Preparation and Performance Evaluation of a Self-Degrading Bridging Temporary Plugging Agent for Drilling Fluids

In drilling fractured low-permeability reservoirs, bridging temporary plugging agent particles, because of their large sizes, are difficult to achieve tight plugging, and these particles exhibit low self-degradability. To address these technical challenges, a self-degradable bridging temporary plugging agent SMNP-1 was prepared via inverse microemulsion polymerization and using liquid microfluidics technology. The plugging agent SMNP-1 was synthesized using acrylamide (AM) and sodium p-styrenesulfonate (SSS) as the monomers, and β-cyclodextrin diacrylate ester (β-CD-AA) as the crosslinking agent. The molecular structure of SMNP-1 was verified with infrared spectrometer, and the particle size distribution and micromorphology of SMNP-1 were characterized with laser particle size analyzer and scanning electron microscope (SEM). Laboratory experiments were conducted to investigate SMNP-1 for its water absorption and swelling performance, adsorption performance, temporary plugging and unplugging performance, self-degradability, pressure-bearing capacity, and compatibility. Experimental results demonstrate that at elevated temperatures, SMNP-1 exhibits a moderate volumetric expansion rate after absorbing water, and a strong adsorption capacity. The SMNP-1 particles, after high-temperature treatment, show a normal size distribution and are morphologically nano-micron sized microspheres. At test temperatures of 100 ℃, 120 ℃, 140 ℃ and 160 ℃, the rates of SMNP-1 to plug natural cores are 96.88%, 96.63%, 96.57% and 95.27%, respectively. After 240 h of high temperature exposure, the unplugging rates of SMNP-1 are 87.22%, 89.95%, 93.27% and 96.13%, respectively, and the rates of self-degradation are 52.05%, 56.40%, 63.04% and 74.11%, respectively, demonstrating remarkable reservoir protection effect. At test temperature of 100 ℃, the maximum displacement pressure difference is 55 MPa, and a maximum displacement pressure difference of 25 MPa can still be achieved when the temperature is increased to 160 ℃, demonstrating an excellent plugging performance. SMNP-1 exhibits little effect on the rheology of drilling fluids, it can effectively enhance the wall-building capacity of a drilling fluid after fluid-loss and the reservoir protection effect. SMNP-1 has been successfully applied in the wild-cat well Xinsheng-1.