The Art of Getting Stress Orientation from Borehole Image Logs

Borehole breakouts and drilling-induced fractures are two of the main horizontal stress indicators observed in wellbore data. Approximately 19% of the stress orientation indicators in the World Stress Map database come from these two types of borehole failure collected in oil and gas and geothermal wells. Borehole image logs are the best data to interpret wellbore failure features such breakouts and drilling-induced fractures.

Basics of Wellbore Failure

Borehole breakouts are stress-induced enlargements of the wellbore cross-section (Bell and Gough, 1979). When a hole is drilled into a homogeneous, isotropic, elastic material, the stresses around the wellbore are reorganized so that there is a greater stress concentration in the orientation of the minimum applied stress and a lesser stress concentration in the orientation of the maximum applied stress (Kirsch, 1898). In Figure 1A, contours represent stress, where greater densities of contour represent higher stress concentrations and lesser densities of contours represent lower stress concentrations.

The stresses around the wellbore wall are described by a radial stress (σrr) acting perpendicular to the wellbore wall at all points, and a “hoop” stress (σθθ) acting tangent to the wellbore wall at all points (Figure 1B). The radial stress does not vary around the well and equals the internal borehole pressure or mud weight (Pw in Figure 1A). The circumferential stress varies around the wellbore wall and is typically measured clockwise from the orientation of the maximum horizontal stress. The minimum circumferential stress occurs at the azimuth of the maximum horizontal stress (SHmax) and the maximum circumferential stress occurs at the azimuth of the minimum horizontal stress (Shmin).

As demonstrated by many authors (Zoback et al., 1985; Bell, 1990), borehole breakouts occur when the stresses concentration around the borehole exceeds the compressive strength of the rock as illustrated in Figure 2A. Notice that when the circumferential hoop stress exceeds the compressive strength of the rock, breakouts form 90° from the direction of SHmax. The enlargement of the wellbore is caused by the development of intersecting conjugate shear planes that cause pieces of the borehole wall to spall off (Figure 3).? If the hoop stress becomes tensile and exceeds the tensile strength of the rock, tensile fractures will form in the direction of SHmax (Figure 2B, Aadnoy, 1990). DIFs typically develop as narrow sharply defined features that are sub-parallel or slightly inclined to the borehole axis in vertical wells and are generally not associated with significant borehole enlargement in the fracture direction (note that DIFs and breakouts can form at the same depth in orthogonal directions).

Compressive and tensile wellbore failure is a direct result of the stress concentration around the wellbore that results from drilling a well into an already stressed rock mass. In a homogeneous and isotropic elastic material in which one principal stress acts parallel to the wellbore axis, the effective hoop stress and radial stress at the wall of a cylindrical, vertical wellbore (overburden stress, Sv is a principal stress acting parallel to the wellbore axis) is given by the following equation:

where q is an angle measured from the azimuth of the maximum horizontal stress (SHmax), Shmin is the minimum horizontal stress, Pp is the pore pressure, PMud is the mud weight, and σΔT is the thermal stress induced by the cooling of the wellbore.

The effective stress acting parallel to the wellbore axis is:

where ν is Poisson's ratio.

Breakouts are evidence that there is a stress difference in the plane normal to the hole axis.? They also indicate that the stress concentration has surpassed the compressive rock strength..? However, they are not a sign of impending full collapse unless they grow in an uncontrolled manner.? Rock mechanics analysis is quite successful in predicting the onset of breakouts, using stresses, strength, and different constitutive models.

Wellbore Failure Observations in Image Logs

Borehole image logs provide unambiguous identification of wellbore failure features, as well as information about breakout width, which can be used to estimate the magnitude of the maximum horizonal stress. When interpreting acoustic image logs, borehole breakouts can be observed as broad zones of low amplitude (Figure 4). Borehole breakouts typically appear on resistivity image logs as broad, parallel, poorly resolved conductive zones separated by 180o (i.e. observed on opposite sides of the borehole) (Bell, 1996). Breakouts are typically conductive and poorly resolved because the wellbore fracturing and spalling associated with the breakout results in poor contact between the tool pads and the wellbore wall, which in turn causes the tool to measure the resistivity of the electrically conductive drilling mud rather than the formation. However, it is important to note that breakouts will appear as resistive, rather than conductive, zones in resistivity images run in oil-based mud.

