Mechanism for Gel Propagation Through Fractures

R.S. Seright – 1999

Abstract

This paper describes an experimental investigation of the mechanism for propagation of Cr(III)-acetate-HPAM gel through fractures. When large volumes were extruded through a fracture, progressive plugging (continuously increasing pressure gradients) was not observed. Effluent from the fracture had the same appearance and similar composition as the injected gel, even though concentrated, immobile gel formed in the fracture. The concentrated gel formed when water leaked off from the gel along the length of the fracture, driven by the pressure difference between the fracture and adjacent porous rock. A simple model was developed to account for the experimental results. Critically, pressure gradients and dehydration factors were the same for fractures in 650-md sandstone, 50-md sandstone, and 1.5-md limestone.

Key Takeaways

  • Gel propagates through fractures via wormholes through concentrated, immobile gel — not by progressive plugging — so pressure gradients do not continuously increase during gel injection as might be expected.
  • Water leakoff from gel into adjacent porous rock is driven by the pressure difference between fracture and matrix, causing gel dehydration and concentration — the dehydrated gel becomes the immobile plug while fresh mobile gel wormholes through it.
  • Effluent from the downstream end of the fracture has the same appearance and composition as injected gel — confirming that fresh gel propagates through the concentrated immobile gel, not around it.
  • Pressure gradients and dehydration factors during gel extrusion were the same for 650-md sandstone, 50-md sandstone, and 1.5-md limestone — demonstrating that the matrix permeability over this range does not significantly affect the gel propagation mechanism.
  • A simple model accounts for the experimental results, providing a mechanistic basis for predicting gel placement volume requirements and penetration distance in fractured treatment designs.
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