Figure 2. (a) Statically
and dynamically normalized resistivity image log (FMI) showing borehole
breakouts and natural fractures in borehole Tuhara-1A, (b) Dynamically
normalized travel time and amplitude images from an acoustic image log
(AFIT) acquired in Orui-1A borehole showing petal centerline fractures
(PCFs), natural fractures, and a fault (observable offset of other
geological features across a natural fracture). (c) Plot of an oriented
four-arm caliper log (C1-C3 and C2-C4) from borehole Kereru-1. The 10%
tolerance of bit size (black line) is shown as a grey shaded zone,
plotted next to pad 1 azimuth and borehole orientation information
showing caliper enlargement indicative of the presence of a borehole
breakout.
Image log acquisition, data processing, and quality assessment is
performed on all image logs, the details of which are documented in the
supplemental information (Text S1 & S2).
From the borehole image logs, we identify stress-induced borehole
failures, including borehole breakouts (BOs; Figure 2a & 2c),
drilling-induced tensile fractures (DITFs), and petal-centerline
fractures (PCFs; Figure 2b). BOs and DITFs are well-known indicators of
horizontal in-situ stress orientations in vertical to semi-vertical
boreholes, in which it is assumed one of the principal stresses is the
vertical stress (Sv). BOs and DITFs develop parallel to
the contemporary minimum (Shmin) and maximum
(SHmax) horizontal stresses respectively (Bell, 2003;
Bell & Gough, 1979; Aadnoy & Bell, 1998). In such cases, BOs and DITFs
can also be used to determine SHmax orientations in
boreholes deviated \(\geq\)20 as long as corrections are applied to
address the impact of vertical stress (Sv) on their
development (Peška & Zoback, 1995). BOs form as enlargements of the
borehole diameter on opposite sides of the borehole wall where the
circumferential hoop stress, induced by non-uniform horizontal principal
stress magnitudes, is large enough to exceed the rock strength (Bell &
Gough, 1979; Zoback, 2007). Borehole breakouts typically appear on
resistivity image logs as a pair of wide, conductive (in water-based
mud) or resistive (in oil-based mud, such as OBMI tool; King et al.,
2010) zones. In acoustic televiewer logs, they appear as low amplitude,
out-of-focus zones (Figure 2a). In both types of logs, BOs are located
~180° from each other around the circumference of the
borehole wall. BOs often correlate with borehole enlargement and
associated large caliper values as the result of the borehole failure
(Figure 2c; Tingay et al., 2008). Oriented four-arm caliper data is also
used to infer the presence of BOs along boreholes. To reliably
distinguish BOs from other non-stress related features that affect
borehole shape, such as keyseats and washouts, we apply the criteria
presented by Reinecker et al., (2003).
DITFs develop on the borehole wall where there is a significant
difference between the two horizontal principal stress magnitudes and
the local stress concentrations around the borehole wall lead to hoop
stresses that overcome the tensile strength of the rock (Brudy &
Zoback, 1999; Zoback, 2007; Barton & Moos, 2010). DITFs typically
appear as narrow, conductive (on resistivity image logs) or low
amplitude and longer travel time (on acoustic image logs) pairs,
~180° from each other around the circumference of the
borehole wall. DITFs are generally parallel or slightly inclined to the
borehole axis in vertical to semi-vertical boreholes (Barton et al.,
1998; Bell, 2003; Tingay et al., 2008; Rajabi et al., 2016a, 2016b). In
this study, all BOs and DITFs are reported as individual feature length
and width, such that a single BO or DITF measurement does not span a
number of separate individual BOs or DITFs, similar to what has been
done in previously analyzed HSM image logs (Lawrence, 2018; Griffin,
2019). This is an important aspect of quantifying induced features from
borehole image logs because geological properties, such as varying
strength associated with variably bedded lithologies, impact the
development and growth (both width and length) of borehole breakouts
(Kingdon et al., 2016; Fellgett et al., 2019). It is also important to
capture each induced feature individually for accurate statistical
considerations of borehole stress orientations.
PCFs are induced fractures that form within the bedrock ahead of the
drill bit in response to stress concentrations at the bottom of the
borehole during drilling, and propagate inward towards the borehole (Li
& Schmitt, 1998; Davatzes & Hickman, 2010; Wenning et al., 2017). PCFs
appear as conductive (resistivity image logs) or low amplitude (acoustic
image logs) partial sinusoids that merge into discontinuous borehole
axial centerline fractures (Figure 2b; Kulander et al., 1990). The
average of the centerline fracture orientations or dip orientation of
the partial sinusoids of a PCF is parallel to the orientation of
Shmin (Davatzes & Hickman, 2010). In contrast to the
DITFs, the centerline portions of PCFs are often less than 180° apart
from each other around the circumference of the borehole wall.
Finally, we use the A–E World Stress Map (WSM) quality ranking system
and circular statistical analysis for stress orientation indicators
(Heidbach et al., 2016). The borehole locations, image log intervals,
SHmax orientation mean, standard deviation, and the
quality classification are based on the length-weighted method (Heidbach
et al., 2016) for individual boreholes are summarized in Table 1 and
Table 2.
4 Results
4.1 Central HSM (Hawke’s Bay region)
A total of 810 BOs with a combined length of 454 m are identified from
borehole image logs and oriented four-arm caliper logs acquired in
Kauhauroa-2, Kauhauroa-5, Makareao-1, Tuhara-1A, Kereru-1, and Whakatu-1
boreholes in the Central HSM region (Figure 3; data set 1; see Fig. 5
for the borehole locations and names). Using only BOs from boreholes
with B-C quality rankings (following the WSM criteria), and so more
likely to display far-field stress orientation measurements, a dominant
157°/337° ± 18° orientation is observed, indicating a NW-SE
Shmin orientation, from which we infer an
SHmax orientation of 067°/247° (ENE-WSW) (Figure 3). The
only exception is borehole Whakatu-1 (WSM D quality ranking), in the
southeast area of the central HSM, which shows a dominant BO orientation
of 054°/234° ±13° (NE-SW), from which we infer a NW-SE
SHmax orientation (144°/324°) (Table 1; Figure 3). 2
DITF pairs are observed in boreholes Kauhauroa-5 and Tuhara-1A with mean
SHmax orientation of 020°/195° (NNE-SSW) and 079°/263°
(ENE-WSW), respectively (Figure 3). No BOs, DITFs, or PCFs are observed
in Waitahora-1 borehole from OBMI image logs (this image log only
provided ~37% coverage of borehole wall) or from
oriented four-arm caliper data.
Table 1. Stress
Indicators From Analysis of Borehole Image Logs and Oriented Four-Arm
Caliper Data in the Central HSM, New Zealand.