reservoir geomecanics

246 Reservoir geomechanics
a.
s min
b. c.
160
180
∆P = 0
∆P = 15MPa
140
w
120
100
80
60
s tmin
40
20
0
−20
0
0 90 q t 180 270 360
q t q t
Figure 8.7. Theoretical illustration of the manner of formation of en echelon drilling-induced
tensile fractures in a deviated well. (a) The fracture forms when σ tmin is tensile. The angle the
fracture makes with the axis of the wellbore is defined by ω, which, like σ tmin varies around the
wellbore. (b) The en echelon fractures form over the angular span θ t , where the wellbore wall is in
tension. (c) Raising the mud weight causes the fractures to propagate over a wider range of angles
because σ tmin is reduced around the wellbore’s circumference.
grows, σ tmin becomes compressive. Raising the wellbore pressure (Figure 8.7c) allows
the fracture to propagate further around the wellbore because the σ tmin is decreased
by the amount of P, thus increasing the angular span (θ t ), where tensile stress are
observed around the wellbore circumference.
Confirming that S Hmax and S hmin are principal stresses
Drilling-induced tensile fractures were ubiquitous in the KTB pilot hole and main
borehole. As mentioned above, along most of the well path, the tensile fractures are
axial. As this is a near-vertical borehole, it indicates that there is a near-vertical principal
stress. However, in a few sections of the wellbore, the state of stress is locally perturbed
by slip on faults and is rotated away from a horizontal and vertical orientation (Brudy,
Zoback et al. 1997). Over the entire depth interval studied in detail in the KTB boreholes
(from ∼ 1kmto∼8kmdepth), axial drilling-induced tensile fractures indicate that one
principal stress is nearly always vertical and the cases where this is not true is limited
to zones of locally anomalous stress (Brudy, Zoback et al. 1997). Near vertical drillinginduced
tensile fractures were observed to ∼7 kmdepth in the Siljan wells drilled
in Sweden (Lund and Zoback 1999). A similar situation was encountered by Wiprut,
Zoback et al.(2000) who documented both axial and en echelon fractures in a suite of
five oil wells in the Visund field of the northern North Sea. While the stress field is wellcharacterized
by a near-vertical and two horizontal principal stresses, there are zones