Effect of a polymer on chromium(III) diffusion during gelant injection in fractured media

The effect of a polymer on chromium diffusion during gelant injection into fractured media was explored with a polymer/chromium(III) (Cr3+) gelant. The capture and barrier effects are two main affecting mechanisms of polymer molecules on the diffusion of Cr3+, and the formation of the polymer leak‐off layer is a key influencing factor. The experimental results show that when the polymer molecular weight (MW) or concentration increased, both the diffusion rate of Cr3+ and the leak‐off degree of the gelant decreased sharply. This resulted in the delayed initial production and advanced final production of Cr3+. Because of the changing diffusion rate of Cr3+ and the dilution effect of brine during gelant injection, the change trends of the Cr3+ production from the fracture outlet and matrix ports reversed after the injection volume exceeded a value; this value was named the critical injected volume. During gelant injection into the fractures, under the effects of gravity action and the disproportionate diffusion of Cr3+, the chromium diffusion profiles measured at the fracture top and bottom were different. The chromium diffusion profile along the matrix varied with increasing gelant injected volume, but it finally reached the diffusion equilibrium state in the matrix.