Fracture-initiation pressure (FIP) and fracture-propagation pressure (FPP) are both important considerations for preventing and mitigating lost circulation. For significant fluid loss to occur, a fracture must initiate on an intact wellbore or reopen on a wellbore with pre-existing fractures, and then propagate into the far-field region. Wellbore-strengthening operations are designed to increase one or both of these two pressures to combat lost circulation. Currently, some theoretical models assume that FIPs and FPPs are only functions of in-situ stress and rock-mechanical properties. However, as demonstrated by numerous field and laboratory observations, they are also highly related to drilling-fluid properties and to interactions between the drilling fluid and formation rock. This paper discusses the mechanisms of lost circulation and wellbore strengthening, with an emphasis on factors that can affect FIP and FPP. These factors include microfractures on the wellbore wall, in-situ-stress anisotropy, pore pressure, fracture toughness, filter-cake development, fracture bridging/plugging, bridge location, fluid leakoff, rock permeability, pore size of rock, mud type, mud solid concentration, and critical capillary pressure. The conclusions of this paper include information seldom considered in lost-circulation studies, such as the effect of microfractures on FIP and the effect of capillary forces on FPP. Research results described in this paper may be useful for lost-circulation mitigation and wellbore-strengthening design, as well as hydraulic- fracturing design and leakoff-test (LOT) interpretation.
