Experimental Investigation of Some Properties of Emulsified Acid Systems for Stimulation of Carbonate Formations

Abstract Acidizing treatments in carbonate formations is of paramount importance to restore or to stimulate the near-wellbore area by creating wormholes. Emulsified acid was found to be especially effective when the injection rate is low. This paper discusses the properties of some emulsified acid formulations. The systems used in the study are mixtures of acid, diesel oil, surfactant and possibly a cosurfactant. Model systems made of anionic and nonionic surfactants are compared to the properties of a commercial emulsified acid. Experimental results are focused on the effect of the acid to oil volume ratio, the acid concentration and the surfactant nature. Rheological properties and temperature stability of the emulsion systems are analysed. Results of coreflood tests in limestone samples are presented which compare the behaviour of emulsified acids to the behaviour of plain HCl acid. It is shown that emulsified acids are effective stimulation fluids at low flow rates: their main advantage is a deep penetration in conditions where plain acid gives compact dissolution patterns. On the opposite, at high flowrates, plain HCl gives better results. These results are confirmed by CT scanner tomography analysis showing highly branched wormhole structures even at low flow rates. Guidelines are deduced to improve acidizing treatments in low permeability carbonate reservoirs.