Gas transport, sorption, and mechanical response of fractured coal.

摘要

Chapter I reports laboratory experiments that investigate the permeability evolution of an anthracite coal as a function of applied stress and pore pressure at room temperature as an analog to other coal types. Experiments are conducted on 2.5 cm diameter, 2.5-5 cm long cylindrical samples at confining stresses of 6 to 12 MPa. Permeability and sorption characteristics are measured by pulse transient methods, together with axial and volumetric strains for both inert (helium (He)) and strongly adsorbing (methane (CH4) and carbon dioxide (CO2)) gases. To explore the interaction of swelling and fracture geometry the evolution of mechanical and transport characteristics for three separate geometries are measured -- sample A containing multiple small embedded fractures, sample B containing a single longitudinal through-going fracture and sample C containing a single radial through-going fracture. Experiments are conducted at constant total stress and with varied pore pressure -- increases in pore pressure represent concomitant (but not necessarily equivalent) decreases in effective stress.;Chapters II presents a mechanistic model to represent the evolution of permeability in dual permeability dual stiffness (DPDS) sorbing media such as coalbeds and shales. This model accommodates key competing processes of poromechanical dilation and sorption-induced swelling. The significant difference in stiffness between fracture and matrix transforms the composite system from globally unconstrained to locally constrained by the development of a "stiff shell" that envelops the perimeter of an representative elementary volume (REV) containing a fracture. It is this transformation that results in swelling-induced permeability-reduction at low (sorbing) gas pressures and self-consistently allows competitive dilation of the fracture as gas pressures are increased. Net dilation is shown to require a mismatch in the Biot coefficients of fracture and matrix with the fracture coefficient exceeding that for the matrix -- a condition that is logically met.;Chapter III reports measurements of deformation, strength and permeability evolution during triaxial compression of initially intact bituminous coals. Experiments are conducted on coal (Gilson seam, Utah) at effective confining stresses of 0.75 to 3 MPa and pore pressure of 0.5MPa. Permeability is continuously measured by the constant pressure differential method, together with axial and volumetric strains for both water (H2O) and strongly adsorbing carbon dioxide (CO2) gas. The coal is an initially elastic, brittle-plastic material with a strain-weakening behavior. Strength and Young's modulus increase with increasing confining stress and permeability is hysteretic in the initial reversible deformation regime. As deviatoric stress and strain increase, permeability first decreases as pre-existing cleats close and then increases as new vertical dilatant microcracks are generated. Post-peak strength the permeability suddenly increases by 3-4 orders-of-magnitude.;Chapter IV presents laboratory experiments to investigate the role of gas desorption, stress level and loading rate on the mechanical behavior of methane infiltrated coal. Two suites of experiments are carried out. The first suite of experiments is conducted on bituminous coal (Lower Kittaning Seam, West Virginia) at confining stress of 2 MPa and methane pore pressure in fracture of 1 MPa to examine the role of gas desorption. The second suite of experiments is conducted on bituminous coal (Upper B Seam, Colorado) at confining stresses of 2 and 4 MPa, with pore pressures of 1 and 3 MPa, under underpressured and undrained condition with three different loading rates to study the role of stress level and loading rate.;Chapter V reports laboratory experiments to investigate the rapid decompression and desorption induced energetic failure in bituminous coal using a shock tube apparatus. By comparing experimental results on coal and those on fragmenting volcanic conduits it is speculated that the characteristics of fracture network (e.g., aperture, spacing, orientation and stiffness) and gas desorption play an important role in this dynamic event as coal is a dual porosity dual permeability dual stiffness sorbing medium. Then a shock wave model is applied to analyze how the fragmentation speed and the ejection velocity of gas-particle mixture change with a variety of parameters, (e.g., coal properties, Langmuir constants). This study has important implications in understanding energetic failure process in underground coal mines such as coal gas outbursts. (Abstract shortened by UMI.)