Summary
Successful acid stimulation of long-horizontal-well intervals in carbonate
reservoirs requires effective acid distribution along the entire reservoir
length. Such treatments also require large volumes of acid and seawater/brine
injection at sufficiently high injection rates to drive the acid wormholes deep
into the reservoir. Under these flowing conditions, significantly large tubing
friction loss is anticipated unless optimal friction reducer performance in the
tubing is maintained throughout the pumping operation. Because prediction of
wormhole penetration and corresponding skin factor depends on analysis of
downhole-injection pressures at the reservoir face, it is crucial to properly
account for these hydrostatic and friction changes prior to evaluation of
wormhole length and skin factor.
In this study, an integrated flow model has been developed to predict the
wellbore-pressure profile and wormhole distribution by tracking the movement of
the acid in the wellbore and the formation. The wellbore-flow model is based on
steady-state, 1D, pressure-based nodal method. The segmented wellbore in the
reservoir interval is then coupled with analytical transient reservoir-flow
models. The wormhole propagation in the formation is calculated based on the
modified Buijse-Glasbergen correlation and upscaling model developed in our
earlier work. The resultant wormholing skin factor is calculated by simulating
and updating the changing well injectivity along the entire injection interval
at every timestep. The model developed in this work is applicable for both
fully completed wells (i.e., radial flow) and selectively completed
perforation-cluster wells (i.e., spherical flow) typically employed in
carbonate reservoirs.
Analysis of injection rates and pressures during acid treatment provides
engineers with a way to determine the varying injectivity and tubing friction
as stimulation proceeds. The model presented here can be used as a forward
model for analyzing real-time treatment rate and pressure histories and can
also be used to review past treatments to improve future treatment designs.
Using actual field-stimulation data, we also discuss key elements to successful
stimulation planning and the diagnosis of matrix-acid treatments to achieve
effective wormhole coverage for horizontal completions in carbonate
formations.
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
2 December 2010
- Meeting paper published:
21 September 2010
- Revised manuscript received:
22 January 2011
- Manuscript approved:
26 January 2011
- Published online:
20 October 2011
- Version of record:
13 March 2012
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