Wellbore geomechanics of extended drilling margins and engineered lost circulation solutions

Wellbore tensile failure is a known consequence of drilling with excessive mud weight, which can cause costly events of lost circulation. Despite the successful use of lost circulation materials (LCM) in treating lost circulation events of the drilling operations, extensions of wellbore stability models to the case of a fractured and LCM-treated wellbore have not been published. This paper presents an extension to the conventional stability analysis of a wellbore to such circumstances. The proposed wellbore geomechanics solution redefines the extended Equivalent Circulation Density (ECD) criteria and margin for re-breakdown of a fractured and, if the case, LCM-treated wellbore. An analytical approach is taken to solve for the related multiscale and nonlinear problem of three-way mechanical interaction between the wellbore, fracture wings, and LCM aggregate. The criteria for unstable propagation of existing near-wellbore fractures, together with those for initiating secondary fractures from the wellbore, are obtained. Results suggest that, in many circumstances, the occurrence of both incidents can be prevented, provided that the LCM blend is properly engineered to recover certain depositional and mechanical properties at downhole conditions. Under such optimal design conditions, the maximum ECD to which the (re-)breakdown limit of a permeable wellbore could be enhanced is predicted.