A Stability Analysis of Two Horizontal CBM Wells in the Deep Mannville Coals, Alberta Basin, Canada

This study describes the main technical challenges encountered while drilling the first horizontal coalbed methane well ever attempted in Canada. The well was drilled in the deep (>1,300 m), low-permeability (1-3 md) coals of the Lower Cretaceous Mannville Formation in the central Alberta Plains. The study also discusses the investigation that followed the occurrence of a few unexpected events during the drilling phase of the well, as well as the stability analysis of the horizontal borehole conducted. Suggestions are made on how to best mitigate the risk of getting the drill pipe stuck and potentially losing the hole along with drilling equipment. The findings are very instructive and should have an application in geological settings similar to those found in the Alberta Basin. Any modifications on how to drill horizontal wells in coal seams present at great depths and under high in-situ stresses must take into account the geomechanical properties of the target seam through triaxial testing, which should include obtaining the non-linear Hoek-Brown failure criterion and testing the coal core for very low confining stress. The lab-derived strength data may also have to be downgraded in order to predict, more realistically, the radius of the yielded zone around the wellbore and the horizontal borehole stability using geomechanical software, such as STABView (R) and FLAC.