Transition Zone Development in a Complex Carbonate System

Abstract

Abstract In a volatile market such as the oil and gas industry, the ability to produce effectively and economically reservoir reserves is crucial. One of the most complex areas of a reservoir is the transition zone, especially in carbonates reservoirs. Transition zone refers to a certain height above the free water level (FWL) where both oil and water usually flow together. Reserves within transition zones vary from one reservoir to the other, in our reservoir we estimate that the transition zone contains about 20% of the total reservoir reserve. As such a study was conducted, to characterise and the defined the best development strategy, in order to produce the transition zone effectively. In characterising the transition zone various core were acquired, for routine core analysis (RCA) and special core analysis (SCAL). Cores collected were acquired to represent both the crest and flank of the reservoir structure. Using the mercury injection capillary pressure (MICP) and the reservoir quality index (RQI) from the RCA and SCAL, 5 reservoir rock types (RRT) were defined. With reservoir rock type 1 (RRT1) representing the best rock type and reservoir rock type 5 (RRT5) representing the worst rock type. Reservoir rock type 3 (RRT3) occurred mostly in the transition zone. The reservoir contains a very complex fluid, as the pressure volume and temperature (PVT) properties varies both vertically and areally, with a variation of bubble point pressure (Pb) between 1400psi and 4000psi. Flank location showed lower bubble point pressure, while crest location showed higher bubble point pressure. However, there are also certain cases where flanks show higher bubble point pressure, and crest show lower bubble point pressure. In order model this complicated behaviour in the reservoir simulator, one equation of state (EOS) i.e. the Peng Robinson (PR), with two regions were used in dynamic reservoir model. Various development strategies were investigated in order to obtain the best development strategy, including but not limited to water alternating gas (WAG), gas injection, water injection, different well spacing, different completion strategy, injection rates, WAG cycles, etc. This paper describes how the transition zone was defined in terms of static and dynamic properties, the various development strategy used and the respective recovery obtained from the most likely development strategy. The best development strategy for this field was found to be water alternating gas (WAG), with wells placed higher up in the transition zone using 6months WAG cycles.