Degradation of high molecular weight polyacrylamide by alkali-activated persulfate: Reactivity and potential application in filter cake removal before cementing

Abstract

The interface between annular cement to wellbore rock is crucial for ensuring the oil and gas well integrity and the environmental protection. Poor bonding quality may lead to gas influx into the annulus or even ground and cause health, safety and environment (HSE) hazards including contamination of freshwater-bearing zones, corruption of the cement and casing, sustained casing pressure (SCP) and formation fluid spill to the ground surface. This bonding interface between annular cement and wellbore is given by a filter cake, which is commonly consisted of a bentonite, barite and high molecular weight polymer such as polyacrylamide (PAM) from the drilling process. Therefore, finding an efficient and environment-friendly method to degrade PAM to remove the filter cake before the cementing is highly desired. In this study, a fast and effective method for high molecular weight PAM (Mw=3×107 Da) degradation by an oxidizing OH-/persulfate (PS) system is presented. By optimizing the PS concentration, alkali-to-PS ratio and temperature of the oxidizing solution, it is demonstrated that a high PAM degradation efficiency (nearly 90%) can be obtained within 20 minutes and maintain a pH range suitable for cement casting. The degradation reactions conformed to pseudo-first-order kinetics with an activation energy of 59.87 kJ mol-1. Dominant radicals were analyzed by EPR that both SO4•- and HO• were found in the reaction system, but the HO• played the dominant role in the degradation process. Structure transformation of PAM was studied via GPC, SEM and FTIR. The backbone cleavage and detachment of side groups caused structural failure of polymer-bentonite and make possible removing filter cake. Filter cake removal test demonstrated at least 81% filter cake could be removed under the optimized condition. It’s more effective than ordinary preflush with only 24wt% on average. The results obtained in this work exhibit that polymer degradation by OH-/PS oxidizing system highly helps to filter cake removal and the OH-/PS system is a promising method for filter cake removal before the cementing process.