Due to the complex geological nature of naturally fractured reservoirs (NFRs), their development is often associated with a lot of challenges, especially when it comes to improved oil recovery (IOR)/enhanced oil recovery (EOR) processes, where certain fluids are injected into the reservoir with the aim of increasing the recovery factor. If a large enough contrast in permeability between the rock's matrix and the fractures exists, which is often the case, fractures will dominate the flow, leading to a poorly processed matrix and eventually a poor recovery factor. The Rubble reservoir is one of the shallowest reservoirs in the long-producing Bahrain field that was discovered in 1932 as the first oil discovery in the Gulf Cooperation Council (GCC) region. With the majority of oil in place categorized as heavy oil, it was decided that a steam pilot would be implemented in 2011. Steam piloting in the Rubble reservoir has gone through multiple stages since 2011, and different thermal recovery processes have been tested using vertical, deviated, and horizontal wells.
The initial pilot tested cyclic steam stimulation (CSS) using high-pressure steam injection and low-pressure production in vertical wells, which was successful in mobilizing oil from the low-permeability Rubble reservoir. Based on the initial technical success, the CSS pilot was expanded to 17 additional wells drilled on three well pads in different parts of the field with different reservoir properties. The CSS expansion pilot also mobilized oil from the reservoir. Subsequently, continuous steam injection and offset oil production were attempted in two of the steam pilot pads. CSS was also attempted in horizontal wells; however, although oil was mobilized, the produced rates and volumes were not enough to have a low steam/oil ratio (SOR). These efforts ultimately led to the forced imbibition (FIM) pilot. In this current pilot, steam is simultaneously injected into three parallel, closely spaced horizontal wells and then produced simultaneously using large pumping units to reduce the bottomhole pressure to a minimum.
Results have been encouraging, with SORs of less than 4 barrels of steam per barrel of oil (bs/bo). FIM has proved to be the most promising steam EOR process attempted in the Rubble reservoir.
This paper examines the results of the Rubble steam pilot stages, highlights the challenges faced in each stage, and presents our evolved understanding of the physical processes involved as the pilot progressed.