In addition to the shale gas that is starting to take off, there is another source of gas in the United States that is underutilized – coalbed methane (CBM).
As the market and infrastructure develop, this resource may be just the ticket
for those of us looking to keep our rigs running and be on the cutting edge of
technology. Like any specialized drilling program, we have most of the basic
tools, i.e. rigs, but there are a few other pieces of kit (as my Australian
friends say) that we need to go with it.
The first step in assessing the profitability of a CBM project is a good set of
reservoir data. This is done by vertical drilling and coring to determine the
depth to – and the thickness of – the coalbed, as well as the amount of
cleating in the coal. The coring should be done with a wire-line coring system
to maintain sample integrity as much as possible, and the cores placed in
vapor-tight containers as quickly as possible to help determine the available
gas. If you use a conventional coring setup, the gas will be gone by the time
you trip the core. Drilling should be done under-balanced to minimize formation
damage. In some formations, this means air drilling, and in others, a carefully
controlled mud program. Either way, you are going to have to rig up a diverter
head, at least, and maybe a full set of bops.
The permeability of the cores will determine well spacing and whether or not to
consider horizontal drilling. Good candidates for horizontal completions are
formations more than 3 feet thick with low permeability (less than 5
millidarcies). If permeability is high, vertical wells are quicker and cheaper.
Mud selection can be critical, and a careful review of the geologic setting of
the coal is needed. The lightest practical weight should be used to keep the
hole in an under-balanced condition. The use of surfactants, lost circulation
solids, and polymers will increase formation damage, decrease final
permeability, and should be avoided. Several operators have told me that it is
common to lose one or more test holes before completion to get the mud right.
If drilling with foam, a surfactant will be necessary to provide foaming
properties, but should be kept to a minimum.
If you are going to drill laterals, or horizontal sections of the well, it gets
a little more interesting. The first consideration is rig type. Tophead rigs are
a much easier and more accurate way to orient the tools; the tophead can be
oriented, and locked much easier than a conventional rotary. Drill pipe is a
major concern in any directional drilling.
In vertical drilling, stiffness helps keep the hole straight. In directional
drilling, predictable flexibility is key. Oilfield bottleneck drill pipe works
much better than flush-joint drill pipe because of its flexibility, and the
fact that the tool joints keep most of the body of the pipe from direct wall
contact, which reduces differential sticking. This is another reason to run an
under-balanced mud program. As far as bit selection goes, a good-ol’ tricone
probably is the best choice. They seem to stay in the seam better. This
probably is because a tricone bit will bounce off the floor or roof of the seam
and track better. This is because the upper and lower confining formations
often are harder than the coal. If you have a choice, and your navigation is
good, it is better to stay near the bottom of the coal; this helps with
Another difference you will find is that you are going to have to work closely
with a directional driller. In my experience, they usually are pretty
knowledgeable, but often kinda weird. Besides, most drillers don’t like having
somebody looking over their shoulders. My advice: Deal with it; it’ll lead to a
more harmonious outcome. Often the DD will let the driller drill the straight
sections of the hole, but they like to get on the brake when sliding (or
If you are going to build a multi-lateral well, one that has several horizontal
sections below one vertical section, it is best to drill the laterals from the
top down. Angle building is much easier in virgin formation, and whipstocks
don’t need to be run and recovered. After you have drilled your first lateral,
pull out of the hole, lay down the directional assembly and pick up a
straight-hole, packed-hole assembly. Go in the hole, and drill slowly by the
kick-off point into new formation and the trip out for the directional
assembly. Then you can build the next lateral in virgin formation. Be sure to
allow enough vertical room to make the bend on your deepest lateral; this is
where the shortest radius will be. It is best to enter the coal at a very
shallow angle. If you enter it at near vertical, you may have to drill out the
bottom and come back up in it – a waste of time. Coal often is too soft to
build angle quickly, and going below into something stiffer may be the only
Cementing CBM wells is fairly similar to conventional wells, except fluid
invasion into the cleat system is very important. If you have drilled a good,
under-balanced hole, formation damage will be minimal, but cement almost always
is much heavier, and this will overbalance the hole to prevent gas migration
into the cement sheath. Don’t forget that the casing must be well-centralized,
especially in the angle-building areas, to provide a good cement job, and
properly isolate the producing formation.
Prior to cementing, the mud should be conditioned to reduce the viscosity (Pv)
and yield point (Yp) to get a flat gel strength profile. Fluid loss control
should be lowered to reduce filter cake across permeable formations. If
conditioning and cementing across horizontal sections of the hole, the
viscosity may need to be increased to clean the hole and provide stability.
This should be done with fresh mud with the lowest possible solids
A fairly recent development in CBM wells is to use lightweight foam cement
(11.5 ppg to 12 ppg) to lessen the pressure on the producing formation, and to
lessen the chance of the cement exceeding the fracture gradient of the
formation. Another advantage is that the foam-cement is more flexible, and will
withstand the expansion and contraction of the casing due to internal pressure
changes without damage. Best bet here is to get a Halliburton dude involved. If
it goes well, great; if not, you’ve got someone to blame.
While there are a few extra steps, and a little more iron involved, CBM
drilling can be a very profitable way to keep your rigs busy.
The World According to Wayne: Coalbed Methane Drilling
February 1, 2010