Innovative Approach to Identify PVT Regions in a Sandstone Reservoir in Khafji Field with Geological Events

Abstract

Prior to initiating a reservoir simulation study, the readiness of a complete set of fluid analysis data should be assured and quality checked. This usually includes, but not limited to, identification of any vertical and lateral variation of reservoir fluid properties. Identification of these trends can have tremendous impact on the results of the reservoir simulation project. Recently, a reservoir simulation study was conducted on a brown reservoir in a Middle Eastern field. This reservoir is geologically characterized by various faults which were developed after fluid migration. The presence of these faults has resulted in communicated reservoir compartments. Application of traditional workflow in analyzing the PVT data was insufficient to identify any trend in the fluid properties or account for any vertical and lateral differences in reservoir fluid properties. This paper explains the work done to characterize fluid properties for a structurally complex sandstone reservoir, where most of the downhole fluid samples were collected in the 1960s. Unfortunately, the sampling depth was not reported for most of the samples at that time, with no production logging data available to identify the production zones within the large perforation interval. Laboratory measurements indicate under-saturated oil with variation that could be correlated with depth. Samples were obtained from three geological subzones; Layers-1, 2 and 3, where a shale layer is believed to be partially separating Layer 1 from the other layers. Part of the challenge is that a large number of samples were collected from different sublayers as a combination, which increases sampling depth uncertainty. Defining properties trend versus depth was the main challenge in the study. This uncertainty was addressed by developing an iterative "what-if?" analysis scenarios by integrating geological parameters. A new model was prepared that considered the initial conditions of the reservoir fluid before any migration or tectonic activities took place. Applying the proposed approach, resulted in a clear trend of hydrocarbon properties variation with depth and provided a clear observation of the presence of one PVT region in the reservoir. Based on the results of this study, one PVT region with a solution gas-oil ratio trend versus depth was applied into the reservoir model, which resulted in a reliable dynamic simulation model.