Summary
A 3D numerical model of fracture initiation from a perforated wellbore in
linear elastic rock is developed, which allows one to determine the
fracture-initiation pressure (FIP) and the location and direction of an initial
rupture. The model assumes that the fracture initiates at the point at which
the local maximal tensile stress exceeds the rock tensile strength. The 3D
boundary-element method (BEM) is used for stress analysis.
The model aims to predict the location of initial fractures and the
difference in FIP between different perforation intervals in arbitrarily
oriented noncemented wellbores. There are many practical applications for this
knowledge, but of particular interest for this research is the employment of
differently oriented perforations for creating heterogeneity of FIP between
wellbore intervals in multistage fracturing treatment. This can enable
stimulation of these intervals in a sequential mode and significantly simplify
current treatment diversion and completion practices.
Comprehensive analysis revealed that the main parameter that can be used for
controlling FIP during multistage fracturing treatment is the angle between the
direction of the perforation channel and the preferred fracture plane (PFP).
The model allows obtaining the range of the angles that is the most suitable
for designing and implementation of diversion between the perforated wellbore
intervals. The influence of geometrical parameters of perforation (such as
length, diameter, and shape) on FIP is substantially less. In addition, we
found that against all expectations, increase of perforation diameter can
result in higher FIP. It was also discovered that the influence of the
intermediate in-situ stress on FIP is comparable with the effect of perforation
misalignment, especially in the situation of a horizontal wellbore and properly
aligned perforations. On the basis of the model developed, an approximate
approach to the evaluation of the effect of wellbore cementation on fracture
initiation was suggested. It was discovered that taking into account the state
of stress within the cement before well pressurization can result in both an
increase and a reduction of FIP, depending on the parameters of perforating and
the wellbore orientation.
The presented model is a necessary step toward predictable and controllable
fracture initiation, which is vital for multistage-fracturing-treatment
diversion.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
11 May 2012
- Meeting paper published:
6 February 2012
- Manuscript approved:
11 September 2012
- Published online:
28 December 2012
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