Optimal Oilfield Development of Fields With a Small Gas Cap and Strong Aquifer

Abstract

SPE Members Abstract Production practices for oil fields with gas caps usually centre around conservation of the gas cap (energy) to maximise oil recovery. A less conventional but effective reservoir management approach involves an early gas cap blowdown phase in situations where the gas cap is small and a strong aquifer is present. This paper describes the critical parameters and the benefits from a less orthodox depletion plan. After discussing this subject from a general point of view, the reservoir management plan for the Skua Field, located in the Timor Sea, is cited as a successful application. Introduction In developing oil fields the aim is usually to maximise ultimate (oil)recovery and at the same time minimise capital expenditure (Capex) and operating expenditure (Opex), the optimum plan resulting in a maximum Net Present Value (NPV).To achieve this goal, oil fields may be produced in a variety of ways, constraint by the physical situation, commercial considerations and government regulation. In terms of reservoir considerations, the most important factors tend to be initial (reservoir) conditions, that is pressure, temperature and depth; and fluid and formation (and rock) properties. Overall, of these the dominant reservoir drive mechanism(s) and its effect on pressure maintenance and sweep efficiency (effectiveness of pushing oil towards the producing wells) are often the chief criteria for implementing the chosen subsurface development plan the number, type and location of wells and production policy. Drive mechanisms may be natural -(gas in) solution drive, (primary)gas cap drive, aquifer drive and compaction drive; or the reservoir energy and sweep efficiency may be supplemented by injecting fluids commonly water or gas, or more exotic substances, leading to enhanced oil recovery (EOR).In this paper the above mentioned natural drive mechanisms are reviewed for the purpose of comparing the benefits from a primary gas cap and a natural aquifer. In other words, when is the size of a primary gas cap more important than a certain aquifer? More specifically it is shown by means of a field example, the Skua Field located in the Timor Sea, how the ultimate recovery is being maximised by "blowing down' the primary gascap! Figure 1 compares the latter production strategy utilised for the Skua Field Development with a more conventional approach. RESERVOIR ENERGY One measure of the relative importance of the various drive mechanisms is the intrinsic energy of the different substances, more specifically the compressibility-volume product, which compensates for reservoir voidage(production) in maintaining reservoir pressure. In order to appreciate the wide range of compressibilities, Figure 2shows these for an "average" situation. P. 307^