Managing Geomechanics Principles for Proper Handling of Wellbore-Stability Problems in Shale and Depletion Effects on Formations While Drilling

Abstract

This paper describes two types of hole problems and wellbore stability solutions for drilling operations in North Sea fields: (1) severe shale cavings and wellbore collapse; (2) drilling through cap rock and reservoir formations impacted by large pressure depletion and reservoir compaction effects.Field cases of wellbore collapses and failures in shale formations during high-angle drilling are presented along with a systematic program of comprehensive geomechanics investigation and evaluation necessary to prevent such events. The program includes analyzing drilling and log data analysis for rock strength and earth stress model construction, laboratory triaxial stress-strain tests on well core samples considering bedding plane inclination effects, 3D analytical modeling and 3D elastic-plastic numerical modeling analysis for determining optimum mud-weight (MW) windows for secure and stable well drilling.Continuous well production can also present problems for in-field drilling as the severe pressure depletion and reservoir compaction will cause significant reduction in stresses and formation fracture gradient. These effects may occur not only in the reservoir, but may also extend significantly upward into the cap rock formations depending on the reservoir/formation stiffness contrast, reservoir size, thickness and depth, etc. Therefore, the optimum MW must be examined and recalculated based on the insitu stresses that can be altered by this effect. In summary, this paper will include the following:1. Analysis of actual drilling cases of severe shale caving and wellbore collapses/failures; 2. Laboratory triaxial stress-strain tests on well cores considering bedding plane effects; 3. Effective drilling guidelines based on modeling analysis of troublesome shale formations; 4. Coping with reduced stresses and formation fracture gradient due to reservoir depletion and compaction effects; 5. Use of wellbore strengthening material for drilling (and possible reduction in number of casing strings); 6. Case histories of severe wellbore instability and lessons learned. IntroductionWellbore stability is affected by many issues, such as the in-situ stresses, pore pressure, rock properties, formation strength, mud fluid properties, reservoir depletion, well azimuth and inclination, and drilling operations. Most of the drilling problems have occurred in the overburden, especially in the shale formations. These instabilities which are encountered during exploration through development drilling may be affected by the properties of both shale (in-situ stress, mineralogy, strength) and the drilling fluid-shale interactions (fluid density, salinity, ionic concentration and swelling) 1,2,3 . Micro-fracture, fissures and weak bedding planes will also destabilize shale as drilling fluid penetrates the formations. The penetrated fluid will increase pore pressure, reduce effective stress and reduce shale formation strength. Shale stability should take into account both mechanical instability induced by stress and strength, and complex dril...

Modelling of Time-Dependent Wellbore Collapse in Hard Brittle Shale Formation under Underbalanced Drilling Condition

“…where κ ′ is the hardening parameter, which is influenced by the hydration effect. The Drucker-Pracker yield criterion has been widely used in the problem of wellbore stability in shale formations to describe the elastoplastic behavior of shale [24][25][26][27][28]. In this study, the elastoplastic calculation is performed using the Drucker-Prager criterion.…”

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.).…”