Get Stress

The ADVANCED STRESS & failure > Get Stress sub-strip calculates the magnitudes and orientations of horizontal stresses from breakouts or tensile fractures or tensile regions observed in vertical or deviated boreholes. Information on the wellbore orientation, the width and orientation of breakouts, mud weight, pore pressure, vertical stress, and rock properties are required.

Two modes are available on the form: Get Stress_1 and Get Stress_2:

  • Get Stress_1 addresses the typical case of many sedimentary basins where information is available on Sv, Shmin, and Pp, but the azimuth and the magnitude of SHmax are unknown.
  • Get Stress_2 applies to the case where the magnitudes of SHmax and Shmin are unknown, but stress orientation is known from independent observations such as breakouts in vertical wells, or the orientations of locally active faults.

Use Get Stress_1 to determine the magnitude and orientation of the maximum horizontal stress (SHmax) from compressive or tensile wellbore failure,

  • When width and orientation of breakouts or tensile fractures are known in a deviated well.
  • When wellbore orientation is known.
  • When mud weight, pore pressure, vertical stress magnitude, the minimum horizontal stress (Shmin) magnitude, and rock properties are known.

Use Get Stress_2 to determine the magnitudes of the horizontal stresses (SHmax and Shmin) from compressive or tensile wellbore failure,

  • When width and orientation of breakouts or tensile fractures are known in a deviated well.
  • When wellbore orientation is known.
  • When mud weight, pore pressure, vertical stress magnitude, and rock properties are known.
  • When the orientation of SHmax is known or can be estimated.

Both modules let you select the log or depth calculation.

When using Log mode, the check option <every> is enabled. It is useful to check on this option when all input values are used as constant.

Through the Tools options in the ADVANCED STRESS & failure > Setup sub strip or the Data > Miscellaneous strip, you can import any type of borehole failure (Breakout, tensile failure, tensile region) that was analyzed using Imager or any other image analysis application.

Two types of failure can be selected: Break out failure, or Tensile failure/tensile region failure. For either option, you can define or import the position and width of failure.

Breakout failure, or Tensile failure/tensile region failure logs are local measurements. Therefore, you should define the sampling rate for which you want the calculations. For a small number of points, 1 is an acceptable sample value, but for those files with existing multiple points, calculations could take a lot of time, so a greater sampling value is recommended.
  • Breakout Width: Total width of zones failed by breakout and foliation slip. Should be greater than 0 and less or equal to 180 (half of the wellbore.
  • Breakout Position: Azimuth is measured clockwise from the bottom of the hole.
  • Tensile failure (theta angle) / tensile region Position: The measurement is the same as for breakout.
  • Tensile failure / tensile region Width: greater than or equal to zero and less than or equal to 180 degrees.
Be careful selecting the reference for these features. Always reference the bottom of the hole used for the corresponding Get Stress calculations.
The tensile region is a combination of both drilling-induced tensile fractures and pre-existing drilling enhanced natural fractures that are open in shear within the tensile region of the wellbore.

The parameter width TF/TR defines the width of the zone in which an inclined fracture or a set of drilling-enhanced fractures appears on a borehole image. The value of wTF cannot be defined if fractures are axial; in that case, enter zero. The parameter wTF is similar to wBO, as it can be used to relate the width of the region of tension to the tensile strength T0.

If you are importing from Imager, for tensile regions, this value is obtained from the file.
The relation between wTF and T0 is poorly defined. Tensile strength predictions or stress constraints based on measured tensile strength should be used with caution for a number of reasons: (1) tensile regions can occur where the wellbore is not actually in tension, due to mixed-mode failure of steeply inclined fractures; (2) tensile fractures can occur during surge events, when the effective mud weight is higher than the static or background equivalent mud weight; and (3) GSTR ignores thermal effects.

If tensile failure cannot occur because none of the stresses are tensile, a warning dialog is generated. Because drilling-enhanced fractures are still possible, the output window will still be displayed, but the required tensile strength (T0) will be positive. This is appropriate for mixed-mode failure or for enhancement of natural fractures.