Acid treatments in high temperature deep wells drilled in carbonate reservoirs represent a challenge to the oil industry. The high temperature of deep wells requires a special formulation of emulsified acid that can be stable and effective at such high temperatures. At these high temperatures, both the reaction rate between acid and rock, and corrosion rate of tubulars are high. This fact makes protection of tubulars and reducing the reaction rate between rock and acid challenging.
A new emulsifier was used to prepare emulsified acids that can be used in stimulating deep wells drilled in carbonate reservoirs. All emulsified acid systems were formulated at 0.7 acid volume fraction, and the final HCl concentration was 15 wt%. A coreflood study was conducted in order to study the efficiency of the new emulsified acid to create wormholes, and to increase the efficiency of the stimulation treatment. Both low and high permeability Indiana limestone cores were used in the present study. The effect of acid injection rate, rock permeability, and emulsifier concentration on the performance of emulsified acid systems were studied. The reaction between emulsified acid and limestone rocks was studied using a rotating disk apparatus at 230 ºF, and rotational speeds up to 1,500 rpm.
The results showed, for both low and high permeability Indiana limestone cores, the new emulsified acid system created deep wormholes at all injection rates (0.5 to 10 cm3/min), with no face dissolution encountered during acid injection. The reaction rate between the new emulsified acid and limestone cores was measured using a rotating disk at 230 ºF. The dissolution rate increased as the rotational speed increased, indicating that reaction between emulsified acid and limestone is a mass transfer limited reaction. Both reaction rate and diffusion rate decreased as the emulsifier concentration increased from 0.5 to 2.0 vol%. From these results, the new emulsifier can be used in formulating emulsified acid systems that can be used effectively in stimulation of high temperature deep wells. This paper summarizes the results of testing the new emulsified system, and recommends its use for field application in deep carbonate reservoirs.