Coal seam failure during primary/enhanced gas production: How failure develops in fields

Coal failure has been observed in many coal seam reservoirs during gas production, which generated significant impacts to coal permeability and to the efficiency of gas recovery. In this paper we develop a flow-geomechanical-coupled numerical approach and use two genuine field examples to discuss the relevant large-scale coal failure behaviours in fields. One example is with the CBM production in Cedar Hill reservoir, San Juan Basin; and the other one is with the CO2-injected ECBM process in Allison Unit, San Juan Basin. Major observations from the present work include: 1) Laboratory tests and analytical analyses based on uniaxial strain conditions may have considerable deviations in prediction of the coal failure during gas production in fields, because such a condition would not hold there because of non-uniform pressure distributions in the field due to gas production/injection. 2) Field-scale coal strength parameters (e.g., the friction angle) should be remarkably weaker than its counterparts measured with core samples because, if otherwise, the coal failure that has been observed would not occur. 3) In CBM processes, coal failure usually occurs first in the immediate vicinity of a wellbore, and then may expand to the whole field with continuing gas depletion. 4) ECBM processes can have complex strain history including inelastic coal failure due to primary production as well as elastic unloading due to CO2 injection.