For mature oil fields with complicated reservoir architecture, reservoir surveillance is key to track reservoir performance. Reservoir surveillance may include various monitoring tools from complicated horizontal production logging tools down to regular well tests. One of the main surveillance methods is running formation pressure measurement tools such as Formation Pressure Testers (FPT) or as historically known to the industry, Repeated Formation Tester (RFT). This paper describes the use of this important tool integrated with production data to understand reservoir production and depletion behavior and hence support the Bahrain Field development plan.
A study was conducted on the Ostracod and Magwa reservoirs; complicated carbonate reservoirs in the Bahrain Field. The Ostracod Zone is a sequence of inter-bedded limestone and shale in the upper Rumaila formation of the middle Cretaceous Wasia group. It is over 200 feet thick and consists of three main units: B0, B1, and B2. The Magwa reservoir is the lower member of the Rumaila Formation. It is 120 feet thick and conformably underlies the Ostracod reservoir. It consists of three main units: M1, M2, and M3.
The main objectives of this study are:
Evaluating pressure depletion from the initial reservoir pressure for each unit in both reservoirs, which defined the existence of flow barriers in this inter-bedded complicated carbonate. Evaluating the relationship between pressure depletion in each unit and the spacing between offset wells to the FPT location. Evaluating the Ostracod/Magwa pressure depletion per unit with time. Linking the pressure depletion to the cumulative production from the area offset by the FPT data.
Evaluating pressure depletion from the initial reservoir pressure for each unit in both reservoirs, which defined the existence of flow barriers in this inter-bedded complicated carbonate.
Evaluating the relationship between pressure depletion in each unit and the spacing between offset wells to the FPT location.
Evaluating the Ostracod/Magwa pressure depletion per unit with time.
Linking the pressure depletion to the cumulative production from the area offset by the FPT data.
The results of this study helped define the depletion risk on the future infill opportunities in such complicated reservoirs. It also helped in locating highly depleted units and determining the optimal locations for the new infill wells.
Tatweer Petroleum completed the first successful Bahrain Field Forced Imbibition Gas Injection Pilot in 2014. This pilot was completed in the Ahmadi formation and demonstrates classical and economic performance. The paper also describes the systematic effective and efficient Tatweer pilot management approach which allowed an impressive results in a time frame less than one year.
The Ahmadi formation consists of three, carbonate, highly fractured reservoirs designated as Aa, Ab1, and Ab3. The Ahmadi reservoirs have the greatest areal extent of any of the oil-bearing reservoirs in the Bahrain field. The Aa and Ab productive zones cover over 41,500 acres, and are separated by 40-45 feet thick, shale members. The gas injection (GI) pilot development in the Ahmadi reservoir, along with the waterflood pilot, is considered a secondary recovery process. The objectives of this pilot is to test the concept of forced imbibition (FIMB) in Ahmadi reservoir after it was successfully tested in Rubble reservoir with steam injection. The FIMB GI process is performed by injecting gas within the pattern area for certain period while offset oil producers are kept shut-in. The offset oil producers are opened once the GI cycle is achieved and accordingly the gas injector kept shut-in. This process will enhance the stripping of the oil from the matrix to the fracture system, which are connected to the offset oil producers.
The selection criteria to select the optimum FIMB pilot pattern was developed based on an intensive matrix selection criteria. The selected pilot consists of three horizontal wells; one injector and two producers. The wells were completed as open-hole with ~1000m lateral in the Ab3 unit, and the space between the producers and injector is ~180m. The selected pilot was initially a water injection pilot. However, the production after the introduction of this pilot was not promising, and water break-through resulted in a reduction of the base production. Consequently, the pilot injector was suspended. The suspended water flood pilot area was selected since it is located in highly fractured and confined area.
A simulation dynamic model was constructed covering a sector area within the vicinity of the pilot. The model was used to run different scenarios for the number of GI cycles at different GI rates, and assess the required soaking time post GI Cycles. So far, 3 GI cycles have been completed with around 60 MMscf being cumulatively injected. The initial results were very encouraging during the FIMB GI execution. The dynamic scenario output predication was calibrated with actual production data from the executed FIMB GI pilot, and the results were in line with the model's predictions. Thus, the FIMB GI seems to be effective and promising enhanced recovery in a highly fractured carbonate reservoirs. The results of the pilot provided confidence on proposing crest GI pilots to enhance the secondary recovery process in the Ahmadi reservoir.