A New Outlook on the Ideal Packing Theory for Bridging Solids

Abstract

Abstract Carefully engineering the size distribution of bridging solids should enable us to form filter cakes of low permeability. A theory called the ideal packing theory is commonly used to blend bridging solids which suggests the ideal blend of particles should follow the relationship cum vol% ∝ dx. The aim of this work was to experimentally assess this theory. Investigations were carried out to determine if there is an x value which gives a better fluid loss performance than the typically-used x = 0.5. The results suggest that selecting x ~ 1 could be more appropriate than using the conventional x = 0.5. The physical meaning when x = 1 is that there will be an equal volume of particles of each size within the distribution; i.e. the distribution is linear. In addition, the results also showed that a blend with a broad size distribution gave a lower fluid loss than a narrow distribution, even when the narrower distribution consisted of finer particles. The trend was that filter cakes of decreasing permeability were formed as x increased from 0.1 to 0.5. Further increase in x up to 1.25 did not appear to have a prominent effect, while further increases beyond 1.25 appeared to increase the filter cake permeability. Trends were similar for filter cakes formed on filter paper and on simulated rock material. Particle blending is important for the selection of bridging solids for drill in fluids, and also for wellbore strengthening (stress cage) applications. Conventional ideal packing theory has been widely used but a review of this theory backed up by laboratory checks has been overdue. This work should aid in the development of a more consistent approach to the selection of the optimum blend of bridging material for a given application.