Summary
Cementing a string in one stage is a challenging task, especially in the
presence of weak formations. Cement-slurry losses during placement are highly
possible if the equivalent-circulating density (ECD) exceeds 11.0 lbm/gal
during placement. A conventional method to overcome this challenge is to use
multistage cementing by setting the stage tool above the lost-circulation zone.
However, field data indicate that the tool can fail, thus causing serious delay
and economic losses. In addition, stage tools are considered weak points and
are not good for a long-term seal. A second method for zonal isolation is to
use low-density cement (LDC). In this study, we considered cementing the
intermediate and production casings in the S-1 (sandstone) and S-2 (carbonate)
shallow formations and the BJD (dolomite) deep formation in a single stage
using lower-density cement based on hollow microspheres, at 9.4 lbm/gal. The
shallow conditions simulated in the laboratory tests were 150°F curing
temperature, 2,400-psi conditioning pressure, and 1,800-psi confining pressure
for 3 months. The deep conditions were 260°F curing temperature, 5,000-psi
conditioning pressure, and 3,000-psi confining pressure, also for 3 months.
Hollow-microspheres cement was used in oil and gas wells without
encountering any operational problems. However, the high cost of microspheres
cement was a limitation for potential field application. In this study, we
present extensive laboratory work to optimize hollow-microspheres LDC by the
elimination of microfine cement from the blend. Experimental studies
(shrinkage, compressive strength, porosity, gas and brine permeability, and
chemical analysis of cement) were conducted to determine the effect of this
optimization on the properties of cement.
Data generated during the 3 months supported the use of the optimized system
to cement casings at the shallow conditions tested. The removal of microfine
cement did not reduce the compressive strength, with an average of 2,013 psi;
no shrinkage was observed, and no increase in permeability was noted during the
three months of testing. The use of the optimized system did help in bringing
cement to the surface in one stage. The cost of the optimized blend was 40%
less than conventional hollow-microspheres LDC.
This paper presents case histories that include job design, job execution,
and evaluation of the LDC. Field treatments were successful and maintained
isolation for more than 3 years.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
11 February 2008
- Meeting paper published:
4 March 2008
- Revised manuscript received:
11 February 2009
- Manuscript approved:
8 April 2009
- Published online:
17 November 2009
- Version of record:
11 March 2010
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