CPMUL-P serves as a primary emulsifier in oil-base mud systems. It is formulated to give good emulsification, improved thermal stability of the invert emulsion, and enhanced high-temperature, high-pressure (HTHP) filtration control. Through comprehensive tests in a number of oil-base mud formulations with various base oils, mud densities, oil/water ratios and hot-rolling temperatures, it proves that at working temperature up to 177℃ (350℉), CPMUL-P can maintain high ES (electrical stability), low HTHP filtrate and desired rheological property.