Accurate reservoir performance prediction m a structurally complex brown field is very important for generation of reliable production forecasts, location of possible by-passed oil estimation of reserves and optimal well/ reservoir management. Reserves estimation is one of tire key functions of Petroleum Engineers and it requires an integrated approach for tellable estimates to be made. The traditional techniques include Decline curve analysis, Material balance. Volumetric. Analogues and Numerical Reservoir simulation. Reservoir X is a structurally complex reservoir nr Field Y in the Niger Delta Basin. It came on stream in 1970 with 3 wells. Eight additional wells started production between 1972 and 1990. Five infill wells were drilled and completed between the years 2000-2005. However, due to operational and technical reasons (which are beyond the scope of this paper), 2 of these wells are yet to be put on production Over the years the reserves associated with these 2 wells have been estimated by analytical means (Volumetric and Material balance methods). However, there was the challenge of investigating the impact of fluid saturation changes around these wells, occasioned by the production fiom offset wells, on the reserves estimate obtained fiom material balance techniques These challenges necessitated the full field 3D integrated reservoir modeling The reservoir contains 9 blocks in which 8 are densely faulted. The material balance analysis, being, at most, a onedimensional model, was deficient in robustly assessing the subsurface uncertainties which includes fault sealing potential and fluid contacts movement. This paper discusses the techniques employed in building die static and dynamic models and shows a comparison of the reserves estimate results fiom analytical techniques versus 3D dynamic estimates.

In recent years, numerical reservoir simulation (3-dimensional modelling) has become a very useful optimisation tool, not just for field development planning but also for ongoing reservoir management. Although several analytical methods (such as material balance equations, Buckley-Leverett displacement theory etc) are used as computationally fast and inexpensive tools, they have been recognised as being incapable of capturing the details and complexity of certain reservoirs and processes. The field presented in this paper is one of the biggest oil fields in the Niger Delta, with an estimated oil in place of over 2.5 billion barrels and cumulative oil production close to 1 billion barrels. About half of this volume and production come from a single reservoir, which is densely faulted. The large number of intra-reservoir faults and the relatively high offtake rates have inhibited the activity of the otherwise strong aquifer and resulted in the high pressure decline observed in this reservoir. Consequently, a lot of the wells quit at relatively low water cuts of 40-50%, with the reservoir pressure being insufficient to lift the crude to surface at higher water cuts. One of the recommendations of the FDP Update was to increase oil recovery through fieldwide installation of gaslift. However, to quantify the gains of gaslifting and optimize oil recovery through effective reservoir management, an integrated detailed 3D model was required due to the structural complexity of the reservoir. This paper presents the workflow used in constructing, initializing and history-matching the 3D reservoir model; and how the history-matched model was used to assess different development scenarios for improving recovery from this large mature reservoir.

The field presented in this paper is a structurally complex onshore oil field in the Niger Delta, with a densely faulted succession of stacked reservoirs. From previous field studies, three of the reservoir units have been interpreted as a single stacked reservoir complex beneath a top seal, and the vertical and lateral communication between these units is largely controlled by the inter-reservoir shales and intense fault configuration.Integrated 3D reservoir modeling has been used to investigate the production performance from this stacked reservoir complex, with a view to identifying the features that contribute to the observed communication across the reservoirs, quantifying the contribution of each reservoir unit to each well's production, estimating the ultimate recovery for the complex and evaluating the current redevelopment plan for the reservoir complex to identify opportunities for further oil development.The results from the study indicate that oil recovery is majorly influenced by communication across faults. A tracer model was used to demonstrate that some wells produce from adjacent blocks on the same reservoir unit as well as the other reservoir units across the fault, giving an indication of the presence of non-sealing faults across juxtaposed oil-bearing sands. Also the application of the 3D model in evaluating the current redevelopment plan for the reservoir complex resulted in the identification of 5 new opportunities in existing wells, to increase the ultimate recovery of the reservoir complex by 7.8MMstb. More so, there exists an opportunity to recover 7.5MMstb from a by-passed section of the complex with a new vertical well.