Fracking, Natural Gas in Groundwater, and Stable Isotope Forensics

In Part 1 of my post on Hydraulic Fracking and Natural Gas in Groundwater, I explained how the composition of natural gas establishes a basis for differentiating between thermogenic and biogenic gases. Recall that thermogenic gases are formed from the heating of organic matter in high-temperature subsurface environments. Gases that form under these conditions are typically composed of hydrocarbons ranging from one to six carbon atoms, or methane (C1) to hexane (C6).

Biogenic gases form in lower temperature regimes, usually in the near subsurface, as a result of microbial activity on buried organic matter. These gases, which are dominantly methane with very minor ethane (C2) and propane (C3), are commonly associated with coalbeds, swamps, and landfills.

We can differentiate between thermogenic and biogenic gases by considering the molar ratio of methane to ethane and other hydrocarbons (C1/(C2+C3). As noted in my previous article, it is not uncommon for that ratio to be less than 100 among thermogenic gases and to be greater (oftentimes, much greater) than 1,000 among biogenic gases. The difference in ratios is a function of the deficiency of ethane (C2), propane (C3), and higher-chain hydrocarbons in biogenic gases.

m1_768
Figure 1

 

In addition to the molar ratios, geochemistry offers a deeper look into the signatures of natural gases by mass spectrometry.  This gives geochemists a powerful means of differentiating between thermogenic and biogenic gases. Specialized laboratories have the equipment needed to conduct such analyses on samples of dissolved gas in water and samples of gas collected from production wells. Such analyses focus on specific carbon and hydrogen components of the methane (C1) fraction.

More specifically, laboratories measure the abundances of an isotope of carbon (carbon-13 or 13C) and an isotope of hydrogen (deuterium, 2H or D) in methane, relative to their abundances in standards determined by the International Atomic Energy Agency. For 13C, the standard is known as Vienna Pee Dee Belemnite (VPDB) and for D, the standard is Vienna Standard Mean Ocean Water (VSMOW).

In a substance such as natural gas, methane (C1) will be depleted in both 13C and D, with respect to VPDB and VSMOW, and the abundances of the two isotopes in methane are reported as ratios in units of parts per thousand, or per mil (e.g., δ13C‰ VPDB and δD‰ VSMOW). Because natural gas is depleted in 13C and D compared with the VPDB and VSMOW standards, the reported abundances will be negative.

Figure 1 is a plot of the δ13C and δD values for the two samples of Wilcox dissolved gas and the five samples of Haynesville formation gas as discussed in the previous post. There are three shaded fields: One is representative of thermogenic gas and the two other fields are representative of gas produced by microbial activity. The microbial gas field to the left of the thermogenic gas field is the domain within which the δ13C and δD values of gas associated with lignite and other low-rank coals cluster. The shaded area below the thermogenic gas field is representative of microbial gases that form in swamps and landfills.

m2_768b
Figure 2

All of the Haynesville gas samples lie near the center of the thermogenic gas field and the two samples of dissolved gas from the Wilcox aquifer are within the field associated with coalbeds. This separation between the Haynesville and Wilcox gas samples is unmistakable and establishes a second and very sound geochemical basis for differentiating between thermogenic and biogenic gas. Geochemists look for separations such as this when trying to identify the conditions under which a gas originated.

We can take this a step further by plotting δ13C against C1/(C2+C3), as shown by Figure 2 (also known as a Bernard plot). In this figure, the Haynesville samples lie within the thermogenic field, in the lower right. The C1/(C2+C3) ratios are all less than 100 and the associated δ13C values range from approximately –45‰ to –40‰. The two Wilcox gas samples are located within an area of the Bernard plot dominated by microbial activity. For these samples, C1/(C2+C3) is close to 10,000 and δ13C is –68‰ to –64‰. The separation between the Haynesville and Wilcox gases is so distinct on this plot, both with respect to δ13C and C1/(C2+C3), that there can be no reasonable basis for arguing that there is any influence of thermogenic gas in shallow groundwater.

In this brief presentation and the previous article, we have examined some very basic concepts that offer penetrating insight into the origin of natural gas discharging from shallow ground­water. The concepts and associated plotting methods that I have illustrated are well established and widely used. It is especially important to consider the overall composition of gas and the isotopic signatures of gas discharging from domestic wells before concluding that oil and gas operations are to blame.

 

Share and Enjoy

  • Facebook
  • Twitter
  • Delicious
  • LinkedIn
  • StumbleUpon
  • Add to favorites
  • Email
  • RSS

Hydraulic Fracking, Natural Gas, and Groundwater

Anyone who has watched films such as Gasland and Promised Land is aware of the alleged dire consequences to groundwater resources from the petroleum industry’s decades-old practice of hydraulic fracturing of tight formations to release hydrocarbons.

Gasland (a “documentary”) and Promised Land (a movie starring Matt Damon) purport to expose such threats, which are highlighted in the form of natural gas discharging from groundwater or flares from the kitchen faucets of landowners whose water wells are located near fields with recently fractured wells.

