metal cross - linked phosphate ester compositions impart fragile progressive gel structure to a variety of oil and invert emulsion - based drilling fluids at neutral or acidic ph . the fluids are useful for drilling wellbores in subterranean formations without need for organophilic clays to suspend drill solids . at greater concentrations , metal cross - linked phosphate ester compositions enhance fluid viscosity for suspending weighting materials in drilling fluids during transport of the fluids , as from a manufacturing or mixing facility to a drilling site .