Summary
Fracture fluid damage caused by residual polymer gel in propped fractures
results in low fracture conductivity and short effective fracture length,
sometimes severely reducing the productivity of a hydraulically fractured well.
The residual gels are concentrated in the filter cakes built on the fracture
walls and have much higher polymer concentration than the original gel. The
residual gel exhibits a higher yield stress and is difficult to remove after
fracture closure.
In this work we studied polymer gel behavior theoretically and
experimentally in hydraulic fracturing. We developed a model to describe the
flow behavior of residual polymer gel being displaced by gas in parallel
plates. We developed analytical models for gas/liquid two-phase stratified flow
of Newtonian gas and non-Newtonian residual gel to investigate gel cleanup
under different conditions. The concentrated gel in the filter cake was modeled
as a Herschel-Buckley fluid, a shear-thinning fluid following a power law
relationship, but also having a yield stress.
The model developed shows that three flow regimes may exist in a slot,
depending on the gas flow rate and the filter-cake yield stress. At low gas
velocities, the filter cake will be completely immobile. At higher gas
velocity, the shear at the fracture wall exceeds the yield stress of the filter
cake, and the gel is mobile, but with a plug flow region of constant velocity
near the gas/gel interface. Finally, at high enough gas velocity, a fully
developed velocity field in the gel is created.
The parameters for the gel displacement model were evaluated by experiments.
We examined the filter-cake formation by pumping the fracture fluid through a
conductivity cell, allowing leakoff to build the filter cake, measuring the
cake thickness, and flowing gas through the cell to simulate the cleanup
process. The results show that the yield stress of the residual gel plays a
critical role in gel cleanup.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
14 July 2011
- Meeting paper published:
30 October 2011
- Revised manuscript received:
16 February 2012
- Manuscript approved:
20 June 2012
- Published online:
7 November 2012
- Version of record:
13 November 2012
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