Improving Recovery Through Gas Injection Optimization in Complex Mature Oil Reservoir Arab B and D in the Bahrain Field

Abstract

This paper aims to demonstrate ways of improving oil recovery from a mature oil rim reservoir by managing both gas injection into the gas cap and water pump-off from a strong aquifer. Arab reservoir consists of four members: Arab A, Arab B, Arab C, and Arab D. Arab A and Arab C are gas reservoirs, while Arab B and Arab D are oil reservoirs overlaid by gas caps and underlain by strong water aquifer, which makes their drive mechanisms a combination of solution gas, gas cap expansion, and water drive. Arab oil production commenced in June 1938 and peaked at 5,800 BOPD in 1984. Gas injection into Arab D gas cap started with one well in February 1986 and increased to four gas injectors (3 in Arab D and 1 in Arab B) by September 2015. Recent oil recovery has been negatively impacted by a combination of strong aquifer drive and a depleted gas cap, which resulted in the loss of significant oil to the gas cap, while water-cuts increased in most production wells. Gas injection management needed to address these problems were hampered by decreasing reservoir pressure in the Khuff reservoir, the main source of the injected gas. A two-pronged gas injection optimization project was implemented to achieve the following objectives: To reduce the strength of the aquifer drive, and consequently, more gas could be injected into the gas cap given the limited gas injection pressures available; and To increase injected gas volumes to re-pressurize the gas cap in order to push the oil rim back down to the wellbore perforations and reduce water invasion in the oil zone. The gas injection optimization project commenced in 2015 by using idle watered-out producers to pump-off the aquifer water while increasing gas injection rates from 15 to 25 MMscf/d into the gas cap. Most producing wells responded with falling water-cuts and increased oil production rates. After three years of the experiment, Arab oil production increased by 70-80%, reducing the overall production decline rate from 50% to 25%. The project was successful, however, sustaining the higher gas injection rates in the future may be problematic as the wellhead pressure for the gas source wells (Khuff reservoir) has already fallen below the required Arab gas injection pressure. Nevertheless, the results of the project convinced the management to invest in future gas compression facilities to meet the long-term gas injection requirements, which in turn will improve the oil recovery of the Arab reservoir.