MadSci Network: Earth Sciences |
William: Petroleum wells that are not in use and won't be used again to produce petroleum usually have one of two fates. They may be used to inject water or carbon dioxide or something else, to stimulate production from nearby wells. Far more likely, they are "plugged and abandoned." This means, basically, that a lot of cement is poured down the hole and a steel plate is welded on top. The steel casing may or may not be pulled out of the hole before this is done. There are very specific state rules about exactly how this is done, and they differ from state to state. In the "old days" (say, the 1930's) there were few or no rules, and many wells were simply abandoned as open holes. As for what happens to them afterwards, it varies. Most wells probably fill up partially with water and/or petroleum. Rock or sediment also falls off the sides of the hole into the bottom of the hole if the casing was pulled or doesn't cover the entire length of the hole. Over the long term (thousands of years or longer) the holes will probably be squeezed shut by the slow expansion of the rock or sediment around them. There would be no perceptible sinking of the earth, because the holes are too small. They might be more than 4 miles long, but are only a few inches in diameter. They are wider at the top (up to a foot or more in diameter), so if they weren't plugged properly a local depression could form over the hole. Such a depression would probably be only inches deep and inches to feet across. As an aside, wells are never actually "pumped dry." Wells are abandoned when the owners decide that the amount of petroleum coming out per day is not enough to justify the expense of maintaining the well. The amount of petroleum remaining underground that cannot readily be removed is anywhere from 10 to well over 50 percent of the original amount. Exactly how much depends on the nature of the reservoir rock and on the strategy used by petroleum engineers to get the petroleum out. David Kopaska-Merkel Geological Survey of Alabama P.O. Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 www.gsa.state.al.us
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