As rank determination is empirical, the various parameters discussed here can give slightly different ranks, especially near coalification boundaries. Consequently, rank of a given sample is often determined from multiple parameters, with the eventual rank being accompanied by its determinant, such as “high-volatile A bituminous by vitrinite reflectance.”
A rough estimate of water in place can be obtained from any of the three moistures (inherent, equilibrium, or improved), as differences between them are typically less than uncertainties in other reservoir parameters, such as coal seam thicknesses and densities. Original water in place is the product of an area, net coal thickness, coal density, and the selected moisture fraction. Water disposal requirements for a given well or project can be estimated as the product of this water in place value multiplied by a reasonable recovery factor, typically between one-half and two-thirds.
Determination of volatile matter, as detailed in ASTM D 3175, begins with placing a 1 g sample in a covered metal crucible in the 950° ±20°C (1,742° ±36°F) zone of an oven.15 Care must be taken to ensure the sample remains covered after disappearance of the luminous flame. After a total of seven minutes, the sample is removed from the oven, cooled in a dessicator, and then weighed. Volatile matter is defined as the percentage weight loss during this procedure less the inherent moisture percentage.
Proximate analysis is determined from ASTM D 3172-07a, Standard Practice for Proximate Analysis of Coal and Coke.16 It incorporates procedures for equilibrium moisture and ash by reference. Equilibrium moisture, ash, and volatile matter are determined from the procedures described above, and fixed carbon by difference.
For reservoir engineering purposes, comparisons of laboratory and field data are often done on a dry, ash-free basis. For example, comparison of desorbed gas contents with a laboratory sorption isotherm may seemingly indicate undersaturation of a coal deposit, when in actuality the samples in the desorption canisters contained more ash than did the subsample taken for the isotherm measurement. Reporting all gas contents and sorption isotherms on a dry, ash-free basis eliminates this problem.
Ash is the noncombustible portion of a coal as determined by ASTM Method D 3174.17 As described in this procedure, a 1 g sample is heated to between 450°C and 500°C by the end of the first hour and 700°C to 750°C by the end of the second hour. The sample is held at this temperature for two additional hours before cooling in a manner to minimize moisture uptake and then weighed a second time. The ash fraction of the sample is final weight divided by original weight. Incapable of holding significant gas, ash is a diluent in the coal, decreasing gas-holding capacity. During the combustion process, CO2 (from carbonates), SO2 (from sulfates), and H2O (from clays) are driven off, making the ash fraction of a sample less than the mineral matter fraction. Ash typically varies more than equilibrium moisture for any given coal deposit, as the primary controls on ash such as shale partings, overbank deposits, or degraded organic precursors varied vertically and horizontally during deposition and coalification. Ash in a coal reservoir can vary from a few percent to more than 50% of the mass of the reservoir rock. High ash portions of a coal seam often hold sufficient gas and possess sufficient permeability to be considered part of the overall reservoir system, yet this same rock would not be commercially mineable.
Consequently, viewing coals from a mining perspective will provide a conservative assessment of the reservoir potential of a coal deposit.
Data can also be normalized to a mineral-matter-free basis, either wet or dry. The dry, mineral-matter- free basis is the more common of the two in coal gas reservoir engineering. First, the sulfur content of coal is determined via ASTM D 4239-05.18 Then the mineral-matter fraction can be calculated from the Parr formula:
MM = 1.08a + 0.55S where
MM = mineral-matter weight fraction, a = ash weight fraction, and S = sulfur weight fraction.
Gas in place depends on areal extent and thickness of a coal body as well as moisture, ash, and mineral matter.
Uncertainties in area and thickness and the resulting gas in place volume are sometimes carried over to coalbed gas contents, with the daf and dmmf bases being used interchangeably. The error caused by this interchange can be illustrated with an example from the Warrior Basin of Alabama.
Example 2.1. Comparison of daf and dmmf fractions
Proximate analyses and sulfur contents from four samples taken from the Jefferson coal seam of the Black Creek Coal Group in the Warrior Basin coal are collected in table 2–2.19
Table 2–2. Proximate analysis and sulfur content—Jefferson coal, Warrior Basin
Seq no County Moisture, % Ash, % Volatile matter, % Fixed carbon, % Total, % Sulfur, % Mineral matter, % Min. mat./Ash
1 Jefferson 2.3 7.3 31.9 58.5 100.0 3.1 9.6 1.31
2 Marion 5.2 3.3 37.3 54.2 100.0 1.3 4.3 1.30
3 Tuscaloosa 1.4 4.6 32.6 61.4 100.0 1.4 5.7 1.25
4 Walker 4.1 4.2 36.7 55.0 100.0 1.5 5.4 1.28
× Average 3.3 4.9 34.6 57.3 100.0 1.8 6.2 1.28
Source: Rightmire, C. T., Eddy, G. E., and Kirr, J. N. 1984.
On the basis of volatile matter, coal rank is high-volatile bituminous. Coal purity is high, with ash and mineral matter both less than 10% for all samples. Using average ash and moisture fractions, 0.049 and 0.033, respectively, for conversion of pure coal gas contents to in-situ conditions entails multiplication by
1 – a – w = 1 – 0.049 – 0.033 = 0.918 where
a = ash fraction, wt %, and
w = equilibrium moisture fraction, wt %.
Using the average mineral matter fraction, 0.062, and average moisture fraction for conversion of pure coal results to in-situ conditions requires multiplication by
1 – MM – w = 1 – 0.062 – 0.033 = 0.905 where
MM = mineral matter fraction, wt %.
The resulting in-situ gas contents will differ by 0.918
——— = 1.014 0.905
While the average mineral matter of 6.2% is about one-third larger than the average ash of 4.9%, interchange of the two for calculation of in-situ gas contents gives rise to a variation of only 1.4%. From inspection of the Parr formula above, variation between values calculated on daf and dmmf bases is greatest for high-ash, high-sulfur coals.
Ultimate analysis determines the chemical elements in a coal sample.20 Typically reported as carbon, hydrogen, oxygen, nitrogen, and sulfur, four of the elements are determined by laboratory procedures, while oxygen is determined by difference. The active standard in the United States for ultimate analysis is ASTM D3176-89, ASTM (2002), which references an additional 14 ASTM procedures detailing sample acquisition and preparation as well as laboratory procedures.21 Knowledge of the elemental composition of a coal is important for coal chemistry and conversion processes but gives little information about coal rank or rock properties, such as gas sorption potential or permeability. Consequently, ultimate analysis is of limited use in coal gas reservoir engineering at this time.
Procedures for determination of inherent moisture, ash, and volatile matter all contain qualitative actions.
Such actions include taking care not to overly dry a sample when removing excess water by suction during equilibrium moisture determination or cooling a sample to minimize moisture uptake as part of ash fraction measurement. In addition to these subjective actions, sample history can affect proximate analyses. Coals, especially low-rank coals, oxidize rapidly upon exposure to ambient conditions. Luppens and Hoeft ascribed reproducibility differences in moisture analyses to sample oxidation.22 Consequently, proximate analyses will vary from laboratory to laboratory and between operators within a given lab.