blowdown on the ultimate recovery from both the gas cap and the oil rim, assess the impact of simultaneous gas cap blow down and oil rim production on the ultimate recovery from both the gas cap and the oil rim and finally assess the impact of gas cap blow down followed by oil rim production on the ultimate recovery from both the gas cap and the oil rim. In each case above, sensitivities were run to determine the optimal gas cap blow down rates, production start-up time, well type configuration and impact of pressure maintenance. The results of this work have provided very valuable insights that result in the optimal development of both oil and gas resources from saturated oil reservoirs with gas cap resources.