Coal Exploration and Geophysical Logging, with Insitu Analysis and Selective Mining methods.

 

Coal Exploration Dynamics

Geophysical Logging

And

Insitu-Analysis Selective Mining Methods

By

George S. Green, B.A. Geol.

© 2013 George S. Green

ISBN: 978-0-9919761-7-1 (html)

 

page 2

Preface

This paper represents a compilation of two papers previously written and published on my web site. Those papers are; Coal Exploration Dynamics, Insitu-Analysis Selective Mining Methods, © 2009; and Geophysical Logging, Insitu Analysis and Selective Mining Methods, © 2012. There may be some editing changes and additions as a result.

Those two papers set out to present and answer the following question: Is it possible to read a geophysical log taken from a hole drilled through a coal seam, and from that log, determine with some fair degree of accuracy, the ash, moisture and BTU value of the coal within that coal seam? Additionally can this be done for any distinctive layer of coal within any particular coal seam?

The direct answer to both questions is yes; and for this work, I use the API scale on a Gamma/Gamma Density geophysical log. I short interval sample the coal in accordance with, and within specific sets of API numbers. I do this for each, entire section of coal that is encountered through drilling; and I do it for a minimum number of holes until I am able to generate a representative, API mediated, sampling profile.

On analysis, I found that each separate sample, when compared to every other sample occurring between a specific set of API numbers, maintained the same range of BTU values; and similar results were found in the case of ash and moisture content. Given the right geological setting, this makes it possible to selectively mine a coal seam; separating coals of varying BTU value and selling that coal at marginally higher prices; thus increasing the value of the deposit.

Coal seams can also be evaluated insitu by geophysical means for their coalbed methane content. All of this work can be accomplished by geophysical tools that run inside the drilling rods.

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In-situ Coal Deposit Evaluation Methods

Coal deposits occur in sedimentary strata that have some lateral extent. They can also be affected by deformation processes that can alter their internal geometry. The internal geometry of any coal deposit can range from very little deformation to a very high degree of deformation. For purposes of this discussion we will look at a simple horizontal coal deposit.

Un-like a mineral property, where most of the holes that are drilled into a property are 'cored' for recovery of the sub surface rock, coal seams do not need this concentration of core recovery. With coal, only a certain percentage of the holes drilled into a deposit need to be cored and assayed. The rest need only to be drilled and geophysically logged. Accurate coal quality determinations for mining purposes can be determined from the geophysical logs once a minimum assay profile has been generated.

When I was working on B.C. Hydro's Hat Creek coal deposit I came up with the idea that the deposit might be amenable to a method of mining that I referred to as "Insitu Analysis and Selective Mining". It occurred to me that our geophysical logs could be used to accurately indicate the BTU value of any particular layer of coal within any particular coal seam. This study would go on to indicate that accurate values for insitu ash and moisture could also be determined from our geophysical logs.

To examine this potential a program of selective sampling had to be initiated for a minimum number of holes. This sampling program had to be guided by the geophysics being run on these holes. The samples that were selected for lab analysis came from specific geophysical "picks". These picks were sections of core that occurred between set geophysical points found on the API scale of the Gamma/Gamma geophysical log.

You must look very closely at the coal in your core box to find these points. Lower API numbers will indicate less ash content and the coal will become brighter, harder and cleaner. Higher API numbers will reflect increasing ash content. Finding these cut-off points in the core requires looking very closely at the lithology of your coal; and you can not rely entirely on simple point to point measurements selected along the length of the core; especially if you have to deal with the effect of core loss.

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Figure 1

geo.pic

On the left hand side of a geophysical log there sits a scale from 0 to 50. It is called the American Petroleum Institute scale (API). It was set up by the oil and gas industry in the United States for geophysical logging and it measures the background radiation of the layers of rock that have been drilling. This information is displayed as a wavy line (Fig. 1) that runs down the left side of the log. This line represents a graphical representation of the varying degrees of API value associated with coal.

