Handbook for Methane Control in Mining - AMMSA

Figure 8–1.—Methane content as a function of depth andcoal rank (modified from Kim [1977]).Figure 8–2.—Gas content versus depth and coal rank,Black Warrior Basin, Alabama (McFall et al. [1986]).115In the absence of extensive measuredgas contents in an area ofinterest, an alternative approach toobtain the gas content is to use therelations proposed by Kim [1977]based on gas content determinationsfrom adsorption analysis on differentcoals of various ranks and depths(Figure 8–1). This approach can beconsidered in parts of a basin wherecoal samples are initially not availablefor direct gas content testing.However, it is important to note thatthese are only estimated values andshould be confirmed with subsequentdirect gas content testingwithin the actual area of interest.For estimating in-place gas contentsfor a specific area, regional gas contentdata on individual coal samplescan be used along with data oncoal rank and/or depth to constructcurves such as those in Figure 8–2.Such curves are generated for aparticular coalbed or closely associatedgroup of coalbeds and can beused to estimate gas content valuesonly if the rank or depth are knownfor the coalbed of interest. As anexample, the graph in Figure 8–2presents such curves for the BlackWarrior Basin in Alabama [McFallet al. 1986]. Coal lithotypecharacteristics also affect themethane content of coal. Forinstance, significantly highermethane capacity was observed forbright bands (850 ft 3 /ton) versusdull bands (570 ft 3 /ton) in the samecoalbed during an evaluation ofcompositional effects on coals from western Canada [Lamberson and Bustin 1993]. Total gascontent varied with the amount of vitrinite and liptinite, which usually offer high methanestorage capacity, whereas no obvious relationship was observed with the inertinite content.Some studies report increases in gas yield with fusain content, which tends to allow rapiddesorption of methane [Creedy 1986]. Despite these examples, while the general influence of