Homework No. 6 – Constraining the Maximum Horizontal Stress from Wellbore Failure
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The condition for a tensile fracture forming in the wellbore wall in a vertical well when a thermal stress is ignored is (Zoback, 2007),
Shmin +SH max −2(SH max −Shmin )−2Pp −(Pm −Pp)=To Eqn. 1
Shmin +SH max −2(SH max −Shmin )cos(π−wbo)−2Pp −(Pm −Pp)=Co Eqn. 2
If faults are in frictional equilibrium (Zoback, 2007),
S1 −Pp 2 +1) +µ)2 =( (µ
S3 −Pp Eqn. 3
in which Shmin is a minimum horizontal stress, SHmax is a maximum horizontal stress, Pp is a pore pressure, Pm is a mud pressure, To is a tensile strength, wbo is a breakout width, Co is an unconfined compressive strength, S1 is a maximum principal stress, S3 is a minimum principal stress, and µ is a coefficient of sliding friction.
Use the following unit in your calculation: ‘psi’ for maximum horizontal stress and minimum horizontal stress
I. Answer the questions on the page below
In a vertical well, assuming a pore pressure of 4400 psi, an overburden stress of 11000 psi, a coefficient of sliding friction of 0.6, a mud pressure of 5900 psi, a tensile strength of 0 psi, a
breakout width of 0°, and an unconfined compressive strength of 14000 psi, calculate values of the minimum horizontal stress and the maximum horizontal stress at points 1, 2, 3, 4, and 5 in Figure 1.
Figure 1. A stress polygon assuming a pore pressure of 4400 psi, an overburden stress of 11000 psi, a coefficient of sliding friction of 0.6, a mud pressure of 5900 psi, a tensile strength of 0 psi, a breakout width of 0°, and an unconfined compressive strength of 14000 psi. A Co line represents a breakout, and a To line represents a drilling induced tensile fracture.
Please do not write a unit in an answer, just write a number.
doi:10.1017/CBO9780511586477
Barton, C. A., Zoback, M. D., and Burns, K. L. (1988). In-situ stress orientation and magnitude at the Fenton
Geothermal Site, New Mexico, determined from wellbore breakouts. Geophysical Research Letters, Volume 15,
Issue 5, doi: 10.1029/GL015i005p00467
