Rock Mechanics and Wellbore Stability Analysis While Drilling using LWD Sonic, Density and Caliper Measurements

摘要

The art of rock mechanical and well bore stability analysis hasrnmoved to the real-time domain with the use of Logging-rnWhile-Drilling tools. Rock mechanical analysis basedrnprimarily on sonic logs (compressional and shear slowness) isrna well established technique to identify wellbore stabilityrnissues. Logging-While-Drilling logs are slowly replacingrnwireline logs in large numbers of wells. Not only has thernLWD data quality equalled wireline data quality, these datarnare now available in real-time while drilling. The rockrnmechanical analysis previously restricted to post drillingrnanalysis, can now be done while drilling. The typicallyrnrequired sonic data (DTP and DTS), the bulk density and arnshale indicator are sent to surface in reasonable timernincrements and the rock mechanical evaluation programs arerninstalled on the rig site LWD systems. Alternatively, the datarncan be sent to service company or oil company’s offices andrnanalysis performed there.rnIn addition to traditional rock mechanical computations, a newrntechnique is introduced to evaluate borehole break-out and itsrnorientation while drilling using an LWD caliper measurement.rnThe LWD caliper tool used in the North Sea examplesrnemploys three ultrasonic transducers to measure the stand-offrnbetween the logging tool and the borehole wall. The caliper isrncomputed from these three measurements. In the new methodrnwe fit an ellipse through the three measurements and computernthe minimum and maximum axes of this ellipse. With the toolrnface information provided by the LWD survey tool, we canrncompute the orientation of these axes. Where borehole breakoutrnis resulting from stress relieve at the borehole wall, therndirection of the long axis coincides with the direction of thernminimum horizontal stress in the formation. Threerndimensional borehole caliper images can be createdrnafter logging.