Comprehensive Approach for Confirming and Modelling Residual Oil Distribution Below Current Free Water Level in Brown Giant Oil Field, Offshore Abu Dhabi

Abstract Optimizing the development of oil and gas, which are finite underground resources, is indispensable for human activities. Historically, humanity has prioritized the extraction of conventional oil and gas, which are easier to produce. However, recent technological advancements have enabled the efficient extraction of unconventional oil and gas, such as shale oil. The target reservoir of this study has similarly focused on conventional oil that is located above current free water level (FWL). Nevertheless, historical core data, log data, and production data of the reservoir indicated the presence of oil below current FWL. The oil below current FWL is considered to be residual oil, which is the result of an imbibition process that occurred after the oil charge. The exact distribution and development potential of residual oil, however, were not well understood. Additionally, since the late 1970s, peripheral water injection (PWI) from outside the current FWL has been implemented in this reservoir to maintain pressure, but it has been ineffective. A hypothesis was formed that the cause of this pressure discontinuity might be the residual oil. For the above reasons, it became necessary to accurately capture the distribution of residual oil below current FWL to optimize future development plans in this reservoir. Previous reservoir models had not successfully modeled the distribution of residual oil below current FWL. Therefore, this study adopted a new static modelling technique that is most suitable for the heterogeneous and mud-dominant reservoir. To accurately model the distribution of residual oil below current FWL, we created a paleo FWL surface in addition to current FWL and constructed a water saturation (Sw) model based on the paleo FWL surface with saturation height function (SHF) for each static rock type (SRT). This approach allowed us to model the distribution of residual oil below current FWL. As a result, the matching accuracy between the Sw model and Sw log significantly improved compared to previous model. Moreover, the distribution of residual oil obtained from this modelling study was consistent with regional tectonic events. Furthermore, accurately modelling the thick Sw transition zone of residual oil allowed us to reproduce the pressure difference between peripheral water injectors and oil producers in the dynamic model without arbitrary modelling methods. The results of this study led to the optimization of future development plans such as water injector location for pressure maintenance.