Borehole Televiewer Interpretation: Acoustic and Electrical Imaging for Fractures, Breakouts, and Stress Orientation
A borehole televiewer is a wireline logging tool that generates a continuous, oriented, 360° image of the borehole wall by recording either the acoustic reflectivity of the formation surface (the acoustic or ultrasonic borehole televiewer, BHTV or ULTC) or the electrical micro-resistivity contrast of the formation face (the formation microresistivity imager, FMI or FMS), with both tool types serving as the primary data source for borehole geomechanical analysis, natural fracture characterization, and in-situ stress orientation mapping in WCSB Montney, Duvernay, and Devonian reservoir programs. The acoustic televiewer rotates a piezoelectric transducer at 2-6 revolutions per second while logging at 30-150 m/hr, transmitting 200-500 kHz pulses outward through the borehole fluid and simultaneously recording the two-way travel time of the returning echo (which maps borehole radius variations and produces a continuous acoustic caliper image) and the amplitude of the echo (which encodes the acoustic reflectivity and surface roughness of the borehole wall). Open fractures filled with borehole fluid reflect weakly and appear as dark sinusoidal traces on the amplitude image; cemented fractures filled with calcite or silica reflect strongly and appear as bright sinusoidal traces; stress-induced breakouts appear as dark patches at consistent azimuths across a depth interval where the borehole wall has spalled and roughened due to compressive failure; and drilling-induced tensile fractures appear as narrow, vertically continuous dark traces at opposite azimuths marking where wellbore hoop stress has gone tensile and the formation has failed in tension parallel to SHmax. The formation microresistivity imager (Schlumberger FMS/FMI, Baker Hughes STAR, Halliburton XRMI) uses 192 button electrodes arranged on 4 or 8 articulated pads pressed against the borehole wall, measuring micro-resistivity at 0.5-1.0 mm resolution — twice to five times finer than the 3-5 mm effective resolution of most acoustic televiewers — but requiring electrically conductive water-based mud for the current return path, which limits FMI application to WBM wells. Because the majority of WCSB Montney and Duvernay horizontal wells are drilled with synthetic oil-based mud (SOBM) to manage shale instability in the Doig and Montney overlying sequences, the BHTV is the standard borehole imaging tool in horizontal WCSB completions — it operates in both OBM and WBM systems provided the borehole fluid is acoustically clear (free of gas or fine particulates that scatter the acoustic pulse). The 360° borehole wall image from either tool type is presented as a rectangular "unrolled" display of depth versus azimuth from 0° (north on oriented surveys) to 360°, so that any planar feature intersecting the cylindrical borehole produces a sinusoidal trace whose amplitude equals the true dip angle and whose minimum azimuth corresponds to the dip direction, enabling structural dip, fracture orientation, and breakout azimuth to be read directly from the image using interactive dip-picking software integrated with the log display system. The measurement is highly sensitive to tool positioning: the BHTV must be centralized within the borehole to produce a geometrically accurate acoustic caliper image (an off-center tool records false radius variations), while the FMI pads must maintain full contact with the borehole wall even in rugose or enlarged sections, which is why both tool types include caliper arms and centralizer systems as integral components.
Key Takeaways
- BHTV versus FMI: mud type is the primary selection criterion in WCSB horizontal wells: The BHTV operates in any borehole fluid — OBM, SOBM, or WBM — as long as the fluid is acoustically transparent, making it the standard borehole imaging tool in Montney and Duvernay horizontal wells drilled with synthetic oil-based mud. The FMI requires water-based mud for electrical continuity between the pad electrodes and the formation, and produces higher resolution images (0.5-1.0 mm vs 3-5 mm) when mud type permits. In WCSB vertical exploration wells drilled with WBM for cost reasons, FMI is often preferred for its superior resolution in Devonian carbonate fracture characterization. A WBM vertical well with both a BHTV run and an FMI run is the most comprehensive borehole imaging dataset available, with the acoustic caliper from the BHTV providing independent borehole geometry data to validate the FMI pad contact quality.
- Breakout orientation as a direct measurement of minimum horizontal stress direction: Borehole breakouts form when the compressive hoop stress at the borehole wall (concentrated at the azimuth perpendicular to maximum horizontal stress, SHmax) exceeds the rock's unconfined compressive strength. The breakout appears on the 4-arm caliper as C1 significantly exceeding C2 at a consistent azimuth throughout the breakout interval, and on the televiewer image as dark roughened patches at that same consistent azimuth. Because breakouts are perpendicular to SHmax, the breakout azimuth defines the direction of minimum horizontal stress (Shmin) directly. Alberta Geological Survey stress orientation compilations show NE-SW SHmax (approximately 045-060° azimuth) across most of the Montney fairway, meaning WCSB horizontal wells are drilled NW-SE (135-150°) to generate transverse hydraulic fractures perpendicular to SHmax for maximum fracture-wellbore intersection.
