Experimental Study on Degradable PGA as Temporary Plugging Agent in Drill-in Fluids

Polyglycolic acid (PGA) has good environment protection property. In laboratory study, the mechanical property, the particle sizes, the thermal stability of the PGA molecules and the crystal density of PGA before and after high temperature soaking are analyzed, the thermal stability of PGA in water and oil is studied, and the application potential of PGA as a temporary plugging agent for drill-in fluids is evaluated. It was found that the physiochemical properties of PGA are highly affected by the liquid environment of a drilling fluid and temperature. In water-based drilling fluids, PGA will degrade at 100 ℃ to oligomers because of the pH of the drilling fluid, and the thermal stability of the PGA molecules and the mechanical strength of PGA are all decreased as well. Strong acid (pH< 5) and strong base (pH< 9) both accelerate the degradation of PGA, and PGA degrades faster in strong base environment than in strong acid environment. In high temperature oil-based drilling fluids, the PGA particles first swell and then peel and break, and the main body of the PGA particles turns from plastic to brittle, resulting in a decrease in the tensional and compressive strengths of the particles to some extent. PPA plugging experiment shows that after 8 d of soaking in an oil environment, the PGA particles exhibit good plugging performance. These studies show that PGA is more suitable for use in oil-based drilling fluids than in water-based drilling fluids.