A class of water muds that use partially-hydrolyzed polyacrylamide (PHPA) as a functional additive, either to control wellbore shales or to extend bentonite clay in a low-solids mud. As a shale-control mud, PHPA is believed to seal microfractures and coat shale surfaces with a film that retards dispersion and disintegration. KCl is used as a shale inhibitor in most PHPA mud designs. In low-solids muds, PHPA interacts with minimal concentrations of bentonite to link particles together and improve rheology without increased colloidal solids loading.

Reference:
Clark RK, Scheuerman RF, Raoth H and van Laar H: "Polyacrylamide-Potassium Chloride Mud for Drilling Water Sensitive Shales," Journal of Petroleum Technology 28, no. 6 (June 1976): 719-726.
Reference:
Fraser LJ: "New Method Accurately Analyzes PHPA's in Muds," Oil & Gas Journal 85, no. 27 (July 6, 1987): 39-42.