Opportunities to improve the standard three-stage wellhead gas-lift compressor design for application to unconventional shale reservoirs are presented. A two-stage design is presented, with two field installations in theEagle Ford Shale reviewed as a case study.
The shale revolution began with gas reservoirs, most notably the Barnett, and preceded development of shale oil resources. This resulted in the need for many new compressors, and the rental compressor industry accelerated production of the standard three-stage compressor. No substantive design changes were made, as compressors that could meet either wellhead/gas-lift or gathering applications were preferred due to their versatility.
As the shift was made to horizontal oil, problems appeared with the standard compressor design in handling natural gas liquids components (primarily propane, butanes, pentanes, and hexanes plus). The standard design provides extra aftercooling, in part to support glycol dehydrator operation. As a result, these components often condense in compressor gas coolers, resulting in operational and environmental problems including frozen dump lines, heavy tank vapor emissions, and hydrates. Downtime and emissions related to these problems contributed to some operators viewing gas-lift as the artificial lift method of last resort, despite its superior ability to handle sand production, deviated wellbores, and high fluid volumes.
The capabilities of two-stage versus three-stage compressors for gas-lift are compared in the case study. The lower suction pressures afforded by three-stage compression are negligibly beneficial to horizontal shale oil wells, where slugging is an issue, and higher separator pressures are selected to mitigate slugging and aid liquid displacement from separation equipment. Additionally, the incremental pressure drop achieved by three-stage compression can provide little production improvement when compared to the total pressure drop from the reservoir to the wellbore. The phase diagram is used to show the necessity of temperature control on each stage of gas cooling to prevent process problems for gases including NGL components.