Washout of Cr(III)-Acetate-HPAM Gels from Fractures

R.S. Seright – 2003

Abstract

This paper investigates washout of mature Cr(III)-acetate-HPAM gels from fractures. After gel placement, the pressure gradient for gel washout during brine or oil flow was similar to the gradient during placement. The mechanism of gel failure involved displacement of relatively mobile gel from wormholes — only a small fraction of the gel (<5%) was displaced during the washout process. Washout resistance increases with more concentrated gel. A constriction in a fracture inhibited washout during the first pulse of brine flow but not subsequently. Oil-wet polyethylene cores showed similar washout behavior to water-wet Berea sandstone. Lower placement rate and secondary crosslinking reactions (post-placement Cr(III) acetate) both substantially increased resistance to washout.

Key Takeaways

  • Only a small fraction of the gel (<5%) is displaced during washout — gel failure in fractures is not catastrophic wholesale removal but displacement of mobile gel from wormholes, leaving the majority of the gel plug intact.
  • The mechanism of gel washout is displacement of relatively mobile gel from wormholes during flow, not erosion of the immobile concentrated gel — this distinguishes gel failure from simple erosion of a solid plug.
  • A gel placed at lower injection rate is approximately five times more resistant to washout than a gel placed at high rate — because lower rate promotes more dehydration during placement, creating a denser, more immobile gel.
  • Post-placement secondary crosslinking with Cr(III) acetate increased washout resistance by a factor of two to three for a resorcinol-formaldehyde-HPAM system — providing a practical method to improve durability after gel placement.
  • Wettability of the fracture walls (oil-wet polyethylene vs water-wet Berea sandstone) did not significantly affect washout behavior — making washout resistance reasonably predictable across different lithologies.
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