Parametric analysis of wellbore strengthening methods from basic rockmechanics
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The authors previously presented the industry's first SPE paper in wellbore strengthening by proposing a new idea for lost circulation prevention while drilling. During our attendance of the 2010 SPE FORUM (Lost Circulation and Wellbore Strengthening) held at Park City, Utah, we presented and discussed about various wellbore strengthening methods as currently employed and practiced in the industry. These methods include well cooling, stress cage, and tip screening of induced fractures. Some participants requested parametric analysis of these methods using equations of rock mechanics principles. A set of analytical equations are developed for parametric analysis of four typical wellbore strengthening methods. They may be classified by the size of cracks to be stabilized as follows: (A) Borehole strengthening with heating by stabilizing 0 to 0.1 in. cracks. (B) Borehole strengthening by stabilizing micro cracks (with mud cake or fine particles) for 0.1 in. to 1 in. cracks (C) Borehole strengthening by stabilizing macro cracks (stress cage method) for 1 in. to 2 ft cracks (D) Borehole strengthening by stabilizing a large fracture with tip screening method for more than 10 ft cracks Simplified equations are developed for the above four methods. These equations reveal limitations and advantage of each method. The following items are a part of the parameter studies: (1) Water base mud mixed with 25-40 mesh particles stabilizes borehole by plugging micro cracks with mud cake. Some solid return from shale shaker or mixing 25-40 mesh crushed nut shell enhances borehole stability. (2) The stress cage method is safely applied if the formation permeability is not too small. However, as the permeability is small, it requires to reduce extension of induced fracture with a high fluid loss pill to place the granular materials, while an ultra-low fluid loss mud is required during drilling after strengthening to reduce the pressure build-up in the fracture section behind the seal. (3) The tip screening method does not require squeezing particles by inducing a fracture. Particles mixed with drilling fluid prevent fracture initiation, and, if a fracture is induced, it prevents further fracture propagation by tip screening. It is effective if a lost circulation zone has some permeability, while it is not effective if the lost circulation zone has no permeability. In this paper, a set of simplified equations are presented to clarify the wellbore strengthening methods and parameter studies are conducted for each method to clarify the advantages and disadvantages of each method. Equations developed in a previous paper by current authors are significantly improved for easier implementations of wellbore strengthening methods. Copyright 2011, Society of Petroleum Engineers.
