A New Emulsified Acid to Stimulate Deep Wells in Carbonate Reservoirs

Abstract

Abstract 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. At temperatures above 200 °F, there is a need to add more corrosion inhibitor and corrosion inhibitor intensifier, which increases the cost of the treatment too much. A new emulsifier was developed and used to prepare emulsified acids that can be used in stimulating deep wells drilled in carbonate reservoirs. In the present paper, the rheology of the new acid is compared to the rheology of another system formulated by a commercial emulsifier that has been used extensively in the field. All emulsified acid systems were formulated at 0.7 acid volume fraction, and the final HCl concentration varied from 5 to 28 wt% HCl. The rheology measurements were conducted at temperatures up to 300 °F for emulsifier concentration ranges from 0.5 to 2.0 vol%. The reaction between emulsified acid and rock was studied using a rotating disk apparatus at 230 °F and rotation speeds up to 1,500 rpm. A core flood study was conducted in order to study the efficiency of the new emulsified acid to create wormhole, and increase the efficiency of the treatment, especially at a high temperature (300 °F). The results showed that the new emulsified acid system had higher thermal stability and higher viscosity than the old one. Also, the new emulsified acid system created deep wormholes at all injection rates covered (0.1 cm3/min up to 10 cm3/min), with no face dissolution encountered during acid injection. The reaction rate between emulsified acids formulated using the new emulsifier was measured using a rotating disk at 230 °F. The dissolution rate varied between 2.57E-6 gmol/cm2.s at 100 rpm and 1.09E-5 gmol/cm2.s at 1500 rpm. The diffusion rate was measured for these emulsified acid systems and was found to be around 2.73E-7 cm2/s. 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 reservoir.