Print this pageCoal Bed Methane: Gas Saturation and Cleating Properties
KHAVARI KHORASANI, GANJAVAR
The coal bed gas prospectivity depends on two key factors: (a) gas saturation, and (b) whether the coal contains an open cleat system. Kinetic compositional modeling of coals from different age and depositional settings show that the rank at which a coal would be saturated (at the reservoir conditions) by in situ generated methane, vary from R[o] approx. 0.6 to R[o] > 1 %. A major risk is related to post generation diffusive losses, which is controlled by the timing of uplift, as well as the nature of the sediment package surrounding the coal seams. The importance of biogenic gas for saturation of undersaturated coals is evaluated. It appears that the presence of biogenic gas could represent a warning signal of a sediment package too "leaky" to allow retention of a high methane concentration over geological time. The origin of cleats has been attributed to various factors. It will be shown that the existing theories regarding the origin of cleats fail in most cases outside the "calibration" areas. Quantitative data show that the capability of coals to form and maintain an open cleat system is at least partly related to a delicate balance between the horizontal shrinkage due to mass transfer from the coal to free petroleum and compaction due to expulsion of petroleum, effective stress, and the relative concentration of "brittle" vs. "ductile" polymers in the coals. It will be discussed how the timing of uplift will influence the coal bed gas prospectivity both in terms of gas saturation and of cleating properties.