Wellbore strengthening techniques are commonly used to prevent drilling fluid losses. Current methods generally require that particles are added to the drilling fluid to hinder fracture propagation, which creates practical difficulties as the particles are often relatively large. An alternative approach is the idea of using the filter cake that forms against the wellbore rock to create a robust seal, the efficacy of which will depend on cake strength. However, little is currently understood about filter cake strength and how it is impacted by typical particulates in the drilling fluid.

In this work, the strength of drilling fluid filter cakes is assessed. Furthermore, filter cake properties such as porosity and thickness that are altered by constituent particles and that affect cake strength are explored. The cake strength was measured using the hole punch test and particle properties such as particle concentration, size distribution and shape were evaluated.

Representative water-based and oil-based drilling fluids were analysed to establish benchmark results, which were based on the rheological and filtration behaviours as well as filter cake properties. These results produced similar trends to those of model water-based drilling fluids composed of typical drilling fluid components, such as an increase in cake strength and a decrease in cake porosity as the barite volume fraction in the fluid increased. For these model fluids, the cake strength also increased as the particle size and cake porosity decreased whilst calcium carbonate cakes were stronger than the barite equivalents. Cake strength may have been influenced by interparticle contact surface area, which was affected by cake porosity and thickness.

The relationships between particle size, pore distributions and thickness were better understood by visualising the internal structure of filter cakes, using images captured via X-ray computed tomography. The images showed that the size of the pores decreased as the particle size decreased, the cakes had a more porous bottom layer than top and the porosity decreased with filtration time. Discrete element method simulations were compared with experimental results, and relationships between cake strength and the interparticle contact surface area, determined using cake porosity and particle size, were found.