Enhanced methane storage and generation in the shear zones along coal seams: a potentially significant coalbed methane reservoir

摘要

Shear zones along coal seams have long been considered as a main place where gas outbursts occur during mining, but it might be a positive aspect to coalbed methane prospecting and production for the great quantity of fractures and higher methane content. Fractured coals in shear zones can be further classified into cataclastic series and mylonitic series based on their particle size and texture. Mercury penetrating experiment on a series of variously deformed coal samples from the Pingdingshan coalfield, China was performed, resulting that tectonically sheared coals possess 3-6 times porosity, 2-8 times specific surface area more than their normal counterparts. Furthermore, the cataclastic coals represent the most favorable gas extraction condition due to high porosity and average aperture, whereas the mylonitic coals often show low rate of gas extraction because of small average aperture and worse connectivity of fractures. Electron paramagnetic resonance experiments on normal coals and varing sheared coals showed that the free radical concentration of coals rise with incresing tectonic disturbance, implying that shear strain can lead to some extent to mechano-chemical degradation in sheared coals and thus generate additional gas. The occurrence of a large number of tectonic fractures in shear zone will lighten the burden on the work of artificial fracture for increasingthe permeability of coals in methane production. Therefore, shear zones along coal seams are the most ideal source and reservoir for methane gas production.