Summary
In the acid-fracturing process, the fracture conductivity created by acid
etching of the fracture walls is because of the surface roughness created by
the acid’s nonuniform dissolution of the fracture surfaces. The acid-fracture
conductivity is dependent on surface etching patterns, which are determined by
permeability and mineralogy distributions. That is, the spatial distribution of
fracture roughness affects the fracture conductivity, which cannot be
considered in laboratory measurements of acid-fracture conductivity, which use
core samples that are too small to observe such macroscale heterogeneities, or
in typical acid-fracture simulators, in which the gridblock size is much larger
than the scale of local heterogeneities.
An accurate prediction of acid-fracture conductivity necessitates the
detailed description of the acid etching profiles on the fracture surfaces,
which depend on acid transport in the fracture, leakoff because of local
permeability, and acid/rock reactions. In this paper, we developed a 3D
intermediate-scale acid-fracture model with gridblock sizes small enough
(gridblock sizes comparable to the core-sample size in experiments) and total
dimensions large enough (the total dimensions comparable to a gridblock size in
an acid-fracture simulator) to capture local and macroscale heterogeneity
characteristics. The model predicts the pressure field, the flow field, acid
concentration profiles, and fracture-surface profiles as functions of acid
injection volume. In the model, we use a front-fixing method (Crank 1984) to
handle the irregular, moving boundaries in numerical simulation. Spatially
correlated permeability and mineralogy distributions were generated by using a
semivariogram model.
The model was validated by comparing simulation results with experimental
results from an acid-fracture conductivity cell. With the model, by extensive
numerical simulation, we analyzed the relationship among
fracture-surface-etching patterns, conductivities, and the distributions of
permeability and mineralogy. We also illustrated the formation characteristics
necessary for acid to create channel-caused high acid-fracture conductivity. We
found that a fracture segment with channels extending from the inlet to the
outlet of the segment has high conductivity because fluid flow in deep channels
causes a very small pressure drop. Such long and highly conductive channels can
be created by acids if the formation has heterogeneities in either permeability
or mineralogy or both, with high correlation length in the main flow direction,
which is the case in laminated formations.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
17 November 2008
- Meeting paper published:
19 January 2009
- Revised manuscript received:
7 May 2009
- Manuscript approved:
19 June 2009
- Published online:
17 December 2009
- Version of record:
17 June 2010
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