The purpose of proper well abandonment goes far beyond just covering it up so your horse won’t trip in it. The purpose is to restore the geologic conditions to as close as possible to original before the well was drilled. To do this properly sometimes takes more steps and engineering than the local government hand has written down in his complete book of instructions. You could follow his instructions, but you might not make a proper abandonment, or it might not be doable. In this case, negotiation and education is the key.
Well abandonment in hard-rock aquifers where the production is from large
fractures, vugs or sometimes extensive caverns is particularly challenging.
Historically, the most common abandonment material is neat cement. This is
cheap, available and accepted by most bureaucrats, but it won’t always work
without some “enhancement.”
One of the first steps is to know what you’re dealing with. Good well logs may
or may not be available, but if they are, they’re a start. From the log, you
can calculate approximate well-bore volumes to figure your cement. If logs are
not available, sounding or logging and some knowledge of the local geology will
help. Now we get to the tricky part: Since cement has a much higher specific
gravity than water, when pumped in a well, it may overcome the natural
formation pressure, and leak into the formation faster than you can pump it. In
vugged formations, this sometimes can be overcome by adding gel to the cement
to 1) lighten the hydrostatic head, and 2) increase the viscosity of the mix.
This will allow it to stay where you put it until it sets and develops enough
mechanical strength to stay where you need it.
If the formation is highly vugged, fractured or cavernous, a different method
usually is employed. Since the formation usually won’t take much hydrostatic
pressure, pumping a huge column of cement is impractical, as it will leak away.
If lightening the hydrostatic head with gel is not enough, staged cement may be
the answer. This is done by pumping small amounts of cement (a few sacks) at a
time, and letting it develop some compressive strength before the next stage.
This may take a long time, and if you anticipate this condition, be sure to
figure it in your bid; you may be there for a while. I’ve seen wells that took
more than a month to properly cement. In states that require an onsite witness
to all abandonment procedures, this will make you very popular with your
inspector if you pump 20 sacks, wait 3 hours, pump 20 more sacks, wait 3 more
hours until you have the entire 1,200 sacks in the hole.
One way to cut the WOC (waiting on cement) time is to accelerate the cement.
One of the most common methods: calcium chloride. This will cut the waiting
time considerably, depending on how much you add, but be careful, too much and
the cement will flash-set in your pumps and tremie, and ruin your day. If you
see the pressure start to spike, and the pump is grunting and groaning, it’s
time to do something right now. An old trick that still works is to have 10
pounds or so of sugar on hand. Sugar will prevent cement from setting for a good,
long time, long enough to wash out your pumps and save the job, if not your
tremie line. Just add it to the pump suction, and let it work. If you don’t use
it, you always can take it home for your wife to use to make
If the hole is so fractured or cavernous that staging cement is not practical,
there are other methods. One of the purposes of proper abandonment is to
restore the hole to pristine condition geologically. This means sealing any
interconnection between discrete, separate aquifers. If the section you are
sealing is all one aquifer, sometimes it is acceptable to pump gravel of the
same formation as native. This works well and is fast, but is not always
allowed by the regulations. If not, bentonite chips – if allowed – are an alternative.
They are more expensive, but the make an excellent seal between aquifers. If
you are dumping or pumping them deep, it is important to screen out the fines,
or they will swell before you get them where you need them. Additional
pump/dump time can be gained by mixing some polymer such as Baroid E-Z mud
(usual disclaimer) to the mix water.
Once you have cemented up to the casing shoe, it is easy to calculate the
displacement and pump to surface in one batch. Allow enough excess for your
inspector to see, and you are almost done. Keep in mind that cement is at its
greatest volume when it is pumped, and will shrink. When you go back the next
day, don’t be surprised if the cement level is down somewhat; this is natural.
Actually, it’s probably a good thing. It allows you to cut off the casing below
ground level, and fill to surface so your horse won’t fall in
Hope this helps. If you need any help abandoning a well or with general
cementing, give me a call.
Just in case ya’ll think I’ve gotten all serious or something, I’ve got to tell
you a story about my brother Willard. Ya’ll know my brother Willard; he’s not
the sharpest bit at the rig, but he likes to go moose-hunting in Alaska. He’s
been a couple times and done pretty well, too.
Last year, he went to a little camp on Lake Whodathunkit. Hired a plane out of
the Yellow Pages. I guess he didn’t
understand exactly what “bush pilot” meant. The pilot delivered him and his
gear, and he had a wonderful week and shot a good-sized moose. After a few
days, the pilot showed back up to get him, and Willard started loading a
1,400-pound moose in the little ol’ single engine plane. The pilot told him
that the moose was too heavy, and he couldn’t take all of it: “You can take the
head to mount, or a haunch to eat, but not all of it.”
Willard told him, “I kill’t a moose last year, and the pilot let me put in the
plane. It’s about the same size as yours.” The pilot finally gave in, and they
loaded the moose and all Willard’s gear. The plane roared, struggled mightily
to take off, but didn’t quite clear the tree line on the other side of the
lake. Down they went. The crash pretty much destroyed the plane and knocked out
the pilot. Willard wasn’t hurt – probably because he was drunk – and got out to
check the damage; it was totaled. About that time the pilot came to, and asked
Willard, “Where are we?” Willard said, “’Bout a 100 feet farther than we made
it last year.”
The World According to Wayne: Well Abandonment in Hard-rock Formations
October 1, 2009