C.W. Crowe, R.C. Martin & A.M. Michaelis, Dowell Division of Dow Chemical
Various polymers and other thickening agents have been evaluated as gelling agents for hydrochloric acid. These materials were compared on the basis of thickening efficiency, acid stability and residue formation upon spending of the acid. Thickeners tested include xanthan polymers, guar gum, hydroxyethyl cellulose, hydroxyethylcarboxmethyl cellulose, polyacrylamide, polyvinyl alcohol, polyvinylpyrrolidone, acrylamide/sodium-2-acryamido-2-methylpropane sulfonate copolymer, and a cationic surfactant which thickens acid when used at high concentrations. Of the thickening agents tested, the xanthan polymer showed the greatest overall potential for use as an acid gelling agent. This polymer is an efficient viscosity builder and has good stability in 15% hydrochloric acid at temperatures up to 150_F. Computer model studies, which take into consideration cool-down and gel degradation rate at various stages of acid spending, indicate viscosity is maintained when treating wells that have bottom-hole temperatures up to 220_F. Since the gelled acid retains its viscosity during spending, both reaction rate and fluid leak-off are retarded. Laboratory studies, simulating downhole spending conditions, yield a 3.6-fold increase in spending time for gelled 15% hydrochloric acid (22 cp) as compared to ungelled acid. Well test results are presented that demonstrate the effectiveness of acid fracturing treatments employing hydrochloric acid with xanthan polymer.