This paper highlights the basic steps taken to improve the history match of GOR and Water Cut in order to make the Ofon reservoir model more reliable for the forecasts necessary to optimise the revised scope of the OFON phase-2 project. The simulation study results were used to validate an optimal project case which involved drilling of 24 new wells, increased water injection capacity and implementation of gas lift injection in both existing and new wells.

Since saturation matching is mainly a mass transport (fluid flow) problem, it was decided to concentrate on dynamic parameters with known uncertainties. Based on the results of an uncertainty study performed with the in-house EST software, it was possible to determine which dynamic parameters had the most impact on water production and GOR within the limits of their uncertainties. It showed that relative permeability and critical gas saturation were the most influencing on water cut and GOR respectively. Sensitivities around experimentally acceptable limits of these parameters were undertaken leading to obtaining a best-case history matched model.

As the model was very pessimistic in water cut calculation (over-estimating observed water production), the relative permeability curves were thus adjusted by imposing Krw-Sw of the rock type with least water mobility on all the remaining 4 rock types in the model, while respecting the existing Kro-Sw functions of all rock types. This significantly improved field BSW although it was still necessary to perform local modifications around some individual wells to fine tune their water cut matches.

GOR matching was improved by sensitizing on the critical gas saturation uncertainty limits as determined from core experiments, leading to an improved field GOR match.

A test of the history matched model with recent production history confirmed its reliability for use as a predictive tool in the optimisation study for the Ofon phase-2 project.

Update of the GOR and BSW match improved the economics of the phase-2 project leading to the validation of a more cost-effective project scope.

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