In coal mines, the Penitent really was

This is a little technical meander that occurs with some regularity, on Saturdays. It is largely for background information, and is a very simplified explanation of what happens in drilling oilwells and for related underground resources. Because the series is now getting longer, references to earlier posts are given at the back end of this one. And to explain the title, and give an indication of today's topic; one of the nastier jobs in early coal mines was given to a man called a Penitent. He would wrap himself in wet rags and crawl along the floor holding a long stick with burning rags on it, ahead of him, and up against the local mine roof. The intent was to burn dangerous pockets of methane before they got large enough to explode.

There is a debate in Los Angeles about the risks involved in extending the subway from Wilshire Boulevard to the sea. The major concern is with methane pockets. Because

Millions of years ago, the L.A. Basin was under the Pacific, and centuries of dead sea life created rich reserves of fossil fuel. By the early 20th century, the fuel was being pumped out in a maze of active oil fields. Today, many of the old pumps are gone, but significant pockets of explosive methane and other subterranean gases remain.

The Fairfax area -- home to the bubbling La Brea tar pits -- poses a particularly vexing problem for diggers.

Methane, or fire damp as it used to be called in coal mines, is a particular risk for underground workers. It is a gas often found in coal, the discontinued Bureau of Mines had evolved an equation that relates the gas volume to the depth of the coal.

Gas has always been a problem in mines, it is invisible and whether the mixture of carbon dioxide and monoxide called choke damp, or stythe, that lies in hollows in underground roads, or the methane at the roof, both are killers. Ventilation is designed to dilute these gases to the point that they are not dangerous, but as the gas issues from the coal, as it is ground from the face by mining machines, it can pass through the concentration range of 5 - 15% at which it can explode. If, at that instant, a mining pick strikes a layer of hard rock in the coal, the spark may be sufficient (or the hot rock) to ignite the methane. At that point the mine tunnels can become like a gun barrel as the methane blast stirs up the coal dust into an even greater explosive mixture. In such cases without proper safeguards all those in the mine at that time die.

Such a risk has led to a considerable effort to find ways of getting rid of the methane before the coal is mined. One way, with the side benefit of producing a salable product, is to drill wells into the coal, and extract the gas first. This coalbed methane has become a considerable resource with a growing potential as a resource.

But there is a problem. Coal seams (apart from such giant seams as those in Wyoming where they can be 100 ft or greater in thickness) are usually only about man-height. (Although if you have ever tried the historic method of mining with pick and shovel you will find that about 4 ft high is ideal). And the amount of gas (or oil from an oil reservoir) that you can get from a well is a function of the length of the well in the gas bearing layer. As a result if a vertical well was drilled into a coal seam, the amount of gas produced might only come from a 6 ft thick layer in the well. This does not drain much of an area, nor produce that much gas. And so the Federal Agency responsible (at that time the US BoM) carried out a considerable amount of work on developing ways to create a horizontal well from the surface.

Most coal seams are quite shallow (the deepest I have been in was around 3,000 ft in the South of England) at around 700 ft. This means that the wells, which start out vertical, must turn through 90 degrees, by the time that they reach the seam. As an early rule of thumb the build angle for such a well is around 8 degrees/100 ft. So to turn 90 degrees, would take some 1100 ft. Researchers at Pittsburgh found that by controlling the thrust across the bit they could steer it vertically to keep it in the seam, and, over time were able to drill out considerable distances.

Pioneering independents, working in conjunction with the Bureau of Mines and later with the Gas Research Institute, were successful in establishing some coalbed production in the Black Warrior.

At about the same time, the San Juan Basin in New Mexico and Colorado was the site of early pioneering work by Amoco Production, which drilled its first coalbed methane well there in 1977-78 at the Cedar Hill Field. That first field came on line in 1979 with significant production figures, Murray said.

Unfortunately, natural gas prices were soft and coalbed completion techniques were far from perfected, making progress very slow.

But companies continued to further the science of coalbed methane drilling and production through the 1980s -- through persistence and the financial boost of Section 29 tax credits for unconventional natural gas sources -- and by the early 1990s the San Juan Basin coalbed play was an unqualified success story.

. One of the pioneers was a fellow called Maurice Deul. The technology has spread to Australia more recently. It is attractive particularly in Queensland and New South Wales. Within the United States it is now being recognized more and more as a potential resource, with the need for horizontal drilling now well recognized.

In Australia the technology is somewhat further advanced. It already provides a third of the natural gas consumed in Queensland. And Australian investigators, working from an originally American idea, have also dramatically reduced the turning radius, so that the horizontal well can be kicked off in a much tighter radius. In fact the smallest radius achieved is around nine-inches. This uses a high-pressure waterjet drill as the drilling method, a third method to add to the technical toolbag, to go along with roller cones and diamond bits.

