Formation mechanism and location distribution of blockage during polymer flooding

Abstract

The blockage produced during polymer flooding can lead to a significant decline in reservoir productivity. The composition of the oilfield scale is very complicated, and formation mechanism and location distribution of blockage are unclear. In this work, the composition of the oilfield scale produced during polymer flooding was analyzed by thermal gravimetric analysis (TGA), infrared spectrum analysis (FI-IR), X-ray diffraction analysis (XRD), and scanning electron microscopy observation (SEM). The formation mechanism of the blockage was studied by means of water compatibility experiment, core flooding experiment, swelling experiment, and polymer retention experiment. The location distribution of blockage was explored by physical simulation displacement experiment using the long sand pack tube. The results show that the oilfield scale is composed of polymer, inorganic substances, water, and crude oil. Various components are interdependent and entangled in the scale. After the polymer solution is injected into the formation, the “fish eye” scale and inorganic salt crystal/polymer scale are produced in the near-injection well zone. Through scale migration and fine particle exfoliation, there occurs the complex composite scale combined by migrated scale and fine particles as well as the newly-generated fine particle/polymer flocculent scale. When the scale encounters with crude oil, the asphaltene from crude oil can be adsorbed and precipitated on the scale surface, leading to the oil-wrapped composite scale. Besides, the injected polymer can be adsorbed and captured in the oil layer, causing the decrease of the layer permeability. With decrease of oil layer permeability, various scale is deposited and bridge-blocked in the oil layer, causing the blockage during the polymer flooding process. After three displacement stages of water flooding, polymer flooding, and subsequent water flooding, blockage is produced in all zones of oil layer, with the most severe blockage in the middle zone near the injection well. The study can provide a theoretical basis for the subsequent research on the blockage-removing fluid systems suitable for polymer flooding reservoirs.