MadSci Network: Earth Sciences
Query:

Re: What happens to dry oil wells?

Date: Wed Nov 8 08:30:16 2000
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Area of science: Earth Sciences
ID: 973487245.Es
Message:

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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