The Gas-Assisted Gravity Drainage (GAGD) process has been suggested to enhance oil recovery by placing vertical injectors for CO2 at the top of the reservoir with a series of horizontal producers located at the bottom. The injected gas accumulates to form a gas cap while oil and water drain down to the bottom due to their heavier densities. The GAGD process has limitations with regards to the high levels of water cut and high tendency of water coning. This paper provides an integration of water sink into the GAGD process to overcome these limitations.
The hybrid process of Gas Injection and Downhole Water Sink-Assisted Gravity Drainage (GDWS-AGD) was developed and tested to minimize water cut in oil production wells from reservoirs with bottom water drives and strong water coning tendencies. In the combined technologies, the 7 inch production casing are dual completed for two 2-3/8 inch horizontal tubings: one above the oil-water contact for oil production and one underneath for water sink. The two completions are hydraulically isolated inside the well by a packer. The bottom (water sink) completion employs a submersible pump and water-drainage perforations.
The submersible pump drains the formation water from around the well and prevents the water from breaking through the oil column and getting into the horizontal oil-producing perforations. The GDWS-AGD was efficiently adopted to improve oil recovery at the upper sandstone member/South Rumaila Oil Field, located in Iraq. The Rumaila field has an infinite active aquifer with very strong edge and bottom water drives. Many successive cases were conducted to obtain the clearest image about optimal setting of the combined processes. These cases include oil and water production only, oil and water production with constant pressure gas injection, and oil and water production with decreased pressure gas injection. The injection pressure needed to be periodically decreased in order to ensure immiscibility of CO2 flooding. In the GDWS-AGD, the produced water not only reduced water cut and coning, but also significantly reduced the reservoir pressure, resulting in improving gas injectivity. In addition, the GDWS-AGD process improved oil recovery to promising levels. More specifically, the results showed that oil recovery increased from 71% to 85% and water cut decreased from 98% to less than 5% in all the horizontal oil producers.
The novelty of GDWS-AGD process comes from its effectiveness to improve oil recovery with reducing the water coning, water cut, and improving gas injectivity. This leads to more economic implementation, especially with respect to the operational surface facilities.
