Modeling of Hydrocarbons Below Free Water Level in a Major Oil Field in Abu Dhabi UAE and its Impacts on Dynamic Behavior and History Matching

Abstract

AbstractThe presence of residual oil below Free Water Level is common in Middle Eastern fields. This later is explained by the presence of a paleo oil-water contact. An early water movement and sweep of the accumulated oil induced by tectonic events after its charging, has led to the trapping of residual oil below the curent Free Water Level. This theory is also supported by the presence of tarmat above and below this Free Water Level along the paleo oil-water contact.In the studied field, the analysis of fluid saturation logs, early well tests and initial fluid gradients have led to the definition of the oil-water contact and the Free Water Level at discovery time. The analyses of cored wells and water saturation logs have demonstrated the presence of oil below this Free Water Level which is explained by the presence of a paleo oil-water contact. Furthermore, this analysis has enabled the definition of the geometry of this paleo oil-water contact, which appears to have a curved shape. This geometry has also been confirmed by the presence of tarmat along the defined paleo contact.Modeling of the water and oil saturations below Free Water Level cannot be achieved by a conventional methodology using a saturation height function. Therefore a specific workflow has been developed. Given that oil saturation below Free Water Level is rock type driven, it could be distributed in 3D as any static property. Water saturation logs were consequently upscaled and populated in 3D using a Sequential Gaussian Simulation algorithm with the rock types as main driver. The resulting grid has been extracted up to the base of the transition zone of the main oil column and merged with the oil saturation grid generated using a saturation height function.A thorough dynamic synthesis of the field has highlighted a specific dynamic behavior of the water injectors perforated below the Free Water Level. In some specific intervals of the reservoir, high shut-in pressures have been identified. These high pressures can be explained by the presence of residual oil below the Free Water Level which acts as a baffle inducing a partial confinement of the injectors. The implementation of this residual oil in the simulation model reduces the injected water mobility and drastically improves the pressure match.