Methane Migration

Many different components make up natural gas. The most prevalent is methane (CH₄), which is highly flammable and odorless. Methane is lighter than atmospheric air, so it has a natural proclivity to rise, not only through the atmosphere but also through porous formations underground. Underground methane, including tight gas trapped in shale, can migrate up through the stratigraphy if a pathway presents itself. A natural gas well or oil well is an obvious pathway to the surface, but engineering and operational best practices mitigate the risk of losing the valuable commodity. In cases of natural gas migration either to the surface or into groundwater sources, natural methane migration is often the more likely cause.

Geologic changes can provide pathways for upward migration. For example, damage to structural traps and seals caused by seismic activity may provide a pathway for natural gas to rise from deep subsurface formations. Even tiny changes to caprock integrity or microfractures in bedrock can provide microscopic pores through which light methane gas can travel. Once in permeable stratigraphic layers, the gas continues to rise until becoming dissolved in groundwater or until reaching the surface.

Methane migrates to the surface naturally through groundwater and other porous subsurface channels.

Other sources of natural methane migration include landfills, garbage dumps, and compost piles. These sources release methane not only into the atmosphere but also into the soil where it can enter groundwater or the basements of homes.

Research published in 2011 at the height of the hydraulic fracturing boom in Pennsylvania indicated a correlation between hydraulic fracturing and methane contamination of drinking water.1S.G. Osborn, et al., “Methane contamination of drinking water accompanying gas-well drilling and hydraulic fracturing,” Proceedings of the National Academy of Sciences 108 (2011): 8172–8176, accessed February 25, 2017. However, a more holistic investigation into the composition of groundwater in both gas-producing and non-gas-producing regions of Pennsylvania revealed naturally occurring methane in 78% of groundwater samples.2John Krohn, “STUDY: Naturally-Occurring Methane “Ubiquitous” in NE Pa. Groundwater,” Energy in Depth, June 3, 2013, https://energyindepth.org/marcellus/study-naturally-occurring-methane-ubiquitous-in-ne-pa-groundwater/, accessed February 25, 2017 Especially in regions such as Pennsylvania in which coal and associated methane or hydrocarbon-rich formations lie close to the surface, methane migration is a natural phenomenon, which can be monitored in order to mitigate any associated risks.

Images: “Graphic” by Top Energy Training