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.