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.

This paper presents the results of a 3D model constructed for a giant, densely faulted reservoir in one of the biggest fields in the Niger Delta. The study also showcases how simple analytical techniques applied in a technically thorough manner can achieve close or similar results to those from a calibrated 3D simulation model. In carrying out this work, a detailed 3D model was built and calibrated for a very mature reservoir; incorporating results of the recently drilled wells in the field, revalidated integrated data and latest historical performance data. The reservoir, which is densely faulted by a conjugate system of synthetic and antithetic faults, is the largest in the field accounting for almost half of the field's resource volumes. The large number of NW-SE faults, coupled with the relatively high offtake rates from the reservoir, have throttled the activity of the otherwise infinite aquifer and led to severe pressure depletion in the reservoir. Consequently, a lot of the producers quit at relatively low water cuts (~40 − 50%). One of the objectives of the detailed modelling study was to investigate the benefits and gains of gaslifting existing and future new wells. Prior to this study, simple decline curve analysis (DCA) had been carried out independently on a well-by-well basis using estimated abandonment conditions under gasliftassisted flow to predict the recoveries and gains from gaslifting. The prediction results from the full-field calibrated model compare very closely with the estimates from the individual wells DCA's and suggest that the DCA results can be used as 50/50 estimates for other reservoirs in the field where there are no full 3D models. This work therefore supports the fact that simple analytical methods, which are applied in a technically thorough manner, can still be used in the absence of full 3D models.