Polyanionic cellulose (PAC) is a water-soluble, anionic cellulose derivative that is commonly used as a thickener, rheology modifier, and fluid loss control additive in the oil and gas industry. It is made by chemically modifying cellulose, which is a natural polymer that is found in plant cell walls.

PAC is used in drilling fluids to increase the viscosity and control the flow properties of the fluid. It helps to maintain the stability of the borehole and to prevent the formation of drill solids. It is also used as a fluid loss control agent to minimize the loss of fluid into the formation.

PAC is also used in hydraulic fracturing (fracking) to increase the viscosity of the fluid used in the fracking process and to improve the efficiency of the process by keeping the proppant suspended in the fluid.