The art of rock mechanical and well bore stability analysis has moved to the real-time domain with the use of Logging-While-Drilling tools. Rock mechanical analysis based primarily on sonic logs (compressional and shear slowness) is a well established technique to identify wellbore stability issues. Logging-While-Drilling logs are slowly replacing wireline logs in large numbers of wells. Not only has the LWD data quality equalled wireline data quality, these data are now available in real-time while drilling. The rock mechanical analysis previously restricted to post drilling analysis, can now be done while drilling. The typically required sonic data (DTP and DTS), the bulk density and a shale indicator are sent to surface in reasonable time increments and the rock mechanical evaluation programs are installed on the rig site LWD systems. Alternatively, the data can be sent to service company or oil company's offices and analysis performed there.
In addition to traditional rock mechanical computations, a new technique is introduced to evaluate borehole break-out and its orientation while drilling using an LWD caliper measurement. The LWD caliper tool used in the North Sea examples employs three ultrasonic transducers to measure the stand-off between the logging tool and the borehole wall. The caliper is computed from these three measurements. In the new method we fit an ellipse through the three measurements and compute the minimum and maximum axes of this ellipse. With the tool face information provided by the LWD survey tool, we can compute the orientation of these axes. Where borehole breakout is resulting from stress relieve at the borehole wall, the direction of the long axis coincides with the direction of the minimum horizontal stress in the formation. Three dimensional borehole caliper images can be created after logging.