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By Petroleum Technology Transfer Council |
Ejector vapor recovery unit reliably and economically recovers vent and flash gas
With no moving parts, this venturi-based recovery method captures high-Btu content gas, allowing it to quickly pay for itself.
Kip Melancon, Burlington Resources Inc., Houston, and Jim Nunn, COMM Engineering, USA, Lafayette, Louisiana
Burlington Resources Inc. installed a proprietary technology called an Ejector Vapor Recovery Unit (EVRU), manufactured and installed by COMM Engineering, USA, in their Reddell field located in Evangeline Parish, Louisiana. Using ultrasonic flow meters, COMM determined the loss to be 45 Mcfd of rich (2,475 Btu/Mcf) vent gas under normal operating conditions. Analysis determined the chemical makeup of vent gas. Prior to installation, the estimated daily revenue from capture of the high-Btu gas was expected to be about $557 per day, which would pay out the $126,862 installed cost in less than eight months. Within a few weeks following the installation of the ejector, additional oil completions were placed on production from in-field wells, increasing the 45 Mcfd flash and vent gas volume to 83 Mcfd. This increase in gas volume reduced payout to about four months. With no moving parts, maintenance costs are virtually nonexistent. Burlington Resources has requested bids for installations at several additional production sites.
BACKGROUND
Burlington was exploring options for capturing vented gas in its Reddell production facility, to realize a revenue stream and be environmentally progressive (vented methane has 21 times the greenhouse gas effect as carbon dioxide). The company considered using reciprocating and rotary-compressor vapor recovery systems, but ultimately selected the patented EVRU (the Unit) because of its mechanical simplicity, operational reliability and attractive economics. The Unit was installed in August 2004.
UNIT OPERATIONAL CONCEPT
The Unit, which uses the venturi principle, is a non-mechanical ejector that recovers vent gas by using high-pressure motive gas to entrain low-pressure hydrocarbon vapors, Fig. 1. The high-pressure natural gas is fed through a small, fixed contraction-expansion nozzle that converts its high pressure to a gas jet of supersonic velocity. This high-velocity natural gas jet is used in the eductor to entrain the vent (flash) gas at 0.20 psi above atmospheric. The entrainment-mixing process and subsequent expansion in the diffuser-expansion duct converts the high-velocity gas mixture to a pressure of at least 30 psi gauge for entry into a compressor. A volume ratio of about 3/1 motive to vent gas is required.
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Fig. 1. Schematic of venturi-driven EVRU.
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Understanding the design and operational concept, Burlington saw the following advantages compared to typical compressor solutions:
- No moving parts, very minimal maintenance
- Operates 100% of the time
- Operation not affected by liquids; In mechanical systems, liquids can cause premature failure
- Extremely low operating “cost” is the cost of recompressing the motive gas
- Attractive payback of less than five months
- Minimal space requirements, as simple as mounting on a pipe rack.
Confidence in the Unit's performance was buoyed by tests that verified performance by the Environmental Protection Agency (EPA). These are available at: www.epa.gov/etv/verifications/vcenter3-10.html and paper SPE 80599. After design, fabrication and installation, the ejector is just another steel pipe at the production site, complete with all safety control systems.
ECONOMICS
The recovered gas is rich, at 2,475 Btu/cf. That is, 240% of the heating value of methane of 1,030 Btu/cf. The market value is also 240% as much, that is, $12 gas vs. $5 gas. Flash gas typically varies from 1,500 Btu to 2,900 Btu, depending on the percentages of methane, propane, butane and other C4+.
The 83 Mcfd of recovered 2,475 Btu/cf flash gas represents $997 per day, or at least $363,981 per year, based on $5/MMBtu. The Unit itself cost $126,862. Payout at $5/MMBtu occurred in about four months. Cost to recompress the motive gas is the cost of Unit operation.
OTHER INDUSTRY INSTALLATIONS
The manufacturer has installed Units in a Russian gas production site in Siberia, a Kerr-McGee deepwater tension leg platform and several onshore and shallow water sites. Worldwide, several ejectors are now operating. 
THE AUTHORS
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Kip Melancon is an engineering advisor for Burlington Resources Inc. Melancon is a graduate of the University of Louisiana at Lafayette, Louisiana, with a BS degree in industrial technology. He has worked for Burlington Resources and affiliated companies since 1980, with extensive experience in completion and workover operations onshore and offshore. He is currently assigned to Burlington Resources' Mid-Continent Division with responsibilities directed toward Burlington's South Louisiana Asset Area. He can be reached at: KMelancon@br-inc.com.
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Jim Nunn, vice president of COMM Engineering, is responsible for ejector design, fabrication, installation, operation and customer satisfaction. He has extensive domestic and international experience in industrial design, funding, fabrication, verification, deployment, installation and operation of many systems across several industries. Nunn is a graduate of the US Naval Academy with a BS degree in physics, and also has an MS in business administration from the US Navy Postgraduate School. He also serves as an adjunct professor of economics and finance at the University of Louisiana at Lafayette, Louisiana, and is a board member of the International Trade Development Group of Lafayette. Jim can be contacted at: jimnunn@commengineering.com.
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