Droplet Size Analysis of Emulsified Acid

Abstract

Abstract Emulsions have been used for many applications in the petroleum industry for many decades. Emulsified acid has been used in acid fracturing and well acidizing since 1933. A large number of fracturing treatments that used emulsified acid has been reported in the literature. Another fair number of researchers conducted lab experiments utilizing the emulsified acid. However, all of this work cannot be reproduced because of inadequate characterization of the emulsion. One essential way to characterize emulsions is by their droplet size. Various mixing modes and proportions produce emulsions with different size distributions for the dispersed phase. Those physical variations are believed to generate different properties (viscosity, stability, etc) of the emulsions. The main objective of this paper is to provide a detailed description of the droplet size distribution of emulsified acids. Methods to measure and represent the data are the core of this section. We show that coarse or fine emulsions can be produced by selecting the mode of mixing and speed of shearing. Simple mixing and low shearing produce coarse emulsions whereas atomizing and high shearing produce fine emulsions. It is also demonstrated that the droplet size and specific surface area are affected by emulsifier concentration and acid volume fraction. Average droplet size decreases with increasing emulsifier concentration. The diameter also increases with increasing acid volume fraction. The specific surface area of the droplets increases with increasing emulsifier concentration and decreases with increasing acid volume fraction. The change of droplet size has a practical impact on the stability, rheology and reactivity of emulsified acid. Good understanding and characterization of the emulsified acid by its size distribution will lead to advancements in this area. Introduction An emulsion is a mixture of two immiscible liquids. One liquid is called the dispersed phase because it forms the internal phase and the other liquid is called the continuous phase because it forms the external phase. In the emulsified acid system, a continuous liquid phase which is diesel surrounds droplets of hydrochloric acid forming acid-in-diesel emulsion. It is the practice of the petroleum industry to use a concentrated (15–28 wt%) hydrochloric acid. The emulsified acid system is stabilized by adding an emulsifier. Because emulsions are unstable and thus do not form spontaneously, the emulsifier is needed to reduce the interfacial tension between the acid and the diesel to a value that allows the two phases to mix and form one mixture. Once formed, the emulsified acid system should remain stable for a minimum time that allows it to be pumped to the formation. The emulsifier is a surface active agent (surfactant) that reduces the interfacial tension between oil (diesel) and water (acid) by adsorbing at the liquid-liquid interface.