Water Leakoff During Gel Placement in Fractures: Extension to Oil-Saturated Porous Media

Bergit Brattekås, Randy Seright, Geir Ersland – 2019

Abstract

Water leakoff from Cr(III)-acetate-HPAM gels during extrusion through oil-saturated, fractured cores was investigated. Fluid leakoff deviated from expected behavior when oil was present in the fracture-adjacent matrix. A capillary-driven displacement front formed in the oil-saturated matrix, altering the leakoff rate and the depth of gel penetration into the fracture network. Matrix properties (pore size, permeability, heterogeneity) were varied to evaluate their effect on gel dehydration and leakoff rate. MRI was used to monitor water leakoff in a fractured, oil-saturated carbonate core — verifying the displacement front and confirming wormhole presence in the gel during and after placement, consistent with Seright’s filter-cake model.

Key Takeaways

  • Oil in the fracture-adjacent matrix creates a capillary-driven displacement front during gel leakoff — a phenomenon not predicted by existing gel leakoff models developed for water-saturated rock.
  • The capillary front significantly alters the rate of water leakoff and the depth of gel penetration into the fracture network — meaning leakoff calculations based on water-saturated core data will be inaccurate for oil-saturated reservoirs.
  • Matrix pore size, permeability, and heterogeneity all influence gel dehydration and leakoff rate in oil-saturated cores — these properties must be characterized as inputs to treatment design in oil-producing fractured reservoirs.
  • MRI imaging of a fractured, oil-saturated carbonate confirmed the position and existence of a stable capillary displacement front during gel leakoff, providing direct spatial verification of the proposed mechanism.
  • MRI also confirmed the presence of wormholes in the gel during and after placement — directly supporting Seright’s filter-cake model of gel behavior in fractures and extending its validation to oil-saturated systems.
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