A novel self-degradable gel (SDG) as liquid temporary plugging agent for high-temperature reservoirs

Abstract

In recent years, the liquid temporary plugging agent has drawn extensive attention, especially in the repeated fracturing operation for stimulating unconventional oil and gas reservoir production. This paper developed a novel self-degradable gel (SDG) as a liquid temporary plugging agent for high-temperature reservoirs. The gelation process, viscoelasticity, pressure-bearing capacity, self-degradation, and formation protection performance of the SDG were systematically investigated. Moreover, the gelation and self-degradation mechanism of the SDG were revealed by studying the viscosity variation and the microstructures. Considering the gelation time and gel strength, the composition of the SDG was optimized as 6.0–10.0 wt% monomer, 0.004–0.012 wt% initiator, and 0.2–0.4 wt% crosslinker. The SDG transformed from liquid to solid-like gel within 30 to 90 min at 90–130 °C. The rheological results showed that the duration of the low viscosity state exceeded 75 % of the total gelation process at high temperatures, which was conducive to the injection of the SDG. In addition, the viscosity of the SDG after gelation was more than 100 Pa·s, which ensured its effective plugging in fractures. The experimental results with fractured cores showed that the bearing pressure of the SDG remained above 30 MPa/m in fracture with a width of 2.05–5.04 mm, which could meet the requirements of the repeated fracturing operation. Moreover, the SDG could spontaneously degrade from solid-like gel to liquid with a viscosity of lower than 10 mPa·s in 10–105 h at 90–130 °C. The broken-gel liquid could be completely displaced out of the fractured core, and the core damage rate was less than 5.0 %, which ensured the restoration of conductivity of the previously existing fractures. Micrographs showed that the dense three-dimensional network structure with high viscosity was formed by polymerization and crosslinking. Then, the acrylic acids in the polymer chain were broken at high temperatures, resulting in the gradual destruction of the gel and the spontaneous degradation of SDG.