Enhanced methane desorption characteristics from South Wales anthracites affected by tectonically induced fracture sets

Abstract

Abstract It has generally been assumed that anthracite, despite containing potentially high levels of sorbed methane, is unsuitable for coalbed methane (CBM) production because of low levels of fracture permeability. South Wales anthracites are commonly strongly affected by tectonically developed fracture systems formed during Late Carboniferous Variscan compressional deformation. The fracture systems occur as thrust-related slip, slickenside and feather fractures, and are superimposed on previously formed extensional fracture systems that include cleat, conchoidal and bed-parallel fractures. When these fracture systems are intensely developed, the anthracite becomes incompetent and friable as a result of extremely close fracture spacing. According to current models for gas desorption and migration, methane initially diffuses through the micropore system of the coal matrix before flowing freely through larger pores and fracture systems. The models suggest that the more closely spaced the fractures, the more rapidly will the coal desorb methane. The tectonically fractured South Wales anthracites were investigated to determine the relationship between the rate of desorption of methane and tectonically induced fracture spacing. The former was established using gravimetric desorption isotherm experiments and in situ quantification of the latter was achieved by using a hand-drill penetrometer. The results demonstrate a clear relationship in anthracite between both the rate of methane desorbed and the amount of structural deformation. It is concluded that regions of deformed anthracite, previously considered unsuitable for CMB production, should be re-investigated.