Stability of acid in crude oil emulsion based on interaction energies during well stimulation using HCl acid

Acid stimulation is one of the most common chemical approaches to overcome the reduction of crude oil production due to clogging of oil formation passages. Nonetheless, in this process, the formation of stable and tight acid in crude oil emulsions may lead to further unsolicited formation damage. In this study, the stability of dispersed HCl droplets with concentration of 15 and 28 wt % in a crude oil was characterized by means of the interaction energy theory. Similar experiments were repeated and compared with distilled water. The results showed that water droplets always tend to coalesce in short period of time, while acid droplets resist. This is due to the acid-base interaction energy, which is the most prevailing component among various interaction energies which would profoundly influence the stability of acid in crude oil emulsions. In case of using HCl solution as aqueous phase in the emulsion, this interaction between acid droplets overcomes the others, especially the attractive interactions. The most stable acid in crude oil emulsion would be formed in distances between the droplets shorter than 4 nm because acid-base interactions are short-ranged. For the investigated crude oil, it was also observed that the emulsion with HCl 28 wt % solution was more stable than that for HCl 15 wt %. Bottle tests also agreed qualitatively with the results of this theoretical approach. The practical implications of these results would be that overdosed acidizing with excessive injection rates may lead to unwanted formation damage due to formation of tight acid in crude oil emulsions. Finally, the presented results would serve as a guideline for comparing acids of different concentrations.