Down the right side of our geophysical log there is a space for another wavy line which provides for a graphical representation of the density of the rock that has been drilled. For this work, a different geophysical tool shoots a beam of gamma rays into the rock. By measuring the strength of the beam this tool determines the density of the layers of rock that have been drilled. A skilled reader uses both, side by side graphical representations (logs) to determine something of the nature of the rocks that have been penetrated.

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Between these graphs is an open column where a geologist can pencil-in a sketch of the lithology or type of rock that has been drilled. For my study this space was used to indicate the amount and quality of the coal occurring between a specific set of API numbers. This area was later color coded to reflect API related coal quality.

Figure 2

geo.pic

Recording in a Tertiary aged lignite (for example) may begin at about 25 counts per second on the API scale (Fig. 2). As the coal gets cleaner our graphical representation will begin to trace to the left on the API scale; moving towards a lower API number. At Hat Creek, I decided to selectively sample the coal in accordance with and within several specific sets of API values. I could see in the core that the quality of the coal got better as a lower API number was reached. It became cleaner, with a higher percentage of bright bands.

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Figure 3

geo.pic

In the example shown here (Fig. 3) near the top of this particular hole, we see a (.7 meter) layer of coal that exists between 15 and 25 API. Looking at the next interval, we can see a 3 meter section of coal that exists between 5 and 15 API. The coal between 5 and 15 API is visually distinct. It is hard, bright, with a conchoidal fracture and lots of bright bands.

Immediately beneath this layer is another 3 meter section of coal that runs from 15 API to 25 API. It is black, layered, coal, with minor shale.

Immediately below this, there is a 4.0 meter section of coal that runs from 25 API to 35 API. It is dull, black, coal with significant shale.

Below this, there is one more 3.5 meter sample section that runs from 35 to 45 API. It is essentially, carbonaceous shale.

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For this study, I sampled each instance of coal within a 5 to 15 API range and sent it to our lab for analysis. Next, I took each instance of coal between 15 and 25 API and sent it in for analysis. Then I did the same thing for any coal occurring between 25 and 35 API; and once again for any coal occurring between 35 and 45 API. I did this for the entire hole thus "selectively sampling" the entire hole in accordance with these specific API designations. The same thing was done for a second hole.

I began with a range of 5 to 15 API because this quality of coal when examined in the core box, could be visually identified as predominately clean coal; above 15 API the lithology of the coal began to change. Hat Creeks' best coal occurred between 5 and 15 API. At Hat Creek, these various ranges in API and coal lithology were designated as follows:

5 to 15 API: clean coal; (color coded: black)
15 to 25 API: shalely coal; (color coded: orange)
25 to 35 API: coaly shale; (color coded: pale blue)
35 to 45 API: carbonaceous shale; (color coded: green)

On lab analysis we found the following results;

5 to 15 API = 14,000 to 12,000 Btu's
15 to 25 API = 12,000 to 10,000 Btu's
25 to 35 API = 10,000 to 8,000 Btu's
35 to 45 API = 8,000 to 6,000 Btu's

Because I color coded these layers of coal (Fig. 3) we could easily see where these various qualities of coal were in our cross sections; and depending on the geometry of the deposit itself, this allowed us to look at the possibility of selectively mining the coal; so as to selectively remove coals of variable Btu value and sell that coal at marginally higher prices. Thus increasing the value of the deposit and giving those concerned a better handle on the amount of waste rock being generated.

There was a consistency between API related assays and relative ash and moisture contents as well; although ash and moisture accuracies widened with rising API numbers.

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The American school of thought recognizes two types of organic material in coal; Anthraxylon and Attritus. The British recognize four; Vitrain, Fusain, Clarain, and Durain. The Australians prefer counting "bright bands".

The British designations may have more relevance for my studies and if you look very closely at your coal, you may see that the API “picks” you select may indeed correspond to these particular variations in coal lithology.

