An Integrated Subsurface Approach to Reveal Reservoir Connectivity for a Mature Field

Abstract

Abstract Reservoir compartments are segments that are effectively isolated by structural, sedimentologic or diagenetic barriers that hinder fluid flow laterally between compartments. However, in cases of sandstone reservoirs incising each other, the boundaries of the compartments are poorly defined and huge uncertainties are imposed on the connectivity and fluid flow behavior between them. Various geological and engineering approaches are used to work out the sand-to-sand juxtaposed reservoir compartments in Angsi Field offshore Peninsular Malaysia. In Angsi Field, located in the Oligo-Miocene Malay basin, the I35U and I35M sandstone reservoirs from Group-I stratigraphic unit (coastal-fluvial/deltaic lobes) provide a complicated case of areal and vertical reservoir compartments. The I35U channelized blocky sand incised into I35M mouth bar sand in some parts of the field. Mapping of the interlaced reservoir compartments of both reservoirs is based on conceptual geology, log patterns and seismic data. Although the vertical spacing of the two reservoirs is below seismic resolution, the difference in quality, thickness and fluid type have allowed seismic involvement in delineating compartments. To verify the geological findings, multidisciplinary team seeks out various methods to resolve connectivity uncertainty between the two reservoirs. Fluid gradients supported the idea; however reservoirs fluid contacts are sometimes leveled. Existing oil producers from both reservoirs provides reliable pressure-production data for classical reservoir engineering analysis. Production Decline Curves of both reservoirs vary significantly; seemingly controlled by internal depositional architecture of each reservoir. Depletion Plots further verify no lateral communication between the two reservoirs. Finally, Material Balance proves the understanding of communication between reservoirs; oil-in-place and reserves are aligned with geological studies. Two successful wells drilled lately based on findings from the above approach. A representative simulation model is being built for both reservoirs, incorporating the findings, to plan for further reservoir development and management strategies. This paper shares the geological and reservoir engineering integrated approaches to resolve the reservoir connectivity uncertainties between I35M and I35U, which could be applied in similar coastal fluvial/deltaic settings. The paper also highlights the importance of multidiscipline integration in resolving reservoirs uncertainties.