Similarly, drilling-induced tensile fractures in electric image logs appear as thin dark bands approximately parallel with the axis of a vertical wellbore occurring 180 degrees apart in the image (Figure 4). These bands will appear as resistive in case of tools run in oil-based mud. On a reflected amplitude image, drilling-induced fractures are poor reflectors of acoustic energy and therefore they appear as narrow zones of low reflectivity separated by 180 degrees. DIFs are not commonly associated with any borehole enlargement and thus are often not well exhibited on borehole radius images.

Other Considerations

When natural fractures are present, near-wellbore stresses also may cause natural fractures to widen locally exhibiting maximum width in the tensile quadrants of a borehole (Figure 5A). Such features are referred to as drilling-enhanced natural fractures and can be used, in vertical wells, as proxy for the orientation of the maximum horizontal stress.

Incipient breakouts occur in the early stages of wellbore breakout development where the borehole compressive stress concentration has exceeded the rock strength and initiated breakout development. The failed material within the breakout, however, has not yet spalled into the borehole (Figure 5B). In a vertical borehole these failures may appear as thin “fractures” that propagate vertically in the borehole (Figure 5C) and may be confused with drilling induced tensile fractures.

Mechanical pipe wearing can be misinterpreted as borehole breakouts (Figure 5D), however such features (usually known as key seats) tend to exhibit a consistent width and orientation and mostly develop on one side of the well. This is typical of deviated wells (>2 degrees), where the key seat is usually located in the high or low side of the borehole.

As a final comment, it is important to remember that in a vertical well, wellbore breakouts develop in the orientation of Shmin where the circumferential stress is most compressive, and tensile wall fractures form in the orientation of SHmax where the circumferential stress is most tensile. In deviated boreholes the maximum and minimum stresses resolved on the borehole wall are a function of the far field stress magnitudes and the orientation of the borehole; breakouts and tensile wall failures do not necessarily align with the geographical orientations of the horizontal stresses. In deviated wells (>10 deg) observations of wellbore failure cannot be used as direct indication of the orientation of the maximum horizonal stress. In these cases, additional modeling is required to infer stress orientation.

References

Aadnoy, B.S., 1990. In-situ stress directions from borehole fracture traces. Journal of Petroleum Science and Engineering, 4(2), pp.143-153.

Bell, J., 1996. Petro geoscience 2. In situ stresses in sedimentary rocks (part 2): applications of stress measurements. Geoscience Canada.

Bell, J.S. and Gough, D.I., 1979. Northeast-southwest compressive stress in Alberta evidence from oil wells. Earth and planetary science letters, 45(2), pp.475-482.

Bell, J.S., 1990. Investigating stress regimes in sedimentary basins using information from oil industry wireline logs and drilling records. Geological Society, London, Special Publications, 48(1), pp.305-325.

Dusseault, M.B., Jackson, R.E. and Macdonald, D., 2014. Towards a road map for mitigating the rates and occurrences of long-term wellbore leakage (p. 69). Waterloo, ON, Canada: University of Waterloo.

Heidbach, O., Rajabi, M., Cui, X., Fuchs, K., Müller, B., Reinecker, J., Reiter, K., Tingay, M., Wenzel, F., Xie, F. and Ziegler, M.O., 2018. The World Stress Map database release 2016: Crustal stress pattern across scales. Tectonophysics, 744, pp.484-498.

Kirsch, C., 1898. Die theorie der elastizitat und die bedurfnisse der festigkeitslehre. Zeitschrift des Vereines Deutscher Ingenieure, 42, pp.797-807.

Zoback, M.D., Moos, D., Mastin, L. and Anderson, R.N., 1985. Well bore breakouts and in situ stress. Journal of Geophysical Research: Solid Earth, 90(B7), pp.5523-5530.

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