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Summary

The authors presented the first industry SPE paper on wellbore strengthening, which proposed a new concept for lost-circulation prevention while drilling (Morita et al. 1988; Fuh et al. 1992; Morita et al. 1996). Recently, the authors were invited to present at the 2010 SPE Forum held at Park City, Utah, USA. This presentation focused on wellbore-strengthening methods currently used in the industry, including well cooling, stress cage, and tip screening of induced fractures. During the forum, participants requested a parametric analysis of these methods using rock-mechanics equations and principles. This paper presents a set of analytical equations developed for parametric analysis of three typical wellbore-strengthening methods (Fuh et al. 2007; Alberty and McLean 2004) and provides information about the strengths and limitations of each method. In addition, an updated set of equations developed on the basis of previous work completed by the authors has been provided to make the analysis of wellbore-strengthening methods easier to implement.

The equations can be classified by the length of the cracks to be stabilized, as follows:

• Borehole strengthening by heating to stabilize 0.0- to 0.1-in. cracks. (Not included in this parametric study.)
• Borehole strengthening to stabilize microcracks (with mudcake or fine particles) for 0.1- to 1-in. cracks.
• Borehole strengthening to stabilize macrocracks (stress-cage method) for 1-in. to 2-ft cracks.
• Borehole strengthening to stabilize a large fracture with the tip-screening method for cracks longer than 10 ft.

Parametric studies were conducted on three wellbore-strengthening methods, and the following observations were made:

• Borehole-stabilization method: Water-based mud mixed with 25/40-mesh particles stabilizes the borehole by plugging the microcracks with mudcake. Borehole stability is enhanced by mixing 25/40-mesh crushed nut shells into the mud. Some drilling engineers enhance borehole stability by intentionally returning a proper range of cutting particles from the shale shakers.
• Stress-cage method: This method is applicable if the formation permeability is not too low. However, if the permeability is low, a high-fluid-loss pill is required to reduce propagation of the induced fracture and adequately place the granular materials. After wellbore strengthening, ultralow-fluid-loss mud is required during drilling to reduce the pressure buildup in the fracture section behind the seal.
• Tip-screening method: This method does not require the squeezing of particles to induce a fracture. Mixing particles with drilling fluid prevents fracture initiation, and if a fracture is induced, this mixture prevents further fracture propagation by tip screening. Tip screening is effective if a lost-circulation zone has some permeability, but is less effective if the lost-circulation zone has no permeability.