Swelling kinetics of polymer microspheres used for conformance control and their matching mechanisms with oil reservoir fractures

Polymer microspheres are one type of particle gel with swelling property, and the particle size is distributed between nanometer and micron. Because of the good variability and migration characteristics, polymer microspheres are widely used as conformance control agents, especially in fractured reservoirs. However, in the application process, the matching between polymer microspheres and the size of reservoir fractures remains insufficiently studied, leading to prevalent blind applications and low utilization efficiency. Based on polymer microspheres with different elastic modulus (23.6–715 Pa) prepared in the laboratory, the swelling dynamics of polymer microspheres were established according to their swelling ratio data under different reservoir temperatures (20–80 ℃). The fracture plate model was used to simulate the fractured reservoir, the matching mechanism of polymer microspheres in the model was studied. Applying this model and equation p_t = kGʹ + m, we further investigated the quantitative relationship between threshold pressure and elastic modulus of polymer microspheres in the fracture plate model in the migration process. The swelling ratio of polymer microspheres showed that the polymer microspheres absorbed water quickly in the early stage and gradually slowed down in the later stage. The characteristics of the early stage were simulated by Fickian equation and those of the later stage were simulated by Schott equation. A new swelling kinetic equation of polymer microspheres was established considering the whole swelling characteristics. The diameter/width ratio was used as an evaluation index, the matching property results showed that under the experimental conditions, the migration modes of microspheres in the fracture plate model were straight-through passage, deformable passage, and blockage. Under the same elastic modulus, the smaller the fracture aperture, the higher the threshold pressure. At the same fracture aperture, the threshold pressure increased with the increase in elastic modulus. Based on the obtained results, the value range of threshold pressure under different fracture apertures could be quantitatively predicted, and then the particle size and elastic modulus of the polymer microspheres for conformance control could be determined according to the reservoir fracture parameters. The blind use of polymer microspheres for conformance control agents should be avoided, the success rate of fractured reservoir of conformance control could be improved. The results of the study have experimental guidance for the conformance control scheme design of polymer microspheres.