Determining Fracture Orientation
As mentioned above, when formation pressure is isotropic (equal in all directions), the tectonic stress is zero and Pfrx equals Pfry. In this situation, the borehole is round and spalling of the formation is either non-existent or equal in all directions. In stressed regions, such as in the Rocky Mountains, the borehole will erode to an oval shape. The minimum diameter shows the direction of maximum stress and the maximum diameter shows the direction of minimum stress..


Borehole shape indicates stress direction – maximum stress in direction of minimum hole  diameter. Formation microscanner and dipmeters have oriented caliper data.

Many modern logs have an X and Y axis caliper, but not all of them are oriented to true north. When directional data is recorded, as with dipmeters and many modern resistivity tools, the X and Y orientations are known, Statistical plots are helpful in choosing the dominant direction).


Borehole diameter indicates stress direction - this example is from India where the minimum
 stress direction is NE - SW.

A hydraulic fracture will usually penetrate the formation in a plane normal to minimum stress, or parallel to the plane of maximum stress. Any stress anisotropy (tectonic stress) will cause the fracture to be other than vertical.


Natural fractures take the same directions as hydraulic fractures, indicated again by the borehole shape. In addition, the high angle dips seen on an open hole dipmeter or image log, will also indicate this preferential direction. Since most hydraulic fracture jobs are run in casing, it is not possible to run a dipmeter or caliper survey to find the orientation of a hydraulic fracture. The preferential direction can be predicted from previous open hole data. Dipmeter and caliper data can be displayed on rose diagrams to illustrate preferential directions.

If an azimuthal gamma ray log existed, the fracture orientation could be located by a tracer survey. I am not aware that such a tool exists, but it would not be difficult to design one..

Azimuth frequency (rose diagram) plots show direction of dips seen on dipmeter and image logs. When steep dips caused by fractures are isolated from lower angle bedding dips, the direction of maximum stress xan be determined. In this case, the direction is N30E.

Stress direction is not constant over geological time scales. Differences in the direction of induced fractures (present day stress direction), open fractures (some time ago), healed fractures (older than open fractures), and small faults (could be any age) will help to show the stress history of a region. An example log and rose diagrams are shown below.


Image log in fractured reservoir: gamma ray (left track, shaded red), image track (middle) with open fractures (red sine waves and healed fractures (yellow sine waves), dip track (right) shows red amd yellow dip angle and azimuth. There are no induced fractures in this short interval. Bedding planes are near horizontal. Imagine trying to locate these steep dips without the aid of a computer.

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Induced fractures (top left) show current stress direction. Open fractures (top right) show stress direction when fractures were created, healed fractures (lower left) show different direction at an earlier phase in geological time, and micro faults (lower right) shows another stress regime was present when the faults occurred.

The newest dipole shear sonic log is also an azimuthal tool with dipole sources set at 90 degrees to each other. The example below shows the shear images for the X and Y directions. This log can be run in open or cased hole.

 

 


Dipole shear image log shows directional stress - the Fast Direction is centered on
90 degrees (east - west) which is also the maximum stress direction.

Resistivity and acoustic image logs also provide assistance in locating fracture orientation before the well is cased.
 

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