Geomechanics of wellbore strengthening revisited: A combined theoretical and experimental approach with field case studies

Wellbore strengthening generally refers to the practice of enhancing the lost circulation limit of a (fractured) wellbore through proper design of the drilling fluid system and downhole circulation program. Particulatebased additives, known as lost circulation materials (LCMs), are commonly and often successfully used for this purpose. The use of geomechanical solutions and wellbore stability analysis is currently well established in the industry as standard practice for predrill and real-time estimation of the drilling margin of a nonfractured wellbore; however, an appropriate methodology that quantifies the possible gain in extending drilling margins of the well using the wellbore strengthening technique is lacking in the literature. This paper proposes a combined theoretical and experimental approach to quantifying the possible extension in the breakdown limit of a fractured wellbore undergoing the LCM-based wellbore strengthening operation. The approach comprises a verified geomechanical model and solution for the stability of the fractured and LCM-treated wellbore, together with the use of in-house experimental data from uniaxialstrain compression testing on LCM products. The model identifies the maximum possible extension in the wellbore breakdown margin, along with an optimal blend type and composition for use in the drilling fluid system. The extension allows for drilling through the troublesome formation without secondary failure of the wellbore wall or unstable propagation of the already existing fracture wings. The obtained extended margin is often recognized to be higher than the wellbore tensile limit or minimum component of the farfield principal stress. A set of subsurface and operational field parameters from (depleted) sand sections of four wells located in two different regions is used to assess the proposed wellbore geomechanics solution. The maximum drilling fluid density and optimal LCM blend designs that can be reached while maintaining the stability of the fractured wellbore are determined for the selected field case studies. Results compare well with the actual blend design and enhanced drilling margin, allowing the operator to drill through the troublesome formation.