Shear-Type Borehole Wall Shifts Induced During Lost Circulations
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SummaryBorehole wall shifts during lost circulations are studied and parameter studies are conducted for evaluating the magnitude of the shifts under the following typical drilling conditions.slant-crack induced around vertical wells during lost circulations borehole wall shifts induced during the drilling of normal, thrust, and strike-slip fault areas borehole wall shifts induced for an inclined well because of a fracture induced perpendicular to the minimum in-situ stressParameters varied are the frac size/wellbore size, frac angle, borehole pressure, and h1,H2,v ratio. These analyses are significant for the following reasons:A new fracture model from a borehole is coded using a 3D-dual-boundary element method. This method allows different displacement and stress traction at the two fracture surfaces along a fracture plane around a borehole. Note that other boundary element methods for 3D nonplanar hydraulic fracture problems have been developed by another group, but these methods did not include a borehole (Yamamoto et al. 1999). Minor borehole wall shifts occur with small-scale fluid losses as observed with borehole imagers (Maury and Zurdo 1996). Although these minor shifts do not create drilling problems, the in-situ stresses may be evaluated from the borehole wall shift if the fractured area is identified by acoustic devices. The current model quantifies the relation between these shifts and other parameters (such as the geological properties and frac size and angle). Significant borehole wall shifts occur if a lost circulation is significant and the leakoff plane is inclined with respect to the principal in-situ stress direction. Some stuck pipe problems may be caused by shear type borehole wall shifts rather than by borehole breakouts or differential sticking problems.
