Gelant Placement in Anisotropic Flow Systems

Mei Ye and Randall S. Seright – 1996

Abstract

Two theoretical models determine water injection profiles before and after gel placement in anisotropic reservoirs where effective permeability or pressure gradient is greater in one horizontal direction than another. The primary question: how anisotropic must an unfractured reservoir be for acceptable gel placement and profile modification during unrestricted gelant injection? Both analytical and numerical methods were applied. Permeability anisotropy (most-permeable direction divided by least-permeable direction) must exceed 1,000 — and usually exceed 10,000 — before anisotropy can be exploited for acceptable gel placement in unfractured wells. No unfractured reservoir is likely to have this level of anisotropy. In contrast, fractures provide the linear flow geometry and extreme permeability contrast needed to aid gel placement substantially.

Key Takeaways

  • Permeability anisotropy in unfractured reservoirs must exceed 1,000 (and usually 10,000) before the anisotropy can be exploited to achieve acceptable gel placement without zone isolation — no realistic unfractured reservoir approaches this level of anisotropy.
  • The conclusion is practical and direct: in unfractured wells, no realistic level of natural permeability anisotropy can substitute for zone isolation during gel placement. Anisotropy-based selective placement is not a viable strategy in porous rock.
  • Fractures provide the extreme permeability contrast (typically 10^3 to 10^6 times the matrix permeability) and linear flow geometry needed to achieve acceptable gel placement without zone isolation — this is the fundamental reason why fractured wells respond much better to gel treatments than unfractured wells.
  • Both analytical and numerical methods were applied to the anisotropic placement problem, providing quantitative thresholds for the anisotropy ratio required for acceptable placement as a function of gelant properties and reservoir geometry.
  • These results close the door on anisotropy-based gel placement design in unfractured wells and redirect the focus to fracture geometry and zone isolation as the primary design levers for gel treatment efficacy.
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