Jeffrey R. Levine, PhD
Geological Consulting Services
Coalbed Methane • Gas Shales • Oil Shales • Coal Geology • Organic Petrology
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Sorbed gas resevoir systems have suffered from ambiguous, and in many cases misleading or erroneous terminology, which has contributed to misunderstanding of reservoir composition and mechanisms. Some terms that seem ‘intuitive’ at first glace–say, “gas content” for example–convey an erroneous impression of reservoir composition that makes it more difficult to understand the equilibrium that exists between the sorbed state and gaseous state. Many of terms–say “coalbed methane”, for example–are so deeply embedded in the literature that we have no choice but to continue to use them. If this is the case, however, it’s important that their proper meanings be understood.
In these pages, I will discuss a number of terms that I feel are problematic.
► Conversion Factors for Gas Resource Density in 6 useful units
► Convenient GIIP calculations!
Step 1: Calculate GRD in simple metric units
Step 2: Multiply by conversion factor for any desired units
Step 3: Multiply by total area<br?
Density of Coal
Note: The abbreviation for tonne in the SI system is a lower case 't'. To help distinguish the two ton(ne)s, the (US) ton can be abbreviated with a capital 'T', although a lower case 't' is often used, so take care!
Petroleum engineers in the USA measure the density of reservoir rock in “tons per acre-foot” (T/ac-ft). A more typical measure of density in other disciplines and in other countries is grams per cubic centimeter (g/cm3) or tonnes per cubic meter (t/m3).
How do these density units relate to one another? (Easy!)
1 T/ac-ft
x 0.032808 ft/cm x 2.471 x 10-08 acres/cm2 x 907,185 g/T
= 7.3547 x 10-4 g/cm3
Or inverting, 1 g/cm3 = 1359.7 T/ac-ft
To convert from grams per cubic centimeter to tonnes per cubic meter?
Multiply by “1”. (No comment!)
A common “rule of thumb” for estimating tonnage of coal in the ground is: 1800 T/ac-ft. Multiplying by 7.3547 x 10-4, this is equivalent to 1.324 g/cm3. The origin of this value is unclear, although it has been attributed to Wood et al. (1973), who unfortunately provide little insight on how it was derived. Wood et al. point out that it is not a constant, but will be influenced by mineral matter content, organic matter type, and thermal maturity (rank). Of these, mineral matter content (usually approximated by ash yield) will generally have the greatest impact. Despite this uncertainty, 1800 T/ac-ft is widely used in resource estimation.
1.32 happens to be the ‘grain density’ of vitrinite, which is usually the most abundant maceral constituent of coal; however, coal in the ground will always include mineral matter as well, which would increase its density above this value. But coal in the ground also includes fractures and other macroporosity, that will be filled either with water or gases, which will tend to decrease its density. It’s not uncommon for clean coals to exhibit bulk density of 1.28 or even less on geophysical logs (1738 T/ac-ft), particularly if they are very low in ash, whereas high ash coals will have bulk densities considerably greater than 1.32. As an approximation, 1,800 T/ac- ft, can be taken as representative of coal on an ash-free basis. For resource calculations using this value, gas content and thickness should also be adjusted to an ash-free basis, for consistency. (CAUTION: The gas content adjustment is made on the basis of weight fraction, whereas thickness adjustment should be on the basis of volume fraction.)
An alternative approach–usually the most straightforward and logical–is to have all measurements on a “whole rock” basis.
Density of Shale
The density of shale, for the purpose of resource calculations varies considerably, depending especially on diagenetic grade, which affects porosity, mineralogical composition, and organic matter content.
Further discussion of this topic will appear at some time in the future.
(Y’all come back now, y’hear?)
The following sources are recommended for providing further details on topics discussed on this site.
(Note to Visitors: This effort is a “Work in Progress”, and is obviously incomplete; however, but a journey of a thousand miles needs to start with a single step…. so here is is (plus a few more).
Molecular Structure of Coal:
Davidson, R.M., 2004, Studying the Structural Chemistry of Coal: (Online version:
http://www.coalonline.info/site/coalonline/content/browser/81340/Studying-the-structural-chemistry-of-coal
Davidson is an insightful writer who provides a thorough, lucid review, amalgamation, and explanation of the published literature on coal chemistry. Flawed only by a lack of integration with the petroleum geochemistry literature.
