This paper presents the case history of a focused Well and Reservoir Management (WRM) effort in a giant densely fractured carbonate reservoir in Oman. The field is currently producing under the gas oil gravity drainage reservoir process. The fracture network in the reservoir is largely gas-filled and has a thin oil rim underlain by active aquifer. Horizontal and multilateral producers are positioned in the oil rim while aquifer influx is limited to the optimum rate by water withdrawal from flank water producers. Delicate balance of crestal gas injection, aquifer water withdrawal and oil production is required to keep the oil rim at its optimal position. As the fracture oil rim has been lowered over a period of several years, a large number of wells have gassed out and started to circulate injected gas. Initial analysis of the production trends indicated that additional compression capacity was required in the field.
An integrated WRM effort was undertaken in 2003 to manage this complex field. Active reservoir surveillance planning and execution was carried out to map current fracture fluid contacts across the field. Strategic well optimizations and recompletions were performed in selected wells to capture production from oil rim and avoid short-circuiting of the injected gas. Also, several gassed out intermittent producers and very high GOR continuous producers were shut-in. An eight-well production logging campaign provided better understanding of well and reservoir behaviour. Well integrity tests in over 110 wells further assisted in problem diagnosis and workover planning.
These focused WRM efforts have resulted in sustained 10% production gain over a one-year period. A major capex benefit was also realized when a better understanding of the reservoir and improved GOR performance of the field eliminated the previously perceived need for additional compression.
Introduction
This field is the fourth largest oil field in Oman in terms of STOIIP. It comprises a 6 km wide by 8 km long anticline bound by major reverse thrust to the north-east. The structure map of the field is shown in Figure1.
The reservoir is composed of intensely fractured platform carbonates of the Upper Cretaceous Formation. The main producing units are A and C reservoirs. The field has been on production since 1967 and has undergone several phases of development including depletion, water flood and gas injection / gas oil gravity drainage (GOGD). The latest development phase is based upon GOGD process as the way forward for optimum recovery from this highly fractured field. Moreover, the reservoir studies and modeling work in 1990 showed that the recovery can be increased by lowering the oil rim to enhance the GOGD. Later, the oil rim lowering project was initiated in the field. The downward moving oil rim posed a challenge to maintain production as the perforations in the vertical wells gassed out. This required chasing the rim with re-perforations/ recompletions.
In order to overcome the problem of chasing the oil rim, a ring of dual lateral wells were drilled in A and C reservoirs. The upper laterals were placed in the intermediate rim position while the lower laterals were placed at the final rim position of ~ 750m TVDSS. There is limited production from D and E Formations, which is only from vertical wells.
This current development is considered optimum and almost fully implemented. Future activities will be focused on completion of the producer rings in the existing producing layers. A study on oil rim lowering was concluded in 2004 and endorsed the current reservoir management strategy.
