Thanks for getting back to me on this.
Here are a few links to sites that provide a quick overview of how oil
wells are regulated, and what happens to wells at the end of their
California Department of Conservation
DEPARTMENT OF CONSERVATION BEGINS PLUGGING
LEAKING OIL WELLS IN LOS ANGELES NEIGHBORHOOD
...In the plugging process, a portable rig is placed over the well.
Cement and special drilling mud are alternately pumped deep into the
well casing through tubing. When the cement hardens, it stops oil, gas
and water from entering the well and migrating to the surface. The mud
acts as a secondary barrier.
Alaska Oil and Gas Conservation Commission
WELL PLUGGING REQUIREMENTS.
(a) Plugging of the uncased portion of a wellbore must be performed in
a manner that ensures that all hydrocarbons and freshwater are
confined to their respective indigenous strata and are prevented from
migrating into other strata or to the surface. The minimum
requirements for plugging the uncased portion of a wellbore are as
(1) by the displacement method, a cement plug must be placed
(A) from 100 feet below the base to 100 feet above the top of all
(B) from the well's total depth to 100 feet above the top of all
(2) by the displacement method, a cement plug must be placed from 100
feet below the base to 50 feet above the base of each abnormally
geo-pressured stratum and from 50 feet below the top to 100 feet above
the top of each abnormally geo-pressured stratum...
(3) by the displacement method, a cement plug must be placed from 150
feet below the base to 50 feet above the base of the deepest
[the description continues, but I didn't want to overwhelm you with
the whole thing!]
As I mentioned, specialty bridge plugs are sometimes used -- a
mechanical device, usually used in conjunction with cement, that helps
to seal an abandoned well.
Here's an example, again from Alaska, of a regulation requiring a bridge plug:
E) if the perforations are isolated from open hole below, a mechanical
bridge plug set no more than 50 feet above the top of the perforated
interval, and either a minimum of 75 feet of cement placed on top of
the plug by the displacement method or a minimum of 25 feet of cement
placed on top of the plug with a dump bailer...
and here's what some of the suckers look like:
I trust that's the information you needed.
However, please don't rate this answer until you're fully satisfied
with the information you've received. If there's anything else I can
do for you, just post a Request for Clarification to let me know, and
I'm at your service.
Google searches on:
abandoned oil well regulations cement
abandoned oil well regulations "bridge plug"
and a Google Images search on [ "bridge plug" ]