Page 210 - Reservoir Geomechanics

191 Compressive and tensile failures in vertical wells

Tensile fractures and strike-slip faulting

Consider the state of stress in a strike-slip faulting environment previously discussed
in Chapter 4.Byrewriting equation (4.46) for the case of frictional equilibrium and
µ = 0.6, we have

S Hmax − P p 2 2
= ( µ + 1 + µ) = 3.1 (6.16)
S hmin − P P
which can be simplified to

S hmax = 3.1S Hmin − 2.1P p

and, for reasons that will soon be evident, rewritten as
S Hmax = 3S hmin − 2P p + 0.1(S hmin − P p ) (6.17)

If we now revisit equation (6.7) that describes the formation of tensile fracture in the
wall of a vertical wellbore and assume that the cooling stress, σ
T ,excess mud weight,

P, and tensile strength are negligible, a tensile fracture will form at the wellbore wall
when
min
σ θθ = 3S hmin − S Hmax − 2P p = 0 (6.18)
or

S Hmax = 3S hmin − 2P p (6.19)
It is obvious that because the last term in equation (6.17) (0.1(S hmin − P p )) is extremely
small, equations (6.17) and (6.19) are nearly equal in magnitude. In other words, for
any combination of S hmin and S Hmax which results in frictional equilibrium in the crust
for a strike-slip domain (and µ = 0.6), the wellbore wall will go into tension at the
azimuth of S Hmax ,even without excessive wellbore pressure or cooling of the wellbore
wall.
This can be illustrated graphically using the type of plot shown in Figure 4.28 that
defines possible stress magnitudes at any given depth based on the frictional strength
of the crust. In Figure 6.13a, we illustrate the fact that for
T =
P = 0, the line on

the periphery of the polygon (indicating the magnitude of S Hmax as a function of S hmin
for strike-slip faults that are active) is almost exactly the same as the line representing
equation (6.14) for zero rock strength as shown.

Thermal effects

It was noted above that additional stresses are applied to the rock at the borehole wall
if the wellbore fluid is at a significantly different temperature than the rock. These
stresses can be compressive or tensile depending on whether the temperature of the
fluid is higher or lower, respectively, than the ambient in situ temperature. When the