The Marcellus Shale and Natural Gas

Some layers of rock underlying New York State contain large quantities of natural gas, including the Marcellus shale and the Utica shale. Gas wells have been drilled in the Marcellus shale in the past, but because of the great depth of the shale and the manner in which these wells were drilled, many of them did not yield much gas. Due to a relatively new drilling approach, called horizontal hydraulic fracturing (or hydrofracing), which allows much more gas to be recovered from a well, many new wells are now being drilled in the Marcellus shale in southern New York and Pennsylvania.

Hydrofracing is a common technique used in constructing or reviving wells drilled for either water or gas. Basically, water is forced down the well bore under very high pressures and this causes many fine fractures to form or expand in the rock around the well bore. In the case of the Marcellus shale, the natural cracks in the rocks are vertical. Therefore, a vertical well, even with hydrofracing, does not access many cracks. Having the well access many cracks is important, because the gas, which is mostly stored in the small pores of the rocks, has to travel from the pores to these cracks, where it can then be extracted. Horizontal hydrofracing involves drilling into the shale horizontally at the bottom of the well. This L-shaped well intersects many more cracks than a single vertical well bore. Many further cracks can then be formed through hydrofracing.

For a diagram of this process go to http://www.propublica.org/special/hydraulic-fracturing.