Summary
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.
Summary
Previous lost-circulation models assume either a stationary fracture or a constant-pressure- or constant-flowrate-driven fracture, but they cannot capture fluid loss into a growing, induced-fracture driven by dynamic circulation pressure during drilling. In this paper, a new numerical model is developed on the basis of the finite-element method for simulating this problem. The model couples dynamic mud circulation in the wellbore and induced-fracture propagation into the formation. It provides estimates of time-dependent wellbore pressure, fluid-loss rate, and fracture profile during drilling. Numerical examples were carried out to investigate the effects of several operational parameters on lost circulation. The results show that the viscous pressure losses in the wellbore annulus caused by dynamic circulation can lead to significant increases in wellbore pressure and fluid loss. The information provided by the model (e.g., dynamic circulation pressure, fracture width, and fluid-loss rate) is valuable for managing wellbore pressure and designing wellbore-strengthening operations.
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