Imagine it’s early in the morning and you’re collecting water from the kitchen faucet to start a cup of joe.
Suddenly, the water ignites.
Scary, huh?
Such far-out stories and more continue to circulate as foes of the myriad shale plays ratchet up their attempts to curtail drilling and, perhaps above all, hydraulic fracturing.
The principal oft-repeated storyline is that this oil patch activity is causing potentially flammable hydrocarbon gas to travel into fresh water aquifers.
This scenario remains unproven.
“When hydrocarbon gas is found in a public or domestic water supply in significant concentrations, determining the origin of that gas sometimes can be critically important to avoiding conditions that might threaten public safety,” said AAPG member Mark McCaffrey, geoscience manager of interpretation services at Weatherford Laboratories.
He noted that when this gas is found in groundwater in high enough concentrations to represent such a potential threat, the gas is called “stray gas,” citing Fred Baldassare et al, 2014.
There are varied possible sources of hydrocarbon gas dissolved in groundwater, and they all must be considered at the start of an investigation.
The industry has a collection of tools to determine the specific origin of any such gas found in aquifers.
Avoiding Assumptions
According to McCaffrey, the possible sources of the hydrocarbon gas in a given location can include:
- Naturally occurring biogenic gas, which is formed in the shallow subsurface via bacterial decay of sedimentary organic matter.
- Naturally occurring petroleum seeps containing hydrocarbons from deep beneath the subsurface. Seeps like these have guided geoscientists to explore for oil worldwide for more than 100 years.
- Modern petroleum development or coal mining activities in the area.
- Legacy conditions from discontinued past petroleum development or coal mining activities.
- Nearby municipal landfills where bacteria produce gases via decay of the organic matter present.
“Determining the specific origin of a stray gas typically requires an investigation in which geochemical, geological, engineering and/or historical data are integrated into a comprehensive, site-specific evaluation of the gas source,” McCaffrey reiterated.
“In the geochemical portion of the study, the geochemistry of the stray gas is compared with the geochemistry of samples from each of the possible sources of the gas,” he noted.
He tagged two types of geochemical parameters that are ordinarily measured in the gas samples:
- Molecular composition of the gas – how much of each gas species, such as methane, ethane, propane and others, is present.
- Gas isotropic composition – the ratio of specific isotopes of carbon and hydrogen that comprise the individual gas species.
These data can enable biogenic gas to be distinguished from thermogenic gas and also allow thermogenic gases of differing thermal maturity to be distinguished from each other, according to McCaffrey.
“A common mistake that some people make is to assume in advance that all naturally occurring hydrocarbon gases in aquifers are biogenic,” he said, “and that all thermogenic gases in aquifers are related to petroleum development activities.
“That assumption is commonly false.
“Naturally occurring thermogenic gases are quite common in the aquifers in petroliferous basins worldwide,” he noted.
No Easy Answers
In determining the origin of stray gas, McCaffrey emphasized that it’s essential for the investigator to consider all possible sources of the gas, rather than only the potential sources that may initially be identified most easily.
“The investigator should then attempt to I.D. geochemical characteristics that distinguish those discrete gas types,” he said. “Then those data must be integrated with other data types, commonly geological, engineering and/or historical, in order to identify the stray gas source.
“In certain cases, consideration of the possible mechanisms of gas migration is essential to uniquely identifying the gas source,” he added.
Which of the geochemical parameters will distinguish the gases from each of the potential sources is unknown until samples from each potential source are analyzed, according to McCaffrey.
“Parameters that are useful for separating the possible sources of gas in one location,” he said, “are not necessarily the same parameters that would be useful in separating the potential sources in another location.”