Coal-bed methane reservoir characterization using well-log data

Abstract The coal-bed methane (CBM) industry invests substantial effort into extracting as much information as possible from wellbore log curve data in order to supplement the limited information available from laboratory analysis of wellbore core samples. Coal's unique lithological properties mean that coal seams can be readily distinguished from other lithologies by using a standard suite of well logs. However, the complex compositional, rank and structural variations displayed by coals, even within a single seam and/or across a coal province make accurate predictions of gas content and potential flow rates and ultimate recoveries from specific coal seams fraught with challenges. Empirical relationships help to an extent but tend to work well in some coal provinces but not in others. This means that the relationships between well log curves and coal properties typically need to be calibrated with core analysis for each coal basin to provide reliable prediction accuracy. The brittleness, horizontal stress differentials and coal structure have a strong influence on a coal seam's “fracability.” Fortunately, several useful indices can be calculated using the geomechanical properties of coals estimated from well-log data to meaningfully predict their propensity to respond positively to fracture stimulation. Fracture stimulating coals and handling the increased formation water produced, and potential surface and subsurface water contamination risks it poses, require careful planning to enable enhanced CBM production to be achieved in a sustainable manner. This chapter addresses how well-log assessments help to select the optimum zones to fracture stimulate thereby minimizing risks of water contamination. Banner headline The properties of subsurface coal seams and their potential for coal-bed methane exploitation are greatly assisted by well-log data calibrated with core analysis. The basic suite of well logs can be used to not only distinguish coals from other lithologies but also predict with reliable accuracy their gas content and proximate analysis components. Additionally, logs can be used to derive geomechanical variables to generate indices that determine a coal's fracability and commercial potential.