Summary Owing to the narrow drilling margin that exists between the pore-pressure and the fracture-pressure gradient, drilling in depleted-reservoir, high-pressure/high-temperature, and deepwater environments is universally recognized as being technically challenging. A number of field techniques are available for mitigating many of the drilling problems encountered. Included among these are specialized fluid engineering that involves the use of chemical- and particulate-based treatments for minimizing or preventing losses. In many instances, these techniques can be used to strengthen or stabilize the wellbore when drilling at or near the fracture gradient, thereby potentially eliminating the need for intermediate casing strings. This paper discusses particulate-based-treatments design for sealing fractures. Substantial experience gained from innovative laboratory testing has highlighted the mechanisms and many factors that determine the effectiveness of the fracture seal. The particle-size distribution (PSD) relative to the fracture aperture, particle morphology, volumetric concentration, rheological properties of the fluid, and fluid-loss control influence whether the seal is established within the fracture or at the fracture mouth. Understanding this distinction is important with respect to selecting the optimum treatment and its application for given field conditions. Parameters critical for optimizing the treatment have been identified and are discussed in the context of laboratory and field experience.