Microscopically visual study on the plugging mechanisms and injection parameter optimization of coated swelling particles in fractured-vuggy carbonate oil reservoirs

In the Tahe Oilfield, it is a typical fractured-vuggy carbonate oil reservoir with the oil reservoir temperature of 120−150 ℃ and the total formation water salinity of (20−25)×10⁴ mg/L. In this paper, the screened XN-T temperature-tolerance and salt-resistance swelling particle was coated physically, and its swelling performance and plugging performance were evaluated. The experimental results show that the coated swelling particle has better swelling-delayed performance and the maximum swell multiple (about 5) is reached after 24 h. Under the temperature of 130 ℃, the swell multiple is still 3.6 after 240 h. It is recommended to apply XN-T coated swelling particle in field. The plugging ratio of the swelling particle under 130 ℃ is 98.4 %. Then, the microscopically visual model was prepared from the glass, and the water plugging mechanisms and plugging migration laws of coated swelling particles in fractures and vugs were investigated. Finally, the particle injection parameters and modes were optimized. It is verified microscopically that coated swelling particles can form effective “accumulation”, “bridge” and “attachment residence” in the high-permeability pores and fractures of fractured-vuggy oil reservoirs, so as to drive the fluids into low-permeability small pores and fractures to increase the sweep efficiency and improve the recovery factor. It is indicated that the plugging performance gets worse under the action of multiple compression, shear and deformation as the migration and plugging times of swelling particles in reservoirs increase. The breakthrough pressure is firstly 0.48 MPa, then decreases gradually and ultimately drops to 0.03 MPa. The injection parameters of the coated swelling particle are optimized as follows. The particle size is 0.4−0.6 mm and the particle concentration at the solid/liquid ratio of 10% is adopted. It is recommended to apply the binary in-depth profile control technology of organic coated swelling particle + temperature-tolerance and salt-resistance naphthol gel to the field test in the Tahe Oilfield. In the field test, the mixed injection process of gel and particle is used and the target of stimulation is reached.