Because inorganic scales reduce permeability near wellbores, they cause an additional pressure drop in this region (skin damage) and reduce production. NaCl precipitation, particularly in gas-producing wells, is one of the formation damage problems which impair the permeability. The use of backwashing water not only requires a large quantity of water, but is also inefficient. This study examined the effect of the ultrasonic waves in NaCl scale removal. Twenty core samples with different permeabilities were saturated with NaCl. After precipitation of NaCl within the cores, the samples were first subjected to water injection and then to water injection with ultrasonic wave radiation. At each stage, the permeability of cores was measured and recorded. Experimental results reveal that applying ultrasonic waves restores the permeability of cores more than water injection alone. In samples with permeability lower than 20, 30-100, 100-700 md and cores with permeabilities higher than 1000 md, ultrasonic waves recover permeability to 80, 42, 87 and 81%, respectively. In contrast, water injection alone improves permeability to 29, 18.5, 62 and 77%, respectively. The effect of these waves is much more obvious than water injection alone in low-permeability cores. SEM images from cores show that ultrasonic waves destroy the structure of deposits of sodium chloride. This phenomenon is due to propagation of these waves and the resulting temperature increase in the surroundings, which leads to the increased solubility of these scales. After exposure to ultrasonic waves, sodium chloride deposits inside the induced fractures of cores could be removed from the fracture, whereas water injection alone could not wash and remove the deposits from the fracture. According to the results of this paper, ultrasonic waves can be used as a novel and efficient technology in gas wells to remove inorganic scales in the near-wellbore region.

Inorganic scales are one of the most important causes of formation damage, which causes pressure drops near wellbores; these in turn impair permeability and severely reduce production in oil and gas reservoirs. This paper examines the effectiveness of ultrasonic waves in removing potassium chloride (KCl) scales. Twenty core samples with different permeabilities were exposed to KCl precipitation. After measuring the permeabilities of the saturated core samples, the samples were first subjected to water injection, and then to water injection with ultrasonic wave radiation. At each stage, sample permeabilities were measured and recorded. The results showed that water injection with two pore volumes did not significantly improve permeability, especially in low-permeability core samples. Ultrasonic wave radiation with water could efficiently improve permeability; this result is more obvious for samples with lower permeabilities. SEM images taken from thin sections of the core samples under water injection and water with ultrasonic waves showed that ultrasonic waves distorted the crystal lattice of the KCl scales, causing cracking and delamination. Creation of wormholes in KCl deposits within fractures also resulted from the application of ultrasonic waves. Analysis of chlorine in the output water from core samples in the core-flooding process showed that ultrasonic waves increased the solubility of scales in water, improving the recovery of permeability in the samples. Results of this study show that using ultrasonic waves can be considered a novel and practical method in the removal of inorganic scales in the near-wellbore region of oil and gas reservoirs.

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