Correlating Gel Rheology with Behavior during Extrusion through Fractures

Ying Wang and R.S. Seright – 2006

Abstract

This paper investigated whether rheology measurements could substitute for fracture extrusion experiments to assess gel properties in fractures. The rheology behavior showed a strong qualitative parallel to prior gel extrusion results. However, for a given fracture aperture, pressure gradients measured during extrusion were much higher than anticipated from rheology measurements. Extensive experiments ruled out wall slip and first normal stress difference as explanations. The pressure gradient discrepancy was explained by noting that gel flows through narrow wormholes in concentrated immobile gel — the effective aperture for mobile gel is much narrower than the fracture width. Two models using shell momentum balances explain why (1) pressure gradient varies inversely with the square of fracture width, and (2) fracture gradients exceed viscometer predictions.

Key Takeaways

  • Gel rheology in viscometers shows a strong qualitative parallel to fracture extrusion behavior — qualitative trends can be predicted from viscometry — but quantitative pressure gradients in fractures are much higher than rheometry predicts.
  • Wall slip and first normal stress difference were rigorously ruled out as explanations for the discrepancy between viscometer and fracture measurements — the difference is real and mechanistic.
  • Mobile gel flows through narrow wormholes in concentrated immobile gel — the effective aperture for flowing gel is much narrower than the fracture width, which is why fracture pressure gradients far exceed bulk-rheology predictions.
  • A shell momentum balance model explains why pressure gradient in fractures varies inversely with the square of fracture width (not linearly with 1/width as might be expected from simple Poiseuille flow).
  • A second model developed in this work explains why pressure gradients measured during fracture extrusion exceed values calculated from viscometer data — completing the mechanistic picture of gel propagation in fractures.
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