Insitu Stress - Shmax Orientation

Importance of Shmax

In general, fractures which strike parallel to Shmax tend to dilate and be open and those normal to Shmax tend to be closed (Barton et al. 1995). Field development plans often involve drilling horizontal reservoir sections across Shmax (orientation) to allow the well to be exposed to as many fractures as possible, running in this direction. Water injection wells however may benefit from having an azimuth (close to) parallel to Shmax - the flood front is often in the direction of Shmax.

Deriving orientation of Shmax

The World Stress Map (WSM), [1] is usually my first port of call (Fig. 1) - wellbore breakouts, (Fig. 2) and drilling induced tension fractures (DITs) (Fig. 3) on image logs are a common way to derive Shmax orientation and it contains other sources such as earthquake focal mechanisms. It s bit thin on data from development wells though (I think ! ).

Figure 1. Shmax orientation in NW Europe from the World Stress Map. Heidbach, Oliver; Rajabi, Mojtaba; Reiter, Karsten; Ziegler, Moritz; WSM Team (2016): World Stress Map Database Release 2016. GFZ Data Services. http://doi.org/10.5880/WSM.2016.001

Figure 1. Shmax orientation in NW Europe from the World Stress Map.

Heidbach, Oliver; Rajabi, Mojtaba; Reiter, Karsten; Ziegler, Moritz; WSM Team (2016): World Stress Map Database Release 2016. GFZ Data Services. http://doi.org/10.5880/WSM.2016.001

Figure 2. Hole ovalisation tends to occur in the direction of Shmin This is called a breakout where compressive shear fracturing forms. Shmax is normal direction to that. The parallel to Shmax black lines are drilling induced tensional fractures.

Figure 2. Hole ovalisation tends to occur in the direction of Shmin This is called a breakout where compressive shear fracturing forms. Shmax is normal direction to that. The parallel to Shmax black lines are drilling induced tensional fractures.

Figure 3. Drilling induced tensional fractures, 180 deg apart on a resistivity image log (wireline), N Sea.

Figure 3. Drilling induced tensional fractures, 180 deg apart on a resistivity image log (wireline), N Sea.

It is possible to get a continuous record of Shmax from log data [2] - as fast azimuth from a dipole sonic log is a good proxy for Shmax orientation. Compare this data to a breakout which is a single data point, not always at the stratigraphic level of interest. We can interpret a NW-SE direction of Shmax from the (right hand most) sonic log (fast direction) in Fig. 4, although it should be considered as one line of evidence along with breakouts and DITs from image logs etc.

Note, that this well crosses a fault and there is a disturbance to this regional NW-SE trend (circled in Fig.4). Its quite common to see deviations from the regional stress direction around faults. In fact, maps of breakouts (or Shmax derived from them) displayed on structure, often show deflections close to faults.

Figure 4. Fast azimuth can be a proxy for Shmax - here it has been derived from LWD sonic (red curve far right). There is some disturbance when the well crosses a fault (red circle). From ref [2].

Figure 4. Fast azimuth can be a proxy for Shmax - here it has been derived from LWD sonic (red curve far right). There is some disturbance when the well crosses a fault (red circle). From ref [2].