A class of muds that contain potassium ion (K⁺) dissolved in the water phase. Potassium muds are the most widely accepted water mud system for drilling water-sensitive shales, especially hard, brittle shales. K⁺ ions attach to clay surfaces and lend stability to shale exposed to drilling fluids by the bit. The ions also help hold the cuttings together, minimizing dispersion into finer particles. The presence of Na⁺ ions counteracts the benefits of K⁺ ions and should be minimized by using fresh water (not sea water) for make-up water. With time, Na⁺, Ca⁺² and other ions accumulate from ion exchange with clays, making the mud less effective, but regular treatment to remove Ca⁺² improves polymer function. Potassium chloride, KCl, is the most widely used potassium source. Others are potassium acetate, potassium carbonate, potassium lignite, potassium hydroxide and potassium salt of PHPA. Use of bentonite clay is restricted because of its strong affinity for K⁺. Instead, various polymers are used. XC polymer and PHPA are used for rheology. For fluid-loss control, mixtures of starch and polyanionic cellulose are often used. CM starch, HP starch, carboxymethylcellulose and sodium polyacrylate (SPA) are also used. PHPA is widely used for shale encapsulation. Potassium, lime and starch-like polymers have also been used as potassium mud systems. Although three API methods exist for determining the K⁺ ion concentration, the centrifuge method (for K⁺ >5000 mg/L) is the most accepted field method, and essential for daily monitoring of potassium in a mud. Regular additions of potassium salt maintain shale stability. K⁺ ion is rapidly consumed while drilling shallow, soft and highly dispersive (gumbo) shales, but maintaining sufficient K⁺ ion to stabilize gumbo can become expensive when drilling large holes. Researchers, notably Dr. Dennis O'Brien and Dr. Martin Chenevert (while at Exxon Production Research), evaluated different shales, their clay mineralogy and the concentration of K⁺ needed to stabilize them. Potassium muds above about 1 wt. % K⁺ ion usually fail the mysid shrimp (US EPA) bioassay test. Therefore, K-muds currently find low acceptance in offshore drilling in USA waters.

Reference:
O'Brien DE and Chenevert ME: "Stabilizing Sensitive Shales with Inhibited Potassium-Based Drilling Fluids," Journal of Petroleum Technology 25, no. 9 (September 1973): 1089-1100.