Such representations are intended to create the impression that fracturing opens pathways from deeply buried strata for the migration of natural gas, brine, and drilling fluids to shallow formations that are sources of groundwater for ­cities and rural areas where fracturing is a common method for oil and gas extraction. The print and broadcast media are complicit in the matter, as both have piled on to fan the sentiment of Americans against this method of developing much needed natural gas resources.

A consequence of anti-fracturing films and reporting is confusion among the public and attempts by environmentalists to ban fracturing.

Origins of Natural Gas

Natural gas is derived from different sources. Most of what we think of as natural gas is produced within the deep subsurface under high-temperature and high-pressure conditions (thermogenic gas). A much smaller component is derived from the production of gas in the shallow subsurface under low-­temperature and low-pressure environments (biogenic gas, that is, coal beds, swampy environments, and landfills).

Geochemists can differentiate be-tween thermogenic and biogenic gases by looking at their carbon compositions (as illustrated by gas chromatographs) and their carbon and hydrogen isotope signatures.

This column focuses on the role of gas chromatography as a differentiator of biogenic and thermogenic gases. Carbon and hydrogen isotope ratios will be the subject of another column.

Gas Chromatography

Because of the different environments in which natural gases are generated, thermogenic gases and biogenic gases can be expected to have distinct compositions. All hydrocarbons are carbon-based compounds with the number of carbon atoms of the components ranging from one to as many as 30 (C1 to C30+). In this post, I am interested only in those components with one to six (C1 to C6) carbon atoms, which are methane through hexane, respectively.

m1_768a
Figure 1 – Composition of thermogenic and biogenic gas samples from Haynesville formation and Wilcox aquifer wells, Caddo Parish, Louisiana

Biogenic gases are primarily methane (C1) and very little ethane (C2) and, in some cases, propane (C3), with methane usually accounting for 99% or more of total gas on a molar basis. Thermogenic gas consists of measurable components of fractions heavier than methane (that is, C2 through C6 gases).

The difference between thermogenic and biogenic gases is illustrated in the figure on the left, which shows the composition of the gas samples collected from two Wilcox aquifer water wells and produced-gas samples from nearby fractured wells that were thought to have been the source of natural gas in the aquifer.  All of the wells are located pproximately 10 miles north of Shreveport (Caddo Parish), Louisiana. The figure shows the percentage (by mass) of each hydrocarbon and nonhydrocarbon gases detected in the samples.

Different Sources, Different Compositions

There are distinct differences between biogenic and thermogenic gases. The samples of biogenic gas, collected from the aquifer, include large percentages of the nonhydocarbon gases argon (Ar), oxygen (O2), and nitrogen (N) (all of which are dominant components of atmospheric gas), a large percentage mass of methane, and very little ­ethane and propane. In these samples, the ratio of methane to ethane and propane is much higher than 1,000. These high ratios are characteristic of biogenic gases, which are considered to be “dry.”

The samples of thermogenic gas, collected from recently fractured wells within one mile of the water wells from which the samples of biogenic gas were collected, are deficient in the atmospheric gases, but are dominated by methane through hexane. Furthermore, the molar ratio of methane to ethane and heavier hydrocarbons in the samples is less than 50. Such low ratios are common to thermogenic gases.

The hydrocarbon composition illustrated in Figure 1 can be considered to represent the “fingerprints” of the different samples of gas. Clearly, there are distinct differences between the fingerprint patterns of the biogenic and thermogenic gases.

My further investigation of this matter pointed directly to an association with lignite beds in the Wilcox as the source of the natural gas in the groundwater wells. Lignite, a low-rank coal, is widespread throughout the shallow subsurface of northwestern Louisiana.

Heavy pumping of water wells during an extended period of below-­normal rainfall (drought) caused lowering of water levels in domestic wells throughout the area. The lower water levels reduced pressures just enough to allow biogenic gas to migrate toward pumping water wells. Also found, in the driller’s notes on one of the water wells, were descriptions of lignite within the 50-ft screened interval of the well.

I have looked at many such cases in Louisiana in which natural gas is detected in groundwater. In every such case, whether in northwestern Louisiana, central Louisiana, and southeastern Louisiana, all of the geochemical signatures have been consistent with the biogenic gas produced in shallow subsurface environments, and not with thermogenic gas migrating from fractured formations deep within the subsurface.

This underscores the importance of not rushing to judgement on the assumed effects of fracturing, but to take a close look at local and regional geological factors that easily account for the generation of biogenic gas. In Part 2, I will delve further into this matter, with an examination of the signatures of the stable isotopes carbon-13 (13-C) and deuterium (D or 2H) of the C1 components of the biogenic and thermogenic gases from this investigation.

 

Share and Enjoy

  • Facebook
  • Twitter
  • Delicious
  • LinkedIn
  • StumbleUpon
  • Add to favorites
  • Email
  • RSS