Page 267 - Reservoir Geomechanics

248 Reservoir geomechanics

b.
Bottom Top Bottom
a.
0 45 90 135 180 225 270 315
1075.0
q
w

w
1075.25 Depth

q
1075.5

0 90 180 270 360
Figure 8.8. (a) Drilling-induced tensile fractures in a geothermal well in Japan make an angle ω
with the axis of the wellbore and are located at position indicated by the angle θ from the bottom of
the wellbore. (b) Theoretical model of the observed fractures in (a) replicate both ω and θ for the
appropriate value of the magnitude and orientation of S Hmax .

stress, S hmin , are determined in the manner described previously. For example, iterative
forward modeling of the en echelon fractures seen in Figure 8.8aina moderately
deviated geothermal well in Japan yielded knowledge of the magnitude and orientation
of S Hmax . Three observations constrain the modeling – the position of the fractures
around the wellbore, θ, their deviation with respect to the wellbore axis, ω, and the
very existence of the tensile fractures. The orientation of S Hmax was found to be about
◦
N60 W and S Hmax was found to be slightly in excess of the vertical stress (because of
severe wellbore cooling that commonly occurs in geothermal wells, drilling-induced
tensile fractures are induced even if there are relatively modest stress differences).
In the two cases considered below, the wells are significantly deviated but the occur-
rence of axial drilling-induced tensile fractures in vertical sections of the wells indi-
cates that the principal stresses are vertical and horizontal. In the generalized case of
adeviated well in a deviated stress field, LOT’s will provide information on the mag-
nitude of the least principal stress, but it would be necessary to do iterative forward
modeling of observed wellbore failures to determine principal stress orientations and
magnitudes.
Wiprut, Zoback et al.(2000) observed drilling-induced tensile fractures in the vertical
section of a well in the Visund field of the northern North Sea. Below 2600 m depth,
the well gradually increased in deviation with depth with a build-and-hold trajectory at