A reservoir simulation model, calibrated with 25 years of production history, was used to determine a cost effective reservoir management and production strategy, which optimises future recovery from an oil rim reservoir in the Betty Field, offshore Malaysia. The reservoirs consist of good quality sands in a coastal depositional environment with an anticlinal structure terminated at the crest with a growth fault. The history match confirmed that A6.0 reservoir, unlike all others in the field, which co-exist within a stacked sequence, is surprisingly isolated from the surrounding aquifers. Prior to its premature shut-in, oil production reached 5000 bopd. However, a drastic decline in reservoir pressure caused the evolution of a large secondary gas cap and a steeply increasing producing gas-oil ratio. The recovery factor for this reservoir stands at 25%, significantly less than for the best reservoirs in the field.
After more than a decade shut-in, secondary and tertiary recovery methods investigated in this study included dump flooding from adjacent reservoirs, gas re-injection and water injection based on an extensive prior screening exercise. The limited prospective increase in reserves highlighted the need for a technically sound but also financially feasible solution. Since the current facilities were not intended for any pressure maintenance or enhanced oil recovery scheme, a number of technical limits arose due to space and weight constraints on the platform. Furthermore, fluid separation and export constraints had to be taken into account. Consequently the reservoir management plan required a field-wide optimization of the scheduled activities in order to identify bottlenecks in the gas handling capacity. Simultaneously with the model study, shut-in wells' production potentials were re-tested in order to gather additional data and, a water dumpflood pilot was implemented.
Introduction and Field Overview
The Betty field is located 40 km offshore Miri in the Baram Delta area of Sarawak, Malaysia. It was discovered in 1968, and its production started in 1978. The field consists of 22 stacked reservoirs of varying size and thickness at depths of 7200 to 9650 ft sub-sea. The average water depth is 225 ft. The field is operated by a single unmanned production platform that contains 24 well slots with 48 tubing strings. In the Betty field, some producing reservoirs have experienced high recovery efficiency, while in others the recovery has remained relatively low. One objective of a recent field redevelopment planning study was to investigate and resolve these recovery efficiency anomalies, and to identify areas where recovery could be improved. This is particularly important since, at the present time, no infrastructure such as a compression system or offshore water treatment facilities is available on the platform. Major changes would have to be implemented prior to any secondary or tertiary recovery method and would require economic justification. In addition, existing fluid export constraints, arising mainly from the current regional gas export scheme, had to be taken into account for an optimised field-wide reservoir management plan.
A recent full field review[1] confirmed the presence of a strong aquifer drive throughout the Betty field, aquifer energy provided to the reservoirs combined with the favourable mobility ratio resulted in excellent sweep efficiency. However, one of the reservoirs (A6.0) was found to be almost entirely isolated from its aquifer, resulting in no discernible pressure support. It was estimated that the volume of the aquifer was approximately one hundredth of the aquifer in the adjacent reservoirs with about 0.25 billion barrels of water. Material balance as well as history matching with numerical simulation model confirmed solution gas drive as the predominant energy source for this reservoir. Among all the Betty reservoirs, this particular reservoir was considered the only viable candidate for investigation of secondary and tertiary recovery practices. The decision was based on the anticipated recovery targets and the economic prospects of any method for improved or enhanced oil
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