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EMEPMI and PETRONAS have recently concluded major simulation studies on the application of immiscible Water Alternating Gas (WAG) injection to enhance oil recovery for Tapis and Guntong fields, the two largest waterflooded fields operated by ExxonMobil in Malaysia. Both fields have been under waterflood operation for more than 25 years with about 40% original-oil-in-place (OOIP) recovered to-date.Evaluations of immiscible WAG injection process application at Tapis and Guntong fields were carried out using full-field reservoir simulation models. The scope of the evaluation work included: a) assessment of additional recovery from enhancing waterflood operations, b) assessment of recovery uplift from WAG injection, c) WAG pattern and WAG ratio optimization, d) optimal WAG operating parameters and reservoir management strategy, and e) integrating production and injection well operating conditions with facilities process design.The Tapis WAG evaluation study has supported ExxonMobil's plans to implement a WAG project at Tapis. The study has matured to the point of providing the subsurface design basis for the Project. The Guntong study has provided an initial assessment of WAG potential used for the conceptual WAG development and early project planning.

The Guntong field, the largest waterflood field in offshore Peninsular Malaysia, with an oil-in-place of about 200 mstkl, has been producing since 1985. The field contains 13 stacked reservoirs having small gas caps and limited aquifer support. This paper describes some of the significant reservoir, as well as geologic and facility, challenges faced during development and management of this complex reservoir system. A combination of five-spot and peripheral waterflood patterns was selected to provide the required areal coverage with commingling of reservoirs into two operational groups. Several key reservoir management strategies to maximize performance are: determination and application of optimum target reservoir pressures, development and use of a PC based program and other analytical tools to guide production and injection targets, meeting pattern balancing objectives by implementation of comprehensive reservoir surveillance activities and capacity enhancement programs. To date, the response to the reservoir management efforts has been favorable with an all-time high production rate of 14 kstkl/d having been recorded in 1994. Introduction The Guntong field is located in the South China Sea, 210 km off the east coast of Peninsular Malaysia (Figure 1).

The Tapis field, located offshore peninsula Malaysia, has been producing since 1978 and about 90% of the original development estimated recoverable oil has been produced. Large gas caps remain for future development. Recovery to date has been primarily from line drive pattern waterflood. Continuous implementation of improved oil recovery (IOR) strategies has increased the recoverable reserves and production capacity for economic operation of the field. This paper describes technical studies completed at Tapis and the results of recent IOR work programs that added more than 20 million barrels of reserves. In addition, enhanced oil recovery techniques currently under evaluation for Tapis are described. The Tapis structure is an east-west trending anticline, approximately 16 km long and 7 km wide. Initially, only the northern and eastern areas were developed from four platforms. The other areas were only developed in recent years following studies to re-assess recovery from poorer quality reservoirs and application of more cost-effective technologies to commercialize them. This led to a fifth platform that developed the western area of the field. Recently, the southern flank with marginal reserves was developed from a sixth platform, which adopted the minimal facilities satellite platform concept. Reservoir risk for this development was mitigated by additional exploration/ delineation and development well data. The data confirmed that the reservoirs are oil-bearing and productive. This marginal development was a challenge due to reservoir quality uncertainties (extensive bioturbation), structural complexities (steep bed dips, close to a major reverse fault), and fluid contact uncertainties caused by production. Reservoir simulation study was carried out to address these uncertainties to aid development planning. Reservoir simulation study has also identified the potential for infill drilling and workovers to further improve recovery in the eastern area. This latest simulation study was conducted using finer layers, unstructured grids, and incorporated over 25 years of history matching. Given the advanced state of field depletion, early assessment utilizing compositional simulation model and laboratory tests are being carried out to investigate the potential for enhanced oil recovery processes. Processes under evaluation include double displacement and water alternating gas (WAG) displacement using various types of gas. Introduction The Tapis field was discovered in May 1969 in an average water depth of about 64 meters. It is located in the southern part of the Malay basin as shown in Figure 1. A total of fifteen exploration and delineation wells have been drilled to-date. Hydrocarbon bearing reservoirs are in the Group I, J, and K sandstones as shown in Figure 2. This paper focuses on the oil development in the Upper and Lower J reservoir groups only, which contain about one billion barrels of oil or 90% of the field's original oil in-place. The assessment and application of IOR started early and have been a continuous process; several IOR assessments have been conducted and associated work programs have been implemented. The application of Enhanced Oil Recovery (EOR) has also been evaluated in the past. These were mostly simple screening level assessments. A more detailed evaluation involving laboratory tests and compositional models has recently been initiated.

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