Mechanical behavior of low-rank bituminous coal under compression: An experimental and numerical study

Abstract

The mechanical behavior of coal has an important influence on coal reservoir stimulation and the dynamic change of coal reservoir permeability during coalbed methane (CBM) production. Generally, coal is regarded as an elastic-brittle material, which is reasonable when the surrounding stress is small. However, with the increasing depth of CBM exploitation, the in-situ stress increases continuously, so the coal will inevitably show some plastic behavior. In this work, the typical low-rank bituminous coal from the Baode CBM block on the eastern margin of the Ordos Basin was selected to carry out uniaxial and triaxial compression tests under various confining pressures (Pc) for further investigation of mechanical behavior. The results show that, as a complex porous material, the coal samples have a series of deformation responses during the loading process, which can be classified into four phases. Both the yield stress (σy) and the peak stress (σpeak) increase nonlinearly as Pc increases. Additionally, with increasing Pc, the elastic modulus (E) increases at a decreasing rate, while the Poisson's ratio (ν) decreases at a decreasing rate. Moreover, a change from volumetric compressibility to dilatancy in the coal was clearly observed in the tests, and the stress at the onset of dilation (σd) is also significantly influenced by Pc. Finally, based on irreversible thermodynamics and empirical support, a coupled elastoplastic damage constitutive model for low-rank bituminous coal was established by introducing the non-associated plastic flow laws and the damage evolution rules; the numerical simulation result of the model is in good agreement with the experimental measurements. This model can be used for further investigation of the mechanical properties of coal reservoirs under different in-situ stress conditions during the exploitation of CBM.