Summary
Polymeric viscosifiers are added to cement slurries for a variety of
reasons, including prevention of particle settling and control of fluid loss,
gas migration, and free water. Many of these functions are critically important
after the cement slurry has been placed behind the casing but before the
setting of the cement. Some functions, such as particle-settling prevention,
are also important during the pumping phase. Unfortunately, most of the
viscosifying polymers suffer from thermal thinning at bottomhole temperatures,
especially under shear. The amount of polymer required to maintain the required
level of viscosity at elevated bottomhole temperatures causes excessive
surface-slurry viscosification at ambient temperature. Pumping such slurries
can require higher pump pressures, or in cases where formation breakdown
pressure might be exceeded. This becomes a serious challenge when the window
between the fracture pressure and the pore pressure of the formation is narrow.
It would be a significant improvement to oilfield cementing technology to
develop polymers that do not cause excessive slurry viscosification on the
surface but gradually increase the slurry viscosity as it reaches downhole
temperatures, with the maximum viscosity reached at the time the slurry becomes
static behind the casing.
This paper describes a chemical method, not based on encapsulation, for
modifying biopolymers and their derivatives--for example, hydroxyethylcellulose
(HEC) and xanthan--that renders them insoluble in cement slurries at room
temperature (RT). When the cement slurries containing modified HEC are heated,
the slurries develop viscosity upon heating, as reflected by changes to slurry
rheology with temperature. The method also provides for increased
viscosification efficiency of the modified polymers because of the increased
molecular weights of the modified biopolymer products. Synthesis details,
slurry rheologies at different temperatures, and job-placement simulation
details are presented. A possible reaction mechanism that is operative in the
chemical-modification step is also discussed.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
17 June 2011
- Meeting paper published:
12 April 2011
- Revised manuscript received:
23 November 2011
- Manuscript approved:
29 November 2011
- Published online:
12 March 2012
- Version of record:
15 March 2012
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