A 3-dimensional 3-phase reservoir simulation model was constructed using a black oil reservoir simulator to understand the complex reservoir fluid dynamics, and its effects on ultimate oil recovery. The paper describes the various phases of the model study, which entails the construction of the reservoir model matching of the historical well performance and optimising reservoir operation strategy through prediction of future reservoir performance under various operating scenarios. The findings of the study were used to formulate a cost-effective reservoir management strategy and further development plan to maximize hydrocarbon recovery.
The K5/7 reservoir in the Samarang Field (Figure 1) is an elongated roll- over anticline, with several NNE-SSW trending faults dividing the structure into several fault blocks. It has a small gascap (m = 0.11) and an average oil column of about 120 feet. The gross sand thickness of the reservoir averages some 300 feet with an average net-to-gross ratio of about 86%.The reservoir was first developed in 1975, followed by a work-over and infill drilling campaign in 1986/87. As of year-ending 1990, there are 38 completions producing (or, capable of producing) from the K5/7 reservoir and 4 watered-out completions which have been abandoned. The estimated expectation original oil-in-place (OOIP) in the K5/7 reservoir, based on the most recent geological study, is about 145 MMstb. The original gas-in-place (OGIP) is some 96 Bscf of which 18 Bscf is the gascap gas. The estimated oil recovery factor is 65%, which puts the expectation ultimate reserves (EUR) for the reservoir at some 94 MMstb. Natural water influx from a large aquifer is the predominant drive mechanism aided by expansion of the gascap. The cumulative oil production as of September 30, 1990 is about 75 MMstb, representing some 79% depletion stage based on the aforesaid EUR. The corresponding cumulative gross gas production is some 61 Bscf. The initial reservoir pressure is 2367 psig at the reservoir datum of 5350 ft-ss. At the present depletion stage, the average pressure is about 1550 psig. The reservoir management policy in the past has been to control oil production within a gas oil ratio (GOR) limit of 800 scf/stb on reservoir basis. Based on a review of the reservoir performance in 1986 using simplified engineering calculations, it was proposed that gascap blowdown would maximize ultimate oil recovery and that GOR control was no longer necessary. The proposal was based on the following line of reasoning:-
Migration of oil ultimately into the gascap zone is inevitable due to the strong water drive; and
Gascap blowdown will result in an additional oil recovery of some 12MMstb.
In view of the above, a rigorous reservoir simulation study was undertaken to identify areas of remaining oil and to justify further development via work-over or infill drilling.