SPE Drilling & Completion
Volume 23, Number 3, September 2008, pp. 301-313

Summary

Borehole 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 stress

Parameters varied are the frac size/wellbore size, frac angle, borehole pressure, and σ_H1 , σ_H2 , σ_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.

Introduction

Previously, stuck pipe problems were assumed to be caused by borehole breakouts, differential sticking, and cutting pack offs. However, thorough examinations of borehole walls using borehole televiewers show that some stuck pipe problems are caused by shear type borehole wall shifts (Maury and Zurdo 1996). Most borehole wall shifts are less than 0.5 in. and do not create serious drilling problems. However, it has been speculated (Neda ) that borehole sizes become narrow because of borehole wall shifts, resulting in drillstring stuck problems during massive lost circulations. In this paper, the magnitude of borehole wall shifts is evaluated using a 3D-nonplanar fracture model with a new 3D dual-boundary element method (Giuggiani 1992; Mi and Alibadi 1992; Fedelinski et al. 1993). The model includes a slant borehole, a slant circular, or elliptical fracture from a borehole. Since a 3D fracture induces three stress-intensity factors (first, second, and third type stress-intensity factors), the fracture normally extends in an elliptical rather than circular shape. The model is used to analyze borehole wall shifts under the following three conditions

• Borehole wall shifts induced by a slant crack around a vertical well during lost circulations by varying parameters (such as frac size/wellbore size, frac angle, borehole pressure, and the three principal in-situ stresses).
• Borehole wall shifts induced during a small lost circulation at a fault plane for normal, thrust, and strike-slip fault areas.
• Borehole wall shifts induced for slant wells because of a fracture that is perpendicular to the minimum in-situ stress.

The results show that the borehole wall shifts are normally minor, as observed by borehole imagers, if the induced fractures during lost circulations are small; however, nontrivial borehole wall shifts occur if the lost circulation is serious and the fracture angle is offset from the principal in-situ stress directions. In addition, this analysis shows that if the borehole wall shifts are measured while the fracture is open, the directional in-situ stress may be inversely evaluated.

© 2008. Society of Petroleum Engineers

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History

• Original manuscript received: 4 June 2006
• Meeting paper published: 24 September 2006
• Revised manuscript received: 16 October 2007
• Manuscript approved: 13 November 2007
• Version of record: 15 September 2008