Wellbore Stability in Fractured Rock

Abstract

Numerous wellbore instability problems related to drilling through potentially fractured formations have been reported. Often, these rocks are characterized by the abundance of macro and micro scale bedding planes and/or networks of natural fractures. The presence of fractures weakens the rock mechanically and produces potentially higher-permeability fluid-flow paths within the low-permeability rock matrix. Practically, it is difficult to identify fracture size and fracture density without a costly core sample. A number of wellbore stability case studies have therefore been published where the author relied on anecdotal evidence to postulate failure mechanisms involving fractured rock, and recommendations for how to mitigate the observed instability range from increasing to decreasing the wellbore pressure. This paper presents results from a geomechanical investigation of a wellbore instability incident experienced in a fractured shale formation. As part of this assessment, a preserved core was obtained from the fractured shale interval and the presence of fractures was identified both by CAT scan and visual inspection. A series of triaxial tests were conducted to characterize the mechanical properties and failure strength of this shale. This data, combined with wellbore stability modeling, suggests that the residual strength, rather than the peak failure strength, is a more representative measure of a fractured rock's in-situ strength. The Hoek and Brown (1982) failure criterion was found to be particularly suitable for modeling fractured rock. Multi-arm caliper logs from two boreholes through the same fractured shale suggests that wellbore instability is more complex in the fractured interval than in the over- and underlying intact rock. These caliper logs also clearly demonstrate that the borehole quality was significantly improved by increasing the drilling fluid density, which contradicts the conclusions drawn in a number of published wellbore stability case studies (Santarelli et al., 1992; McClellan et al., 1996; Edwards et al., 2004; Fontana et al., 2007).

Prediction of Mud Pressures for the Stability of Wellbores Drilled in Transversely Isotropic Rocks

“…However, some authors applied this criterion to evaluate the stability of wellbores drilled in isotropic rocks. In general, the results of these analyses show a good match between field evidence and calculated mud pressures ( [46][47][48][49][50]).…”

Influence of Bedding Angle on Borehole Stability: A Laboratory Investigation of Transverse Isotropic Oil Shale

“…However, a number of instability problems are encountered while drilling or excavating shale formations; especially with decreasing angle b between the bedding planes and the borehole axis ( Fig. 1), often referred to as 'angle of attack' (Atkinson and Bradford 2002;Gallant et al 2007;Gazaniol et al 1995;Khan et al 2011;Martin and Lanyon 2003;Ottesen 2010;Økland and Cook 1998;Zoback 2010).…”

Borehole-Quality Design and Practices To Maximize Drill-Rate Performance

“…The concern for pressure penetration into fractured rock has led the industry to limit, or even reduce, the MW. A combination of engineered blocking solids and higher MWs has been shown to be more effective in promoting stability in fractured rock (Ottesen 2010). A similar concept has also proved to be successful in stabilizing cleated coals with higher MW (Zeilinger et al 2010).…”