Mitigating lost circulation: A numerical assessment of wellbore strengthening

Abstract

Drilling in complex geological settings often possesses significant risk for unplanned events that could potentially impede already cost-demanding operations. Lost circulation, a major challenge in well construction, refers to the loss of drilling fluid into formation during drilling operations. When excessive wellbore pressure appears, lost circulation is induced by tensile failure or reopening of natural fractures at the wellbore. Over years of research efforts and field practices, wellbore strengthening techniques have been successfully applied in the field to mitigate lost circulation and have proved effective in extending the drilling margin to access undrillable formations. In fact, wellbore strengthening contributes additional resistance to fractures so that an equivalent circulating density higher than the estimated fracture gradient can be exerted on the wellbore.In this study, a fully coupled hydraulic fracture propagation model based on the cohesive zone model is presented. By implementing the model, an extensive parametric study is conducted to investigate factors involved in lost circulation. The parametric influences emphasizing the mass balance within the fracture reveal mechanisms of lost circulation mitigation. Simulation studies on wellbore strengthening are conducted in two parts, hoop stress enhancement and fracture resistance enhancement. First, a near-wellbore stress analysis characterizes wellbore mechanical responses during lost circulation. The results show elevated hoop stress during fracture width development, which validates the hypothesis of hoop stress enhancement. Also, beneficial influences from poroelastic effect and high rock stiffness are demonstrated. Then, a novel method to simulate fracture sealing is introduced to quantify fracture gradient extension for field practices. With this method, a case study on fracture sealing investigates the roles of sealing permeability and sealing length. The results show inhibition of fracture repropagation and conclude that fracture tip protection is achieved through fracture sealing and fracture fluid dissipation. From the case study, operational insights on wellbore strengthening design are derived.