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Analysis of vertical, horizontal and deviated wellbores stability by analytical and numerical methods
Abbas Khaksar Manshad
0
1
2
H. Jalalifar
0
1
2
M. Aslannejad
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1
2
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M. Aslannejad Department of Chemical and Petroleum Engineering, Persian Gulf University
, Boushehr,
Iran
1
H. Jalalifar Department of Chemical and Petroleum Engineering, Shahid Bahonar University
, Kerman,
Iran
2
A. K. Manshad (&) Department of Petroleum Engineering, Abadan Faculty of Petroleum Engineering, Petroleum University of Technology
, Abadan,
Iran
Wellbore stability problems are known to cost the oil and gas industry billions of dollars each year. However, these costs can be significantly reduced through the application of comprehensive geomechanical models. This paper is relevant and is appropriate in the oil and gas industry. The objective of this paper is the comparison of four rock failure criteria, named the Mohr-Coulomb, MogiCoulomb, Modified Lade and Tresca yield criterion and to apply them to determine the optimum drilling direction and mud pressure. The stability models has been applied to a well located in Iran oil field and leads to easily computed expression for the critical mud pressure required to maintain wellbore stability. Then the finite difference method was used to show the validation and accuracy of predicted mud pressure and investigate the wellbore stability in different states of vertical, horizontal and deviated. The results showed that the Mohr-Coulomb and Tresca criteria estimate the highest minimum mud pressure required for wellbore stability while the Mogi-Coulomb and the Modified Lade criteria estimate the lowest minimum mud pressure. Nevertheless, the mud pressures predicted by all these four criteria are acceptable and can be used.
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Investigation of wellbore stability and advising a sensible
plan before drilling require identification of problematic
regions and improving of drilling operation. The two
important elements needed in a wellbore stability model are
the failure criterion and the constitutive behavior model.
Wellbore drilling in a formation causes stress alteration
around the borehole due to removal of rock. This stress
alteration is important, since it leads to an increase in stress
around the wall of the hole, therefore the induced stresses
should be adjusted by choosing proper mud pressure to
stabilize wellbore. Although the selection of an appropriate
rock failure criterion for analyzing wellbore stability is
difficult and controversial (Al-Ajmi and Zimmerman 2009;
Mclean and Addis 1990), a number of rock failure criteria
and behavior models have been accomplished for the
diagnosis and prediction of wellbore instability. Since there is no
single criterion suitable for all materials and situations,
drilling engineers should be able to choose a suitable rock
failure criterion based on formation rock properties to predict
an optimum mud pressure to stabilize wellbore. Bradley
(1979) was the first to model for compressive wellbore
failure of a deviated well for the purpose of proposing proper
mud weights to preclude borehole failure. However, he did
all of his analyses for the rare case where the two horizontal
stresses are equal and less than the vertical stress. Ewy (1999)
found that the modified Lade criterion predicts critical mud
weight values that are less conservative than those predicted
by the MohrCoulomb criterion yet are not as
unconservative as those predicted by the DruckerPrager criterion.
Al. Ajmi and Zimmerman (Al-Ajmi and Zimmerman 2004)
introduced the fully polyaxial MogiCoulomb failure
criterion, and then proposed a new 3D analytical model (2006) to
approximate the mud weight needed to avoid failure for the
vertical wells based on MogiCoulomb failure mechanism
coupled with elastic theory. Their study shows the significant
role of intermediate principle stress in rock strength, where
using three dimensional MogiCoulomb failure criterion
greater mud weight windows than MohrCoulomb failure
mechanism have been obtained. Zhang et al. (2010)
examined five failure criteria on various rock specimens to
determine the best criterion for the wellbore stability
analysis. Therefore, they concluded that the 3D HoekBrown
and the MogiCoulomb criteria are appropriate for wellbore
stability analysis.
On the other hand, numerical modeling methods provide
an excellent opportunity to analyze the wellbore state of
stress for different applications such as wellbore drilling,
wellbore design or hydraulic fracturing (Lee et al. 2011).
McLean and Addis (1994) used finite element methods to
predict wellbore stability parameters. Chatterjee and
Mukhopadhyay (2003) used ANSYS finite element software
and investigated stress around a wellbore to study the
effects of fluid pressure during drilling. Hoang et al. (2004)
investigated wellbore stability in multilateral junctions
using finite element method and showed that orientation of
junction and in situ stresses both have significant impact on
well completion and stability. Wang and Sterling (2007)
performed numerical analyses named finite elemen (...truncated)