I have discussed the development of horizontal well drilling in coal, for gas recovery, rather than for oilwell use, because, although the techniques are quite similar, in some ways the coal development is somewhat further ahead. Tight radius drilling, or its equivalent, in oilwell use is limited to drilling short laterals. These are typically only some 50 - 100 ft long and drill out from a well. They have benefit in rocks such as the Austin Chalk, which carries oil often in vertical fractures. This means that if you drill a vertical well and just miss one of these your well might be dry, but by drilling a short lateral then you can hit the crack and bring in considerable production. Thus production can be several times that of a vertical well, both in terms of rate and of overall production. More conventional horizontal wells are generated using conventional turbine motors, and steering is done with a bent sub with wells that can now run horizontally out up to 6 miles. (I will try and explain the coiled tubing/bent sub steering method in a later post).

Horizontal well drilling remains more expensive than vertical well drilling, since among other things it can be harder to get the cuttings out of the well, since they tend to settle to the bottom of the hole, and form little dunes. But, increasingly it is becoming the way of the future. And in this application it is being used for horizontal drainage, more frequently than the original oilwell use, which was to extend the reach of the drilling platforms to access oil pools that were close, without needing to move the platform.

This is part of an ongoing weekend series on technical aspects of oilwell (and natural gas) drilling. Previous posts can be found at::
the drill

using mud

the derrick

the casing

pressure control

completing the well

flow to the well

working with carbonates

spacing your well

directional drilling 1

directional drilling 2

types of offshore drilling rigs
As ever, if this is not clear, or if there is disagreement then please feel free to post, and I will try and respond.

Thanks for the explanation about what "coalbed methane" is and the technical difficulties of extracting it. This mining has caused quite a bit of controversy here in the western states. Locals everywhere are upset unless they own the mineral rights ;)

Here's an FAQ I found.
We've advanced from the old days. In the old days the downstreamer or downwinder people had no rights. Nowadays the coal bed methane people can't salinate your water without permission.
If it rains in Wyoming you can pump out more water and get more depressurised methane, but if it doesn't rain the pumps get shut down.
What stops the CBM producers from re-injecting the brine elsewhere in the bed?
It doubles the cost of the coal bed methane. Dumping it into streams requires one well and pump. Pumping it back down to the a saline aquifer someplace else requires two wells and pumps.
If you have to pump it down, the pressure would make the production well artesian, no?  And if you have to pump it up, the pressure would be less than the gravity head and the brine could be allowed to flow by its own weight.

That just doesn't add up.

That's correct. The pump is required to overcome friction losses of moving the water through the coal cleats. The friction loss in the well and the degasifier are much lower and hardly count. It's moving the brine through the coal that takes the power.
Of course, evey well is different.
The PDF I linked mentioned that the pressure in the seam has to be dropped to the point where the methane comes out.  This appears to require at least some water to be removed from the seam and placed elsewhere, for the duration of the production operation.

Pumping oxygen and steam down an injection well and drawing F-T synthesis gas (plus a lot of raw hydrocarbons) from another would appear to be a possibility once the seams have been dewatered.  I'm rather surprised that the British haven't been doing this with some of their deep coal and undersea coal; they're facing a serious crisis with the decline of North Sea oil and gas.  On the other hand, if they burn out the seams near shore, the land above subsides... right into the sea.

Some British mines ran more than five miles out to sea and mined out the coal in what is known as a longwall system.  This allows the roof to collapse after the coal is removed.  But the roof will only break to about one-and-a-half times the seam height above the coal. (The broken rock bulks up as it breaks and fills up the hole).  Above that the rock has the flexibility to bend a bit, and does not break.  This the breaks don't go to the sea bed, and the sea doesn't rush in, providing they are deep enough below it.
Reinjection is listed as an option here, though I don't see any details about the pluses and minuses.
I though that during my time I've had some of the worst jobs in the world, but none of them quite compared to being a coal mine 'penitent'!

Being a coal mine penitent sounds even better than being a nuclear power plant 'sponge'.  I don't know if the practice still continues, but some nuclear power plants used to hire unskilled workers on a day-by-day basis to perform menial tasks in the very hot zones. These part-time workers were called 'sponges' for obvious reasons. The sponges would accumulate a year's worth of permissible cumulative radiation exposure in the course of  less than a week. The purpose of using sponges was to conserve on the permissible cumulative exposure of the plant's full-time skilled maintenance workers so that it could be applied to more important tasks.  

Come to think of it, since my son hasn't had a decent job since he graduated college six years ago, I am going to recommend that he look into be a coal mine penitent.

HO - thanks for another substantial post, a lot more informative than Judy Miller's piece in the Times - one quick question: in the future do you intend to discuss various mid-stream (refining) techniques and processes? - it sure would be enlightening to us civilians out here....
We exist to make this, among other things, an information site.  So we will put this on the list.  There is an overall intention to do wells, then stimulation, then I was going to look at alterative energies, but I had not considered sliding refining into the list. Consider it added.