Summary
Borehole ballooning/breathing is a combined mud-loss/-gain event observed
during drilling operations in naturally fractured formations. Factors
controlling this phenomenon must be well understood to correctly interpret its
symptoms observed during drilling to avoid mixing ballooning with other
formation flow incidents that might lead to unwarranted well-control
procedures.
A mathematical model defining the ballooning process was developed and
solved numerically using finite-difference approximation. It was shown that
fracture roughness and fracture deformation play a significant role in the flow
of drilling fluid in and out of a single fracture. In this study, the focus was
mainly on the effect of fracture roughness (characterized by the fractal
dimension of the fracture surface) and fracture aperture.
The main goal of this work was to compare the numerical-model results with
laboratory-scale experimental observations. Therefore, experiments were
performed to study the mud-loss and -gain events in artificially fractured rock
samples. Cylinders of Berea sandstone, Indiana limestone, and granite were used
for the experiments (1-in. diameter, 3-in. length). Two different fracture
types were used to analyze the effect of fracture-surface roughness on the flow
of drilling fluid in and out of the fracture. To create smooth fracture
surfaces, cores were cut precisely into two equal pieces using a blade.
Alternatively, axial load was applied on the cylindrical rock samples until a
longitudinal fracture with a rough surface was generated.
The results of experimental observations and numerical-model study on the
importance of fracture roughness were provided. Situations where the degree of
roughness becomes critical were identified.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
24 July 2007
- Meeting paper published:
11 November 2007
- Revised manuscript received:
14 June 2008
- Manuscript approved:
21 June 2008
- Published online:
1 June 2009
- Version of record:
1 June 2009
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