Experimental study on the influence of temperature cycle on low-rank coal permeability

The formation temperature is a key factor affecting coalbed methane (CBM) migration in reservoirs. Both the prediction of CBM production and prevention of mine gas disasters require the understanding on the controlling mechanism of temperature on coal permeability. We experimentally examined the evolution of permeability for natural low-rank coal samples under various stresses and cyclic temperature conditions. Apparent permeability and intrinsic permeability decrease significantly when the temperature increases and they only partially recover after the temperature returns. The permeability loss decreases greatly with the increasing number of temperature cycles. The permeability loss due to the rising temperature and the irreversible permeability loss for a whole temperature cycle decrease prominently with increasing confining stress. The impacts of swelling/shrinking of coal matrix, roughness of surface, pore compressibility and weakly bound water in coal on temperature sensitivity of coal permeability are investigated.