The Oil Drum

So, there we have all this oil, sitting in these nice thick oil shale beds out West and just waiting to turn some local in Colorado into the next "world's richest person". All they have to do is to figure out how to get the oil out of the ground cheaply enough to make money from it. (And if you remember from the last post there are over 2,000 patents on ways to do this - if it were that simple there would not be nearly that many). Congress thinks so too, since the Energy Policy Act of 2005 called oil shale a strategically important domestic resource (pdf file).

What's the big deal? Drill a hole down there and it flows it - isn't that how it works? Well not in this case. As I said the oil is really a waxy kerogen that does not want to flow at all. And there is also a problem with the rock. About 40 years ago a guy called Brace (Ref 1) found that the cracks in a rock are related to the size of the grains of the material that make up the rock. A rock with large grains has large cracks, and this gives it a permeability which is the joining of these cracks to give a path through which oil (or water or gas) can flow through the rock. It also gives the rock its porosity which are the holes in the rock into which the oil can collect. Unfortunately the grain size of the average particle in oil shale is around 5.8 microns. This is about a tenth of the thickness of a human hair, medium human hair being about 60 - 90 microns wide. As a result the typical oil shale has very poor porosity, and it is only when it has a high oil content (above 50 gallons/ton) that permeability can be easily measured (Ref 2) , below 20 gal/ton it becomes very difficult, because it is so small. The average grade is around 25 gal/ton.

Part of the problem with coal is that, when it was first grown (as in the peat bogs back when) the region was occasionally inundated with floods, and, as the Hurricanes showed last year, this carried mud and sand into the bog. Over the passage of time, as the bog turned from peat to brown coal, and then into coal itself, these dirt bands turned into sandstone, mudstones and other rocks. The layers are often found inter-layered within a coal seam, either as very thin stringers, or as partings that can separate a single seam into layers that end up several feet apart. The bedding planes and vertical joints (referred to as cleat) provide the permeable paths through the coal, and are often partially filled with additional minerals that deposit out of the water that percolated through the coal at one time. This can also introduce lenses of pyrite and calcite, so that coal is not the simple carbon lump that people often anticipate.

This is another in the short technical posts that show up at weekends, dealing with one aspect or another of fossil fuel production. Given that, as Super G noted the Governor of Montana was on 60 minutes tonight, it seemed like a good time to return to a coal-related theme. A list of related posts will be appended at the end of this one, and relate to the mining of coal, either on the surface or from underground, though it is the surface mining of coal, that currently entices the Montana Governor. It should be noted that the adjacent state of Wyoming produces around 400 million short tons a year of coal, about ten times the current production from Montana.

Today I would like to describe the events that take place when a company chooses to surface mine coal from under the earth. Of all aspects of mining this is, perhaps, the most controversial, in part because of the large surface disruption that often occurs during the time that it is happening. This post will try, however, to just describe the process, without using any of the emotive words that usually surround this topic.

For those wondering what this is, on many weekends I post a small technical talk, which tries in a relatively simple way to explain some aspect of fossil energy extraction. I will direct you to earlier talks at the bottom end of this one. For those that are more knowledgeable I recognize that I am often simplifying considerably, however, by knowing some of the basics it may be possible to achieve a better understanding.

This becomes particularly true when one talks about coal mining, since there is often discussion on these pages about the relative investments in energy in a process, relative to the amount of energy recovered. Part of that evaluation involves the nature and structure of coal as it is mined and treated, and I will go into more depth on that subject next time.

It is relevant here, since it is one of the bases of judgment as to how a seam of coal is mined. If you remember, I had suggested that you might think that coal is found as being similar to a layer of cream in a cake. Separated by the layer of rock above, and more rock below, the coal itself is a relatively even thickness of material that can stretch for miles. However you should note the word "relatively" in that description, and look at the rock layers in a road cut the next time that you drive through one. You will see variations in the layer at the top and bottom all along the length. Also the layers do not have to be flat, geological movements may have tilted the seams until they are at angles all the way up to vertical. (The coal is the Urals is often highly angled, as are some of the seams in Washington State).