BACK ANALYSIS OF COALBED STRENGTH PROPERTIES FROM FIELD-MEASUREMENTS OF WELLBORE CAVITATION AND METHANE PRODUCTION

Direct measurement of field representative strength and permeability c haracteristics of coalbed methane is difficult due to the fact that co al has a cleated structure, These cleats or discontinuities vary in or ientation and size throughout the coal mass. The cleats' network large ly controls the strength of the coal and also provides the passage for fluid flow. Permeability of the cleats varies with the confining stre ss and shear stress level. Laboratory test results are greatly influen ced by the size and orientation of the cleats, and the magnitude and d irection of applied stresses. To obtain the large-scale hydraulic and strength properties of coal, a field problem involving wellbore cavita tion and a concomitant increase in gas production was numerically anal ysed. Calibration of the numerical model against the field observation s which included a 10-fold enlargement of the wellbore radius and a fo ur-fold increase in gas production indicated that the apparent cohesio n of coal is approximately 100 kPa and not 2-5 MPa, as was previously thought. The validity of this finding was later confirmed by performin g a set of laboratory direct shear tests at low effective stress level s. The findings from this study have significant implications in the d esign of wellbores for production of coalbed methane. (C) 1997 Elsevie r Science Ltd.