Migration and plugging mechanisms of polymer gel in 3D fractured media

Key words:  polymer gel  three-dimensional fracture  migration behavior  disproportionate loss  plugging mechanism

Abstract:

Three-dimensional fractured core models were used to investigate the migration, distribution form, and disproportionate loss form of plugging agent, as well as the plugging strength after gelling, through which the migration, distribution and gelation law of gel plugging agent in fractures were studied. It is found show that the migration patterns of polymer gel solution in three-dimensional fractures can be divided into fracture flow, loss flow and matrix flow. Three gel distribution states exist after polymer gel formation, which are gel ball in fracture, gel layer on fracture surface, and dispersed gel block in adjacent matrix. The loss flow leads to the significant component filtration phenomenon when the gel solution migrates in the fracture. Due to the molecular weight difference between polymer and crosslinker, there is an unbalanced filtration diffusion among gel components, in which the polymer molecular filtration is small and the crosslinker ion filtration is significant. The latter reduces the polymer gel strength greatly, and further affects the pugging performance of polymer gel for fracture. When the normalized concentration of crosslinker ion is lower than the critical concentration, the gel strength is low after gelling, and the formed polymer gel shows unsatisfied plugging performance for fractures. Compared with the pre-crosslinking way, when the in-situ crosslinking way is applied to the polymer gelant, it shows excellent plugging performance for fracture.