Maximising Oil Recovery of a Thin Oil Rim through an Optimised Field Development Plan

Abstract

Abstract PDO operates many oil fields in Oman and maintains a periodical update of Field Development Plans (FDP). This paper presents a case study of one such green field where a FDP was performed that proposed numerous development wells targeting a thin oil rim overlain by a gas cap in two clastic transition-zone reservoirs. When drilling commenced, surprises were encountered such as steeper flanks, lower gross rates, higher water cut, faster pressure depletion & uncertain facies distribution. A comparison of earlier models against actual results lead to revisions mainly in vertical compartmentalization, reduced horizontal & vertical permeability. This severely impacted achieving business targets, and sub optimal use of drilling resources & surface facilities. A new FDP Study was initiated to understand the performance and provide a revised development strategy under pressure depletion conditions. The study also evaluated the merit of installing gas lift and the impact of gas cap pressure maintenance. Revised static & dynamic models were built with the current understanding from wells drilled since the previous FDP. The simulation model was history matched with field production & pressure data and then used to identify future well locations and optimum placement within the oil rim. Additionally, the numerical model was used to assess multiple development options such as oil gains arising from gas lift installation, assessment of an additional high-pressure gas compressor for deeper gas-lifting and an additional gas export pipeline. A fit-for-purpose uncertainty analysis of static and dynamic parameters was then undertaken to understand the range of uncertainty on field ultimate recovery. Screening of gas cap pressure maintenance resulted in a sizeable incremental oil volume over the depletion development, thus paving the way for future detailed concept selection of pressure maintenance field development. The study also resulted in identification of future infill well locations, immediate need for installation of gaslift and negated the need for an additional high pressure compressor & invalidated an additional gas export pipeline. Robust static & dynamic models were built through multiple iterations to provide a guiding tool for future development planning. The adopted methodology described in this paper to evaluate the various development aspects is generic and could be useful in typical field developments.