Wellbore instability and the problems it gives rise to are major concerns in the oil and gas industries. One of the main causes of wellbore instability is the penetration of drilling liquid into fractures that are initiated and propagated during the drilling operation. In order to prevent such drilling fluid penetration, a new technique called wellbore lining that works by coating a wellbore with polymers is proposed by Schlumberger Ltd. This research investigates the mechanisms of fracture initiation and propagation in saturated rock formations and the feasibility of the lining technique by numerical simulations of wellbore drilling with and without lining. .
It was found that a fracture propagates fast enough to dismiss the effect of tensile-induced plasticity on the fracture tip as a result of almost identical propagation rate between elastic and Mohr-Coulomb models. Tensile-induced plasticity mainly influences the formation stress field only near the wellbore resulting in the closure of the fracture mouth.
Although the lining changed the stress field around the wellbore, the magnitude was not considerably large. Thus, it appears that a lining does not work by strengthening the wellbore but by preventing the drilling fluid from penetrating. The stiffer the lining is, the larger the breakout pressure becomes because the stiffer lining requires smaller shear strain to carry wellbore pressure.
It becomes clear that drilling fluid penetration is largely influenced by the failure criteria for mud cake and lining properties. There is a need to model local deformation around the dominant fracture mouth to examine actual failure of mud cake and lining. Further investigations are needed.