Impact of Gelation pH, Rock Permeability, and Lithology on the Performance of a Monomer-Based Gel

R.S. Seright and F.D. Martin – 1993

Abstract

Resorcinol/formaldehyde gels demonstrate that gel performance in porous rock depends critically on gelation pH. At pH 9, a strong, clear red gel forms with no free water; as pH decreases toward 7, gel becomes weaker, opaque, and orange-white with residual free water. Gels generally reduced permeability more in low-permeability sandstone than in high-permeability sandstone. Residual resistance factors can be greater in sandstones than in less-permeable carbonate cores, demonstrating that lithology affects gel performance independently of permeability. A mathematical model assesses whether pH effects can be exploited to optimize gel placement in injection wells.

Key Takeaways

  • Gelation pH critically controls resorcinol/formaldehyde gel strength: at pH 9, a strong clear gel forms with no free water; decreasing pH toward 7 produces weaker, opaque gel with residual free water remaining.
  • Gel permeability reduction tends to be greater in lower-permeability sandstone than in higher-permeability sandstone — a counter-intuitive result with important implications for placement design in layered reservoirs.
  • Residual resistance factors can be greater in sandstone than in less-permeable carbonate cores of similar formation — lithology affects gel performance independently of permeability.
  • A mathematical model using pH dependence of gelation can assess whether pH gradients created during injection can promote gelation preferentially in high-permeability thief zones while sparing low-permeability oil zones.
  • The simplicity of the resorcinol/formaldehyde system — Newtonian before gelation, negligible retention, pH-controlled gel strength — makes it an ideal model system for investigating fundamental gel-rock interactions applicable to more complex polymeric gels.
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