Optimizing the production of methane from coal seams will require new thinking and approaches. Coal seam reservoirs are substantially different, both physically and chemically, from conventional reservoir. The major differences and their impact will be discussed with emphasis on chemistry and wettability phenomena.
The special properties of coal seam reservoirs make the generation of coal lines inevitable. Fines may be generated during completion, hydraulic fracturing and production processes. These lines may result in damage to production equipment and reduced fracture conductivity. Since the first attempts at producing commercial quantities of methane from coal reservoirs, more and more attention has been focused on the lines problems which are unique to these reservoirs.
Methods for minimizing the effects of lines on production will be reported. These methods include fracturing design guidelines and new materials which have been shown to be affective in laboratory tests. Laboratory studies reported here have shown that chemical systems may significantly alter the permeability of packs of coal lines.
References and illustrations at end of paper.
The production of methane from coal seams has received much attention during recent years. Most investigations have centered on production mechanisms and fracturing behavior, and several excellent papers have been published in these areas.1–5 Very little attention has been given to some other unique features which are characteristic of productive coal seams. Further, little attention has been focused on the long-term behavior of the coal.
It is believed that coal lines will cause production problems at some point in the life of any hydraulically fractured well completed in a coal seam. In addition to causing damage to production equipment, migrating coal lines lead to a reduction in the conductivity of the hydraulic fracture. This paper addresses the probable causes or coal lines, the problems associated with these lines, and the development of a chemical system that will mitigate these problems.
Coal seam reservoirs are quite different from the reservoirs typically associated with hydrocarbon production. Because of these substantial differences, care must be exercised to ensure that practices developed for conventional reservoirs are not inappropriately applied to coal seam reservoirs. An attempt will be made to briefly summarize the most important differences.
A conventional natural gas reservoir contents gas stored in the pore structure of the reservoir rock. Gas content in this case will depend on the reservoir porosity, reservoir pressure and reservoir temperature. In contrast, the bulk of the methane existing in a coal seam is adsorbed onto the micropore structure of coal grain surface. This very efficient storage mechanism, coupled with the very large internal surface area of coal, allows for huge quantities of gas to be stored at quite low pressure.6,7
Figure 1 shows a typical desorption isotherm.7 As may be seen, lowering the pressure in a coal reservoir by 90% will allow only about 50% of the adsorbed gas to be produced. Conversely, a 90% reduction of the pressure in a conventional reservoir will allow roughly 90% of the compressed gas to be produced.