Wellbore Pressure Management in Unstable Shales: It's Not Just About Rocks

Abstract

Much of the drilling in unconventional resource plays occurs in unstable shales, which are usually fractured and can be easily destabilized. Drilling through them successfully can be difficult at best, and many high-angled holes in these plays are often lost due to mechanical instability. This paper examines the problems of shale gas drilling from the theoretical perspective of Wellbore Pressure Management (WPM) and keys in on the effects of equivalent circulating density (ECD) while drilling and on the effects of equivalent static density (ESD) when there is no circulation.In this paper the following questions pertaining to drilling a typical fractured shale or highly-laminated weak zone are addressed from the WPM perspective: What mud density do I need to drill a fractured shale?  Why can a typical shale gas play well be drilled with no drilling problems, yet becomes very unstable on the last trip out of the hole before wireline logging or running casing?  Why are drilling problems especially acute in laminated shales or similar weak zones?  Why is the wellbore unstable while the drilling density is within the range demarcated by the Safe Drilling Window?  Why does shale instability often not improve significantly when drilling fluid density levels are increased?  Which tools in the driller's toolbox are often used that actually make the wellbore stability issue more problematic?By using a Wellbore Pressure Management approach to understanding instability in fractured shales, the reader can readily see how to best deal with the problem in the field and hopefully improve stability in future wells.