Tight gas is natural gas that is difficult to access because of the nature of the rock and sand surrounding the deposit. Because this gas is so much more difficult to extract than natural gas from other sources, companies require a large financial incentive to go after it; as energy prices rise, so does interest in extracting it. Several global oil and gas companies control significant tight gas reserves, and some have also sunk substantial resources into learning more about extracting it more efficiently.
Normally, natural gas is fairly easy to access. When a deposit is identified, a well can be sunk, and the gas naturally flows into the well, making it easy to pump it to the surface and to distribute it from there. This is because natural gas is normally surrounded by deposits of porous rock, with lots of small holes for the gas to seep through. Sometimes, the gas almost literally pumps itself.
In the case of tight gas, the surrounding sandstone, shale, or other rock is not so permeable, looking much denser in cross-section. The lack of permeability locks the gas up underground, making it difficult to drill a profitable well. This gas is also found trapped in coal deposits. In order to get at the it, it's necessary to find a “sweet spot” where a large amount of gas is accessible, and sometimes to use various means to create a pressure vacuum in the well which pulls the gas out of the surrounding rock.
Historically, such deposits were written off as “unrecoverable,” but as the demand for natural gas has grown, many companies have rethought this assessment, pushing to see if the deposits could be accessed. While tight gas is costly to extract, higher gas prices can make the expense worth it, especially if the gas has a composition that is favorable to distillation, allowing the company to extract several valuable fractions from a single well.
Most tight gas deposits date to the Paleozoic era, which means that they are at least 251 million years old. The advanced age of such deposits is presumably responsible for their inaccessibility; the gas is tight, in other words, because the rock around it has had more time to become dense. These deposits may also be deeper than ordinary gas deposits, posing additional challenges. Companies that work with tight gas use a variety of survey tactics to identify potential sources of gas and to target the best spots for drilling.
By Mary McMahon
Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.