Pore structure of sheared coals and related coalbed methane

Abstract.

Coalbed methane is considered to be one of the major contributors to global warming. Geological prospecting of the rich-storage zone of methane is an important precondition for gas recovery projects. Shear zones along coal seams have long been considered as a major place where gas outbursts occur during mining, but they also have a positive aspect in coalbed methane prospecting and production because of their great quantity of fractures and high methane content. Mercury penetrating experiments on a series of variously sheared coal samples from the Pingdingshan coalfield, China, were performed. The results showed that tectonically sheared coals possess three to eight times more porosity and two to ten times more specific surface area than their normal counterparts. Moreover, brittlely deformed coals possess larger average fracture apertures representing a good condition for methane extraction, whereas ductilely deformed coals show a fracture aperture even smaller than normal coals, which can explain why intensely sheared coals become difficult for methane extraction. The occurrence of a large number of tectonic fractures in a shear zone will make it easier for an artificial fracture to increase the permeability of coals in methane production. Therefore, except for locally ductile deformation areas, shear zones along coal seams are the best source and reservoir for methane gas production and, if this methane is extracted, it will help not only the reservoir but also protect the earth against global warming.