One way of overcoming the challenge of drilling depleted sands is to use so-called “wellbore strengthening” (WBS) techniques, whereby engineered particles are added to drilling fluids to increase the fracture initiation or propagation resistance. Although the mechanisms are not comprehensively understood, most explanations suggest that an increasing proportion of large particles is required to achieve a greater extent of ‘strengthening’. These large particles can create practical challenges while drilling; such as particle attrition, settling, and erosion of equipment. Field examples are presented which highlight these issues. Experimental work to identify solutions to the practical challenges is presented. A laboratory attrition test developed in-house confirms walnut shell as a promising material to resist grind-down when large particles need to be retained in the system. Formulations prepared using controlled size distributions of this material are shown to give a good fracture sealing performance in the laboratory. The accurate maintenance of particle size will require regular solids monitoring at the rigsite to keep the fluid within design specifications. Some data from real-time particle sizing techniques are presented. Overall, the findings presented in this paper should lead the industry towards wellbore strengthening systems which are effective, more efficient and practical operationally.
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Chellappah et al. (2015) studied this question.