This paper describes how the fracture model for Ahmadi was incorporated into a single and dual porosity simulation model. It highlights the approach and methodology that has been used for understanding the reservoir connectivity and its drive mechanism. In addition, the effect of dual porosity model in improving the initial history match of the reservoir performance and its recovery mechanism has been studied.
The two Ahmadi carbonate reservoirs in Bahrain Field (Aa and Ab) are thin, highly faulted and irregularly-fractured. The complexity of these reservoirs has prevented an efficient recovery with average production of wells to 15 Bopd. This has prompted a detailed integrated study to fully understand the recovery mechanism and to increase the wells' productivity.
A conceptual model has been developed for the reservoir connectivity of Ahmadi reservoirs by integrating static and dynamic data. This comprehensive study that integrates transient welltesting and production data with cores and image logs to map fracture network derived using seismic facies has resulted in improving the understanding of reservoir connectivity and flow mechanism. This was accomplished by building Discrete Fracture Model (DFN) that was tuned and validated using sector local well test models to match KH and pressure responses.
In order to reduce uncertainties in the model, different scenarios were built for different fracture intensity and aperture to derive appropriate parameters to pass to reservoir simulator for full field simulation. DFN model output was used in both single and dual porosity models to obtain production performance of the reservoirs.
The Bahrain Field lies beneath the center of Bahrain Island, to the east of Dammanm field and to the west of the Qatar arch (figure-1).
The Bahrain field was discovered in 1932 and is the oldest field in the southern part of the Arabian Gulf. The broad geological structure of the Bahrain field reservoirs consists of an asymmetrical anticline that is elongated in the north-south (N-S) direction. The fault pattern is characterized by a N-S graben with extensional faults along the crest of the dome. Additional set of later-formed NW- trending faults was seen on the flanks of the reservoir. Geology of the field is complex with a large number of faults occurring in the Wasia group formations, which contain the major oil reservoir of Bahrain field, the Mauddud reservoir. Another important member in this group is the Ahmadi formation, consisting of two reservoirs, Aa and Ab. These reservoirs are considered to be some of the most difficult reservoirs in the Bahrain field in terms of productivity.
The Aa and Ab reservoirs are thin Middle cretaceous limestone units in the Cenomanian Ahmadi Formation. They are composed of a variety of marine carbonates and mudstones and separated by about 50 ft of shale 1. They exist at a depth of 1900 ft as shown in the Bahrain Stratigraphic Column (figure-2).