SPE Journal
Volume 16,
Number 4,
December 2011,
pp. 1010-1028
Summary
The effects of filter-cake buildup and/or filter-cake-property variation
with time on wellbore stability have been plaguing the industry. The increasing
use of lost-circulation materials (LCMs) in recent years for wellbore
strengthening in weak and/or depleted formations nessesitates models that can
predict these effects. However, the complexities of effective-stress and
pore-pressure evolution around the borehole while drilling, coupled with the
transient variation of mud-filtration properties, have delayed such modeling
efforts. In this paper, the analytical solutions for the time-dependent effects
of mudcake buildup and mudcake properties on the wellbore stresses and
formation pore pressure, and thus the safe-drilling-mud-weight window, are
derived. The transient effects of mudcake buildup and mudcake buildup coupled
with its permeability reduction during filtration on the
safe-drilling-mudweight window are illustrated through numerical examples. The
results showed that the safe-mudweight windows were greatly affected by the
buildup of filter cake and its permeability variation. For example, the
analysis for filter-cake buildup with cake permeability of 10–2 md
showed that the safe-mudweight window was widened by 0.5 g/cc after 2.5 hours
post-excavation when compared to the case of a wellbore without mudcake. On the
other hand, a lower mudcake permeability of 10–3 md widened the
mudweight window by as much as 1 g/cc. Last but not least, the analyses
revealed that even for mudcake permeability as low as 10–3 md,
neglecting the permeable nature of the mudcake can result in overestimation of
the safe-drilling-mudweight window.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
7 October 2010
- Meeting paper published:
2 November 2010
- Revised manuscript received:
11 December 2010
- Manuscript approved:
11 January 2011
- Published online:
3 August 2011
- Version of record:
23 December 2011
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