UCS T0 Profiles

The second step in the Get Stress workflow is to define which type of failure you will use to calculate either the confined compressive or the tensile strength of the rock. On this form, you also specify the mud weight data to aid in the calculation of the profiles.

After being calculated, the profiles can be displayed in the Get Stress view. Inside the Get Stress view you will find contour lines that are consistent with the input data that has been provided. The colors within the region of allowable magnitudes and azimuths of SHmax indicate the magnitude of UCS, which is consistent with the occurrence of breakouts of the width entered as input.

Through the Tools option in the ADVANCED STRESS & failure > Setup sub-strip or via 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.
Important  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, and tensile strength predictions or stress constraints based on measured tensile strength should be used with caution, for a number of reasons. For example: (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.