Unified Field Compositional Fluid Model for the Bahrain Field

Abstract

The Bahrain Field is characterized by large lateral and vertical variations in fluid properties, with oil gravity ranging between −9 and 80 °API. The lower API crudes are encountered mostly on the structural flanks and within the upper reservoir units, while the highest API crudes are condensates from deeper formations such as Hith and Arab. The deepest reservoir is the gas-bearing Khuff. It has 50 °API condensate and forms a separate fluid type from the rest of the Bahrain Field. The objective of this paper is to derive a single compositional predictor for the entire range of crude gravities. Excluded from this unified model are bitumens from Aruma and Khuff condensates, which are compositionally different. One outcome of this study was to predict the reservoir fluid as a function of well test Gas Oil Ratio (GOR) and API gravity by mathematical recombination of averaged data from abundant well tests across the Bahrain Field. A strong trend of methane fraction in the reservoir fluid versus saturation pressure has been observed, and thus it has been possible to construct the recombined reservoir fluid and then predict saturation pressures, Formation Volume Fraction (FVF), and viscosity. This fluid model was used to initialize compositional models for gas plant evaluations, miscible flood evaluations, and to determine the maximum GOR at which saturation pressure equals reservoir pressure. Another outcome of the unified fluid model was to construct a reservoir fluid composition given a target saturation pressure and API. This information is used to construct representative fluids for laboratory synthesis of crudes and gas for live oil experiments. As part of the process, a number of quality checks were constructed to determine if the fluid encountered is in range of historic produced crudes (e.g. contamination by air or lift gas) and enable construction of fluids for reservoir simulation.