I coordinated this approach with the company providing the geophysical logging services for the 77-78 Hat Creek exploration program. They could see the merit in perusing this line of reasoning and were very helpful in furthering the idea. I gave them the lab results for two holes and they predicted the results for a third hole using only geophysical information. They did quite well.

This company would go on to continue this line of thinking and develop what they identified as their "Coal/Water/Ash Program". A program which allows for the insitu analysis and selective mining of relevant coal properties. It has proven to be a useful program and has been applied to other coal exploration programs in Canada serviced by this same company.

To be clear; the "selective mining" part of this approach was first disgust in 1975 (as per this author's experience) while he was employed by an Edmonton based coal company. At that time, this potential only centered on "selective mining" studies. The potential as a geophysically mediated 'insitu analysis' capability was the author's subsequent contribution to the topic and confirmed with studies carried out by the author in 1977-78 on B.C. Hydro's Hat Creek coal deposit.

Recessions and budget restrictions by B.C. Hydro in the following years limited my ability to follow up and do any refinements to this work. It may be possible to increase the accuracy of this approach by refining the sampling regime in relation to API values.

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An overview of the process.

Initially a coal seam can be core-drilled and the core recovered for study. After a hole is drilled and the core is recovered, the hole is geophysically logged using a gamma/gamma (slim density) geophysical tool. This geophysical tool is small enough to be lowered down the center of the drill rods. Logging through the rods has been done since 1976 when advances in tool design reduced the size of these tools. Logging open hole before that carried with it a risk of loosing an expensive tool and a radio active source down the hole. There are no problems with logging through the rods.

A geophysical log provides a graphic profile of a coal seam. Coal seam profiles have unique characteristics that can be cross-correlated to coal quality. It is possible to read a geophysical log and determine with some fair degree of accuracy, the ash, moisture and BTU (British Thermal Units or Kilo-Joule) value for any seam of coal or any distinctive layer of coal within any seam of coal. This capability makes it possible, given the right geological setting, to separate the various grades of coal within a seam and set that seam (or deposit) up to be selectively mined. This process has been described as an "in-situ analysis, selective mining” method.

The Hat Creek deposit is about eighteen hundred feet thick (600 meters). Its internal geometry is that of an asymmetrical syncline orientated north to south, dipping 17 degrees to the south, with a reverse faulted east limb. There are several reverse faults within the deposit. Even so, it was possible to set this property up for insitu analysis and selective mining methods.

As an initial exploration program defines the coal deposit and the internal geometry of the coal seams, and before infill drilling beings, a certain percentage of holes will have already been cored, logged and assayed to generate an API related graphic profile of every seam. Each API related layer of coal will have been split out for analysis according to its unique graphic profile and mapped in relation to its vertical location within a seam. This sampling and mapping program will display the respective ash, moisture and BTU values for the seams making up the coal deposit.

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If these assays reveal differing layers of coal with different ash, moisture and Btu values, it sets up the possibility of mining these layers of coal as separate products with separate market values. When this study was done for the Hat Creek property it actually increased the value of the property. With a certain percentage of holes drilled, cored and logged in this fashion, it becomes possible to drill off the rest of the deposit without the need to core and assay every hole, thus achieving a significant saving in exploration expenses. You can determine what you need to know straight from the geophysical logs.

Prior to this capability, coal seams were examined as one unit of coal and the analysis on this coal was performed as a ‘bulk sample’ analysis. The end product was a composition of hot and not so hot coals, with some degree of ash thrown in as an admixture from the mining process. Selective mining capabilities make it possible to split out and identify higher quality coal from lower quality coal at the mine face; given the right geological setting.

This capability exists and it has been field tested and proven to be effective. This author is one of the people responsible for pioneering this capability. I do not know how effectively it has been picked up by others in the coal exploration industry.

