Methane Gas in Groundwater
(Updated March 2011)
Why is this indicator important?
Methane gas occurs naturally in groundwater aquifers in most geological sedimentary basins worldwide, including here in Alberta. Methane gas exists in a dissolved state in groundwater underground and will “bubble out” when pumped to the surface. For those on private water well supplies, spurting taps is a common result of this phenomenon. Methane gas can pose an explosion or asphyxiation hazard if allowed to build up in a confined space, so well owners are strongly encouraged to vent their water supply systems when gas is present.
This indicator is an important way to determine if groundwater is being affected by coalbed methane and other oil and gas activity.
Coalbed methane (also known as CBM) is a relatively new natural gas energy source in Alberta. Albertans have expressed concerns that methane gas could migrate from CBM wells to nearby aquifers and water well supplies.
Alberta Environment oversees a network of over 250 groundwater monitoring wells spread across the province. A monitoring program was initiated in 2006 to better understand the quality of groundwater in Alberta. The monitoring wells are tested for a variety of water quality parameters, including methane gas. A total of 126 wells have been accessed for gas sampling to date. Wells will be sampled every few years to monitor long-term trends.
What does the indicator show?
The distribution of monitoring wells sampled to date is shown in the map below. Out of the 126 wells visited, 38 wells contained appreciable concentrations of methane gas (>3000 ppmv), 32 wells contained minor concentrations (<3000 ppmv) and 56 wells contained no measurable amounts of gas. The map shows that many wells with methane gas lie outside the coalbed methane development areas in Alberta and that many wells with no gas lie within coalbed methane development areas.
Methane concentrations (in parts per million by volume or ppmv) for free gas collected from the monitoring wells are shown in the graph below. The graph shows no correlation between methane concentrations and distance to the nearest CBM well drilled or re-completed.
Methane gas can be biogenic or thermogenic depending on its origin.
- Biogenic methane is produced by subsurface bacteria and is a common natural source of methane gas in groundwater aquifers used for water well supplies;
- Thermogenic methane gas is produced at greater depths through high pressure and temperature processes and is characteristic of deep oil & gas reservoirs that conventional energy wells tap into; and
- CBM gas tends to fall somewhere between these two components.
Various data analyses can be used in conjunction to help determine whether gas in a monitoring well is of biogenic or thermogenic origin, or a mixture of the two.
Methane gas typically contains a small amount of ethane. The proportion of methane to ethane in a gas can help determine its origin. Biogenic gas typically contains above 1000 times more methane than ethane. In addition, isotope data can also be used to help determine whether a gas is biogenic or thermogenic. Carbon isotope ratios have been reported for monitoring wells containing concentrations of methane gas above 3000 ppmv. Isotope values tend not to be reliable below this concentration.
In the graph below, methane to ethane concentration ratios and carbon isotope ratios are plotted together. The results show that the gas contained in the sampled wells is primarily biogenic in origin.
Only 13 of the 70 wells with measurable gas contained enough ethane to determine carbon isotope ratios for ethane. The data is shown in the graph below. Potable aquifers in Alberta typically have gas with ratios that fall between -40 per mil and -60 per mil. Most of the monitoring well data lies within this range.
Sampling results indicate that methane gas occurs naturally in aquifers across Alberta, and that groundwater is not being impacted by coalbed methane or other energy well activity.
What actions are being taken?
Alberta Environment continues to track methane gas in groundwater through a province-wide groundwater sampling program. The Working Well program - launched in 2008 - educates landowners on how to manage gas in water wells.