Wringing Value from a Mature Reservoir: An Offshore Niger Delta Case Study

Abstract

The objective of this paper is to share lessons from an intensive study of a mature reservoir and highlight its results. This study also aims to demonstrate the value that can be obtained from a mature conventional oil reservoir when the appropriate assessment processes are utilized. The case study is an offshore oil reservoir with over 50 years of production with waterflooding for almost a decade. The dynamic simulation model from an earlier assessment of the reservoir showed significant deviation following years of waterflooding, as it became more challenging to understand the water front movement, predict water cut changes and deliver more reliable liquid production forecasts. The reservoir had historical challenges with water production and matching the water-cut from producing wells emphasizing the deviation of the existing simulation model from actual data. As a result, a cold-eye review of all available data without anchoring on prior interpretations was required. An updated reservoir characterization and earth modelling methodology was applied during the study with attendant improvements in structural and stratigraphic representation of the reservoir. A full-field assessment methodology was employed in the study both for the fault framework and model building, ensuring the inter-reservoir connections and dependencies were captured. Additional scrutiny was applied to initial fluid contacts, leading to the resolution of longstanding uncertainties. This resulted in the delivery of a new dynamic simulation model with a much better water cut history match compared to the previous study. Fluid contact tracking over time was also better matched using the new model. The new model indicated an increase in original oil in place of 38% leading to the identification of 2 new infill producer opportunities with total estimated ultimate recovery of several millions of oil barrels.