Experimental and mechanism study: Partially hydrolyzed polyacrylamide gel degradation and deplugging via ultrasonic waves and chemical agents

Partially hydrolyzed Polyacrylamide (PHPAM) crosslinked by Cr⁺³ is frequently applied to plug thief zone for the better water management in matured oil reservoir. However, PHPAM gel may certainly cause inevitable formation damage nearby the wellbore. Although various kinds of chemical agents, such as hydrogen peroxide (H₂O₂), sodium hypochlorite (NaOCl), and chlorine dioxide (ClO₂) were employed to mitigate the nearby wellbore damage. But, huge financial investment, poor degelation efficiency, environmentally insecure, corrosion problem, and long time span requirement persuade researchers to look for other effective technique. In this connection, ultrasonic waves is characterized by reliable, environment friendly, and cost effective technology. Current work involves comparative study of PHPAM gel degradation by the individual means of chemical agent and ultrasonic waves. Subsequently, the best-performed ultrasonic parameters and well-performed chemical agent were used independently and then simultaneously to deplug (PHPAM gel) the core sample. Results showed that 20 KHz frequency (1000 W) effectively reduced gel viscosity from original (2495 mPa.s) to 1.37 mPa.s after 10 min irradiation. This degradation is attributed to cavitation, heat energy, and hydroxyl radical (HO∙). However, after 2 min further exposure, the viscosity grew back to 3.29 mPa.s (18 KHz), 1.42 mPa.s (20 KHz), and 3.74 mPa.s (25 KHz). This adverse behavior is owing to hydroxyl radical (HO∙) annihilation. In chemical treatment, H₂O₂ among other chemicals efficiently degelled the PHPAM gel's original viscosity to 2.64 mPa.s after 24 h reaction. Similarly, NaOCl and ClO₂ brought down original viscosity to 6.5 mPa.s and 159 mPa.s respectively. SEM of the samples before and after treatment was performed for the better understanding of PHPAM gel morphology. Considering dynamic experiment, maximum 23.5% and 19.80% damaged permeability recovery (30 × 10^-3 μm² gas permeability) were obtained by applying ultrasonic waves (20 KHz, 1000 W, and 100 min irradiation) and chemical agent (H₂O₂) respectively. Permeability recovery was further increased to 40.90% by the simultaneous application of ultrasonic waves and chemical agent.