53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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ABSTRACT: The poro-mechanical response of porous rocks is usually defined by Biot effective stress concept. Gas sorbing rocks such as coal do not however follow the conventional effective stress law because of swelling associated with gas adsorption and therefore, modification to the law is required. The Biot coefficient of non-reactive porous rocks is readily measured by jacketed-unjacketed experiment however, there is no standard definition neither any procedure to characterize the poro-mechanical response of gas sorbing rocks. This extends to jacketed-unjacketed measurements where it is not clear as to what these measurements represent in a gas sorbing rock such as coal sample and how the results should be interpreted.
In this study, therefore, the conventional jacketed-unjacketed tests were performed on coal samples using adsorptive (CO2) and non-adsorptive (Helium) gases. As expected the Biot coefficients measured by He and CO2 are significantly different. The results of the conventional jacketed-unjacketed experiments showed that the swelling as a volumetric strain is only partly responsible for variation of Biot coefficient when adsorptive gas is used and the Biot coefficient is pressure-stress dependent. The mechanical alteration (bulk modulus in particular) of the sample induced by this gas adsorption seems to be another mechanism influencing poro-mechanical response in a very complex way. In order to shed light on the macro-scale observations, 3D X-ray micro Computed Tomography technique (micro-CT) was used to investigate the internal structural changes of coal sample undergoing stress with a) no pore pressure, b) pressurised Helium and c) pressurised CO2. The micro-scale changes were then linked to macro-scale observations and their effects on measured Biot coefficients were discussed.
The effective stress concept was first introduced by Terzaghi (1923) for unconsolidated granular materials. However rocks have solid skeleton that makes their poromechanical response different from that of cohesionless sediments. Biot (1941) later employed this concept to develop the linear poroelastic theory for cohesive materials such as rocks where the effect of solid skeleton was taken into account. In the Biot theory, the effective stress, σ’ is defined by the total stress σ, the pore pressure p, and Biot coefficient α (a parameter controlling the pore pressure-stress interaction where 0<α<1) (Biot 1941):
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