Not all coal deposits are amenable to these sorts of studies. Some that are highly deformed are not good candidates. Some deposits that have been subjected to hydrothermal alteration may not be good candidates either. Deposits that are primarily low grade lignite may be good candidates. Hat Creek is classified as a "lignite deposit” and was amenable to this kind of an insitu analysis selective mining study. The thickness and geometry of the deposit were beneficial factors.

Coal properties that exhibit this kind of 'value added potential' are going to be viewed as being more valuable as an asset class than property without this kind of work being done.

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Insitu Coalbed Methane.

The other kind of insitu analysis program that can be run during a coal exploration program is one for insitu coalbed methane (CBM). This is done using a "Quad Neutron" geophysical tool.

All coal deposits contain coalbed methane. This tool can detect the insitu potential for coalbed methane in the coal and/or the sedimentary strata enclosing the coal. It can also measure the degree of porosity of those sediments and give some indication as to whether or not any gas present may or may not flow. A core sample (canister test) will be necessary to determine if the detected gas is indeed coalbed methane or carbon dioxide, or some other kind of insitu natural gas.

It is a good idea when evaluating any coal seam that might be a candidate for underground mining, to evaluate that seam for its contained coalbed methane, and do it at the time of an initial exploration program. The determination of insitu coalbed methane, its location and potential concentration during the initial exploration phase might help future miners avoid accidents caused by coalbed methane.

These kinds of studies can help determine if tapping a coal deposit for its contained coalbed methane is a viable and profitable option prior to mining. Any particular coal seam or deposit could be as valuable as a source of coalbed methane as it is for any new coal product. An insitu ‘Quad Neutron’ analysis could help with this determination.

An additional thing to be aware of during a coal exploration program is the potential for intersecting shallow gas. Natural gas as shallow as 300 feet is known to exist in the substrata comprising the Alberta foothills and outer parries. Companies new to coal mining need to be made aware of this potential and the associated dangers when making their exploration plans.

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Logging through the rods.

From my experience logging through the rods began in earnest in 1977, yet still today there are some engineers and geologists that do not know this or believe that it can not be done. It works fine, and here is some insight into what happened to bring it about.

Maybe I had something to do with it or not, I do not know for sure; however, as I was running exploration programs for coal in Alberta in 1975 the geophysical logging company working with me on two occasions got a tool (with a radio active source) stuck in a hole. On one occasion it took 12 hours of work to get the tool out of the hole. I was not happy about this or the prospect of loosing a radio active source down a hole. I subsequently went to the company supplying the geophysical services and I asked them; “Why don’t you just log through the rods?"

The company president looked at me and thought about it for a minute. Two years later, when I encountered them again on another coal property, they were logging through the rods! They had re-designed their geophysical tools to log through the rods, tested them and found that doing it this way worked just fine. They would later go on to invent and design the (afore mentioned) "Quad Neutron" tool to log for coalbed methane, and do it again through the rods!

Now, geologists no longer have to run the risk of loosing a radio active source down a hole; and they can still provide their clients with valuable, accurate, exploration information. There are also occasions where a hole may collapse when the rods are pulled. Logging through the rods enables a geologist to recover valuable geological information before the rods are removed. Other kinds of logs that do not involve the use of radio active material can be run open hole, if the hole will stay open.

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About the author.

The author has been a coal exploration geologist for many years. Participating as an exploration geologist on some ten coal exploration projects; directing exploration activities on seven of those ten projects.

He has logged over 250,000 feet of hole, some 80% of it being coal from the very thick Hat Creek coal deposit.

He has done work for a two Calgary based consulting firms; two Vancouver based consulting firms, two large mining companies and worked as an independent contract geologist, all in coal, in either British Columbia or Alberta.

He has written several papers on coal exploration, advances in exploration practice and the geology of the Hat Creek coal deposit. He is the originator of Riemannian Geology; adapting to geology and exploration practice, Bernhard Riemann’s concept of a geometry of multiple connected manifolds.

Thank you for your interest in my research work.

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