Summary
A critical issue in matrix acidizing of vertically extensive carbonate
reservoirs is the acid distribution along the wellbore. This estimation is very
important, especially for the case where the reservoir properties (permeability
and damage distribution) vary along the wellbore. Several acid-placement models
for oil reservoirs have been developed and applied in the field successfully.
However, when evaluating acidizing in a gas well, the models are not adequate
because of the viscosity contrast between the reservoir fluid (gas) and the
injected fluid (acid) and because of relative permeability effects. We have
developed an acid-placement model for acid injection into vertically extensive
gas wells in heterogeneous carbonate reservoirs that includes gravity
segregation in the wellbore, viscosity contrast, relative permeability effects,
and a wormhole model. The effects of all these factors on the local injectivity
during acid injection are incorporated into a dynamic-skin-factor model.
To test this new model, we analyzed coreflooding data in the literature. The
analysis showed that differential pressure across a gas-saturated core did not
linearly drop during acidizing, but instead increased during a large part of
the acid-injection period. From this analysis, we determined that the pressure
drop in a spent-acid zone ahead of the wormholes dominates the overall
pressure-drop behavior.
Applying this model to typical field conditions, we find that a natural
viscous diversion takes place in heterogeneous gas reservoirs because of the
viscosity contrast between the gas and the injected acid. In particular, we
have modeled thick carbonate gas reservoirs having a few thin,
very-high-permeability thief zones. Before wormholes break through the damage
zone in the high-permeability parts of the reservoir, this viscous-diversion
effect distributes acid more uniformly than expected. However, after wormholes
break through the damaged zone, thief zones will take a large portion of the
injected acid. Another important prediction with this model is that the
injection pressure often increases during acid injection into gas wells.
Without an understanding of the role of mobility contrast in the
injection-pressure response, operators may conclude incorrectly that the
injected acid is not stimulating the formation.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
8 July 2009
- Meeting paper published:
5 October 2009
- Revised manuscript received:
6 January 2010
- Manuscript approved:
25 February 2010
- Published online:
21 May 2010
- Version of record:
11 August 2010
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