A method to predict geomechanical properties and model well stability in horizontal boreholes

A methodology is proposed to quickly assess the stability risk for horizontal wells drilled in coal seams. The first task involves the estimation of strength and deformation parameters of the coal by indirect methods, such as the use of core description to assign a Geological Strength Index (GSI). The second task involves the use of a finite element analysis called FLAC to investigate factors such as the depth of fluid penetration while drilling the horizontal well and the stability of the wellbore during simulated production. These two factors are important because coal fines generated by the drill bit action are carried by the drilling fluid into the cleat system of the coal, thereby plugging the permeability pathways, and causing formation damage. On the other hand, a wellbore that may become unstable during pressure drawdown would require a perforated or slotted production liner. In this study, the conditions of drilling and production necessary for a stable horizontal well drilled into a Mannville coal seam at 750–800 m in the central Alberta Plains, were studied. Modeling showed that the depth of fluid penetration would be at least 2.2 m if there is no filter cake formation while drilling underbalanced. Drilling fluid penetration would be minimal if a filter cake is formed under overbalanced conditions. Furthermore, FLAC analysis showed that drilling a smaller diameter hole (5 cm) would be preferable because this size results in a stable wellbore during production compared to the standard 15-cm diameter hole.