The Synergistic Effects of Ultrasonic Acoustic Cavitation with Chemical Dissolvers on Calcium Sulfate Scale Dissolution

Abstract Scale deposition burdens the oilfields and hinders production, requiring lengthy and expensive operations for pipeline recovery. Despite extensive research for chemical removal methods, all results indicate that incorporating various mechanical aids increases the performance of the chemical solutions. Specifically, calcium sulfate scale, due to its low solubility. This work aims to explore the use of ultrasonic waves to effectively remove scales and potentially reduce treatment time. Calcium sulfate dihydrate (gypsum) crystal samples of length 9±1 cm were exposed to ultrasonic waves to assess the capability of the latter of improving the dissolution. The samples were exposed to 28 kHz ultrasonic waves for 2 hours. According to the specific trial, solid abrasive particles or chemical dissolving agents were added, to evaluate the effect of the dissolution performances. Control experiments were performed by soaking the gypsum sample in the dissolver solutions. The dissolving agents employed in this study were tetrasodium EDTA and potassium carbonate, each with concentration 10 wt.%. When chemical dissolvers were added, the concentration of sulfate ions in the solution was monitored using real-time FT-IR, allowing to reconstruct the dissolution profile of the gypsum sample. Subsequently, the sample was washed with water, when tetrasodium EDTA was employed as dissolver, or with a 5 wt.% solution of hydrochloric acid, when potassium carbonate was used. The sole application of ultrasound brought limited mass loss on the scale samples, however it produced pitting on its surface. This effect is due to ultrasonically-induced cavitation, i.e. formation and consequent collapse of vapor bubbles. These collapses generate microscopic liquid jets, which impact on the surface producing scratches and pitting marks. When the solid abrasive was added, more erosion was visible on the surface of the samples, as the solid particles were strongly accelerated by the collapsing bubbles, vigorously impacting on the surface. As chemicals were added in the system to aid dissolution, the exposure to ultrasound cavitation increased the dissolution 6 times with respect to the control experiment. The sulfate ion concentration profiles measured with the FT-IR highlighted diffusion resistance hindering the dissolution of the gypsum in the control experiments. This diffusion resistance was overcome by cavitation due to the increased small-scale turbulence which enhances the dissolution. Furthermore, the jetting improved the penetration of the chemicals to the deeper layers of the crystal. The gypsum samples were observed under the optical microscope to evaluate the extent of the scratches and pits generated by the ultrasound cavitation effect. This work demonstrates the utility of ultrasound cavitation to the dissolution process, by improving the dissolution of the gypsum crystals and speeding-up the action of chemical dissolvers. The results pave the way for future experiments to reduce the chemical loading and explore more applications of ultrasound-controlled cavitation in descaling.