Wellbore Stability Considerations for Drilling High-Angle Wells Through Finely Laminated Shale: A Case Study From Terra Nova

Abstract

fax 01-972-952-9435. AbstractHigh angle wells drilled into finely laminated shale are often found to be less stable than comparable wells drilled into nonlaminated rock. This can be attributed to a variety of factors including, but not limited to, well trajectory, the in-situ stress field, rock strength anisotropy, shale reactivity issues, chemical imbalances between the drilling fluid and shale pore fluid, and many others. However, due to the lack of available data some of these mechanisms are difficult to quantify and therefore are typically not accounted for during well planning. This can result in significant wellbore instability issues that substantially elevate drilling operation costs.In this paper, a comprehensive approach to account for many of these wellbore instability mechanisms is outlined. A plane-of-weakness model is utilized to account for the effects of weak bedding planes and other discontinuities. The model uses parameters that are obtained by curve fitting triaxial strength test results conducted at various angles with respect to bedding. Additionally, traditional mechanical and chemical effects were also addressed and incorporated into the pre-drill model to assist in well planning. The model was then implemented at the Terra Nova field for the case of a highly deviated wellbore drilled through finely laminated shale nearly parallel to bedding.Real-time monitoring of measurement and logging while drilling data was key to identifying the unstable sections so that the root cause of instability could be diagnosed. The appropriate remedial action was then applied and wellbore instability problems were mitigated.Both mechanical and chemical borehole instability models were applied in a case history to evaluate the potential for wellbore instability. In particular, bedding-plane related and chemically-induced instability were addressed and overcome through comprehensive modeling and the deployment of modified operational procedures.

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

“…In anisotropic rocks bedding plane slip may have a significant effect on borehole and casing stability (Addis et al 1993;Dusseault et al 2001;Gazaniol et al 1995;Økland and Cook 1998;Russell et al 2003;Skelton et al 1995;Willson et al 1999) and has been estimated previously (e.g., Daemen 1983; Gallant et al 2007;Lee et al 2012;Vernik and Zoback 1990). However, the considerable effect of elastic anisotropy has been ignored so far in these analyses.…”

“…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).…”

“…The structural heterogeneity of shale is attributed to the orientation distribution of minerals and organic matter, typically resulting in the formation of weak bedding planes (e.g., Chenevert and Gatlin 1965;Gallant et al 2007) and a pronounced elastic transverse isotropy (TVI) (Dewhurst and Siggins 2006;Johnston and Christensen 1995;Vernik and Liu 1997;Wenk et al 2008), meaning that properties, e.g., elastic constants and strength properties, parallel to the bedding are isotropic but vary across them (Chong and Smith 1984).…”

Automated Alarms for Managing Drilling Pressure and Maintaining Wellbore Stability—New Concepts in While-Drilling Decision Making

“…Real-time pressure management is not new to the petroleum industry as the literature contains numerous publications related to real-time pressure management and wellbore stability (Gallant, et al 2007) (Greenwood, et al 2005) (Meng, Fuh and Spencer 2010). Typically, the mud weight window is created in the pre-drill stage (using all available offset well data) and rigsite personnel would "listen" to the well by monitoring secondary parameters (fluid gains and losses, gas trends, cuttings/cavings, etc.).…”

Analytical Model of Wellbore Stability of Fractured Coal Seam Considering the Effect of Cleat Filler and Analysis of Influencing Factors

“…Specialists have achieved a number of results [7][8][9][10] on the study of the wellbore stability of oil and gas wells, which can give us ideas on solving the problems of a broken coal seam. Zhang et al [11] proposed the coupling system of both flow and solid in rock failure process analysis, in order to…”