Capturing Complex Reservoir Heterogeneity in a Field Under Water Injection – A Case Study of a Niger Delta Field

Abstract

Structural heterogeneity is one of the critical factors that can affect a reservoir's performance, properties and thus, its ultimate recovery. With the decline in oil price and continuous drive for cost optimisation, understanding the impact of such features on reservoir behaviour in terms of pressure communication, fluid movement, production and well placement becomes paramount. In this research paper, the main study subject is a compartmentalised reservoir in a mature Niger Delta field undergoing water injection. Two blocks in the reservoir separated by a fault mapped from the seismic which was believed to be non-sealing as initial pressure data acquired from both blocks through aRepeat Formation Test(RFT) were similar; suggesting that these blocks were in hydraulic communication. Production started from the western block in 2001, and after few years of production, a significant drop in reservoir pressure was observed. To arrest this pressure decline, water injection into the reservoir commenced in 2011. After four years of injection, a substantial increase in reservoir pressure was observed as confirmed from static bottom hole pressure surveys from existing wells in the block. With the successful implementation of pressure maintenance and oil being swept updip structure as suggested by the history matched reservoir model, infill well opportunities were identified in the western and eastern parts of the A block. In order to properly land these wells, reduce production uncertainty and ensure its successful production, a pilot hole was drilled into these blocks to ascertain current fluid contacts and reservoir pressure. Information from the RFT was however surprising as reservoir pressure in the eastern block was found to be significantly depleted contrary to expectations; as similar pressure with the western block was expected since there was no producer in this block. This paper describes how an integrated approach resulted in significant cost saving and how an analytical tool like the multi tank material balance methods was used to effectively characterise a complex reservoir under water injection and revise the field redevelopment plan.