- Natural fracture characterization: open versus sealed versus partially open: Natural fractures appear as sinusoidal traces on the borehole image; the acoustic reflectivity and resistivity contrast of the fracture fill distinguish open from cemented fractures. On the BHTV, a dark (low amplitude) sinusoid indicates an open fracture filled with conductive borehole fluid; a bright sinusoid indicates a fracture cemented with calcite, silica, or dolomite that reflects the acoustic pulse strongly. On the FMI, open fractures are highly conductive (dark on the resistivity image) and cemented fractures are resistive (light). Fracture aperture can be estimated from the width of the anomalous trace on the FMI using a calibration relating aperture to mud resistivity and current concentration effect, with detection threshold approximately 100 micrometres for a typical Devonian reef WBM FMI run.
- Distinguishing drilling-induced fractures from pre-existing natural fractures: Drilling-induced tensile fractures (DITFs) are created by the stress concentration at the borehole wall during drilling when wellbore pressure exceeds the minimum principal stress and the hoop stress becomes tensile. DITFs appear on televiewer images as narrow, vertically continuous traces at fixed azimuths exactly 180° apart (the azimuth of SHmax, opposite to breakouts), with zero apparent dip and no cross-cutting relationships with other fractures. Natural fractures have oblique dip (typically 40-85° in Devonian carbonate reservoirs), variable strike, finite height limited by mechanical stratigraphy, and may cross-cut bed boundaries or intersect other fracture sets. Misidentifying DITFs as natural fractures inflates the apparent fracture density and leads to incorrectly attributing production potential to a naturally fractured reservoir that is actually mechanically intact.
- Image quality control parameters and logging speed constraints: Televiewer image quality degrades systematically with logging speed: the standard for interpretable amplitude images is 128 azimuthal samples per revolution at the logging depth, requiring that the tool log no faster than (azimuthal samples × rotation speed × sample spacing) / depth sampling frequency. At 4 RPS rotation and 0.5 mm desired vertical sample spacing, maximum logging speed is approximately 120 m/hr; logging faster produces helical aliasing where the image appears to spiral along the depth axis rather than producing horizontal bed boundaries. WCSB contractors commonly run televiewers at 100-130 m/hr to balance survey time against image resolution. In severely rugose boreholes (caliper >130% of bit diameter over extended intervals), the acoustic televiewer loses pad-equivalent acoustic focus and amplitude images become unreliable regardless of logging speed.
BHTV Survey in a Montney SOBM Horizontal Well: Fracture and Stress Mapping for Completion Design
A Dawson Creek Montney operator runs a BHTV survey through the 2,800-3,400 m open-hole section of a horizontal well drilled at 90° inclination in SOBM. The televiewer is run on a memory tool (no surface readout in real time) at 100 m/hr. Post-processing identifies 47 natural fracture traces in the 600 m horizontal section, averaging 0.08 fractures per metre. Natural fractures are predominantly NE-SW strike (045-050°, parallel to SHmax) and dip steeply at 75-85° — consistent with the regional fracture system identified in offset vertical wells. Breakouts appear at intervals between fracture zones and show consistent Shmin azimuth of 130° (confirming the NW-SE well azimuth is within 5° of optimal for transverse hydraulic fracture generation). Drilling-induced tensile fractures at 040° azimuth confirm the SHmax orientation independently. The natural fracture intensity map guides perforation cluster placement: 6 clusters per stage are positioned to preferentially intersect the 12 highest-aperture natural fractures identified from the image log, targeting fracture zones where injection pressure will be lowest and hydraulic fracture initiation most efficient.
Fast Facts
The first borehole televiewer was developed by J.T. Zemanek Jr. and colleagues at Mobil Research and published in the Journal of Petroleum Technology in 1969. The original design used a single rotating transducer at 10 kHz and produced images with approximately 2 cm azimuthal resolution — coarse compared to modern 200-500 kHz tools capable of 3-5 mm resolution — but the fundamental principle of recording amplitude and travel time of the reflected acoustic pulse from the rotating transducer was identical to all current BHTV tools. The first commercial FMI tool was introduced by Schlumberger in 1991 as an improvement on the Formation MicroScanner (FMS) pad tool, adding high-resolution resistivity buttons over the full 360° borehole circumference rather than just 4 pad contact strips.
Related Terms
The borehole geometry that determines the quality and interpretability of televiewer images is characterized by the caliper log discussed under borehole, where breakout detection from 4-arm caliper asymmetry, washout severity, and borehole rugosity are identified as the primary factors controlling whether a standard borehole televiewer run will produce interpretable amplitude and dip data or whether the borehole geometry has degraded image quality beyond reliable interpretation. The in-situ stress orientation derived from breakout analysis on the borehole televiewer image is used in horizontal well azimuth selection and hydraulic fracture design for WCSB Montney and Duvernay wells, where the SHmax direction controls hydraulic fracture propagation azimuth, as described under hydraulic fracturing. The natural fracture networks identified and characterized using borehole televiewer data in Devonian reef and Montney tight gas reservoirs contribute to the secondary porosity estimates and flow model inputs described under borehole gravity, where the large-radius density measurement provides an independent check on the fracture porosity estimated from contact log and image log interpretation.