Tomographic formation mechanical stability estimation using borehole radial gradient of bulk modulus distribution

Knowledge of mechanical stability of the near borehole formation is fundamental in: geotechnical investigation • • • defining optimal locations for deviation of horizontal drilling paths reducing risks of borehole collaps through scattering of formation particles in the the borehole while production (sanding) The respective information may be obtained by laboratory measurements directly on cores or in situ as a result of borehole-caliper evaluation or sonic-log interpretation. Normally the borehole-surface is destroyed by the drilling process (microfractures, breakouts) and the measured travel times will be influenced by a medium of lower-velocity. Because of increasing velocity in higher penetration r the seismic energy is traveling as diving waves through the formation in this case. Using a multi-array sonic-probe and low shot-point distances, the high density of travelpaths through the formation allows imaging the formation velocity distribution by an In-Borehole Tomographic Reconstruction (IBTR). These 2D-images especially reflect the sensitive mechanical behaviour in the transition from mud to formation besides an accessible higher vertical resolution and exclusion of caliper effects (Fig. 1).