Liquids Unloading
Summary
Accumulation of liquids (i.e., water and liquid hydrocarbons) in a mature gas well can slow down and sometimes halt gas production. To resume gas production, the liquids must be cleared from the well. The process of removing the liquids is commonly referred to as “liquids unloading.” Practices to remove liquids from the well and restore gas flow can result in methane emissions.
Description
When wells are ready for production (i.e., after completion), most natural gas wells have sufficient reservoir pressure to flow small quantities of formation liquids (i.e., water and liquid hydrocarbons) entrained with the produced gas to the surface. As gas production continues, the reservoir pressure declines and the underlying liquid layers rise closer to the well perforations, resulting in more liquids being entrained with the produced gas. This common progression eventually slows the gas velocity up the production tubing to a point where it is no longer sufficient to lift liquid droplets to the surface. Liquids accumulate in the tubing, creating additional pressure drop, slowing gas velocity further, and raising pressure in the reservoir surrounding the well perforations and inside the well casing. As the bottom hole well pressure (i.e., a combination of sales line back pressure plus accumulated liquid head) approaches reservoir shut-in pressure, flow from the reservoir, and thus gas production, stops.
One approach to remove or unload the liquids and temporarily restore flow is to close the sales line to allow bottom hole pressure to build and then open the wellhead to the atmosphere to blow the liquids out of the well tubing (well “blowdown”). This practice and other methods of removing the liquids include can result in methane emissions.
References
Allen, D. T., Sullivan, D. W., Zavala-Araiza, D., Pacsi, A. P., Harrison, M., Keen, K., Fraser, M. P., Hill, A. D., Lamb, B. K., Sawyer, R. F., & Seinfeld, J. H. (2015). Methane emissions from process equipment at natural gas production sites in the United States: Liquid unloadings. Environ. Sci. Technol. 49, 1, 641-648. https://doi.org/10.1021/es504016r
New Mexico Methane Advisory Panel. (2019, Fall). New Mexico Methane Advisory Panel draft technical report. https://www.env.nm.gov/wp-content/uploads/sites/15/2019/08/MAP-Technical-Report-December-19-2019-FINAL.pdf (6 MB)
Pasci, A., Sullivan, D. W., & Allen, D. T. (2020). Revised estimation method for emissions from automated plunger lift liquid unloadings. Environments. 7(4), 25. https://doi.org/10.3390/environments7040025
Zaimes, G. G., Littlefield, J. A., Augustine, D. J., Cooney, G., Schwietzke, S., George, F. C., Lauderdale, T., Skone, T. J. (2019). Characterizing regional methane emissions from natural gas liquid unloading. Environ. Sci. Technol. 53, 8, 4619-4629. https://doi.org/10.1021/acs.est.8b05546
Please Note: This platform reflects experiences and lessons learned from voluntary program partners. Some of these emission sources and technologies are now regulated at the federal, state, and/or local level in the United States and in other countries. The end user is solely responsible for complying with any and all applicable federal, state, and local requirements. For information on U.S. regulations for the oil and gas industry, refer to eCFR. EPA makes no expressed or implied warranties as to the performance of any technology and does not certify that a technology will always operate as advertised. Mention of names of specific companies or commercial products and services does not imply endorsement.