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Preventing Stray Gas Migration

Having natural gas, more specifically methane, enter your home or drinking water well presents an explosion risk that could have severe consequences. Because methane escapes quickly from water it can accumulate in confined spaces within your home and reach dangerous concentrations. There have been documented cases within Pennsylvania where houses, camps and water wells have exploded due to stray gas migrations that have resulted in serious bodily injury, including death.

The most critical factor in preventing a stray gas migration is a properly cemented and cased gas well. Casing is typically steel pipe that is used to line the inside of the drilled hole or “wellbore.” Each full length of casing is often referred to as a casing string. Wells are typically constructed in multiple casing strings, including a surface string and production string. These strings are set in the well and cemented in place. The multiple layers of casing and cement when properly installed protect fresh water aquifers and help ensure that the producing zones are isolated form the overlying formations. After each casing string is set and prior to drilling deeper into the well, the casing is cemented to ensure a seal is provided between the casing string and the formation or between the two casing strings.

The most critical casing string is the surface casing string. The surface casing string is the primary line of defense in isolating the deepest groundwater zone from the natural gas being produced from the well and/or nonproduced natural gas present in lower formations. After the surface casing is put into place, cement is pumped down the inside of the casing, forcing it up from the bottom of the casing into the space between the outside of the casing and the wellbore, called the annulus. Once a sufficient volume of cement to fill the annulus is pumped into the casing, it is usually followed by the pumping of a volume of fresh water into the casing until the cement begins to return to the surface in the annular space. The cementing of casing from bottom to top using this method is called circulation. The circulation of cement behind surface casing ensures the entire annular space fills with cement from below the deepest ground water zone to the surface.

The two most important factors to ensure that the surface casing will be effective is setting it at the appropriate depth and ensuring that it is cemented to the surface. It is essential that the surface casing be set below the deepest fresh groundwater zone to properly isolate it. However, it is equally important that the surface casing is not set too deep. Setting the surface casing too deep makes it more difficult to cement the surface casing to surface, and also presents a risk or opportunity for natural gas from shallower formations to commingle with the deep fresh groundwater zone. New proposed regulations would require the surface casing be set at least 50 feet below the deepest fresh groundwater zone and not deeper than 200 feet below the deepest groundwater zone, unless necessary to set the surface casing into consolidated rock. Further, if the surface casing is not cemented to surface it is less likely that the cemented casing will be effective in isolating the groundwater than had the cement been returned to surface.

Companies drilling wells to produce gas from the Marcellus Shale formation in Pennsylvania should have a casing and cementing plan specific to each well that it is proposing to drill. This plan should indicate where the surface casing will be set and how the depth of the deepest groundwater zone at that well location was determined. Further, the casing and cementing plan should indicate whether the surface casing will be cemented to the surface.

If you have more questions about stray gas migration and how to protect the groundwater and/or the drinking water well on your property contact an environmental attorney at the Quinn Law Firm.


1. Harper, J. A. The Marcellus Shale. An Old “New” Gas Reservoir in Pennsylvania. Pennsylvania Geology Vo. 38, No.1

2. United States Department of Energy, The Office of Fossil Energy. Modern Shale Gas Development in the United States: A Primer. April 2009.

3. United States Department of Energy, The Office of Fossil Energy. State Oil and Natural Gas Regulations Designed to Protect Water Resources. May 2009.

4. The Pittsburgh Geological Society. Natural Gas Migration Problems in Western Pennsylvania.