Understanding Shale Heterogeneity - Key to Minimizing Drilling Problems in Horn River Basin

Abstract

Abstract More than 70% of nonproductive time (NPT) and increases in drilling cost are related to wellbore instabilities. Shale formations are the primary sources (90%) for wellbore instabilities. Numerous wellbore instability problems have been reported in the Horn River basin (HRB), the largest shalegas play in Canada. As a consequence of the depositional environment, shale formations have laminated structure that results in anisotropic mechanical properties and horizontal stresses. Failure to consider this characteristic of shale can have severe consequences on drilling. Traditional isotropic stress calculation approaches typically used in wellbore stability analysis do not consider 3D azimuthal anisotropy present in shales. Ignoring anisotropy generally results in underestimation of stresses, which can lead to incorrect safe trajectory or mud-weights predictions. In this paper, drilling problems experienced in 15 wells in two different areas of the HRB were examined. Some of these wells had severe wellbore instabilitis due to high pore pressure, mud losses or lost circulation, tight hole/stuck pipe/pack off, or combination of these events. Three borehole assemblies (BHA) were lost in these wells which required side tracking. Most of these problems were experienced in Fort Simpson and upper Muskwa formations. An in-depth postmortem analysis of these wells indicated that shale heterogeneity was not properly characterized (anisotropic horizontal stresses were not considered in the prespud analysis which resulted in incorrect mud-weight predictions and trajectory calculations.