ABSTRACT

Successful hydrocarbon recovery has become increasingly challenging with the oil and gas industry exploring reservoirs under unfavourable geological conditions. Several operations rely on wellbore stability usually provided by suitable mud-weight. Estimating the operating mud-weight window demands sufficient knowledge of in-situ stress conditions, well trajectory and formation material properties to eliminate instability issues. Instability issues become significantly important when drilling in challenging environments, such as depleted formations, highly deviated wells or strongly heterogeneous formations. Analytical calculations can predict the onset of plastic yielding and damage around a wellbore; however, frequently used criteria have two inherent limitations in not being able to capture 1) complex stress distribution around wellbores deviated from the in-situ stress direction and/or in non-homogeneous formations, and 2) material softening/hardening due to formation damage and redistribution of stress influencing further damage or stability/instability. 3-dimensional numerical modelling combined with advanced constitutive material models can capture stress conditions around wellbores of any orientation as well as non-isotropic characteristics and post-yield strength softening. Using efficient modelling techniques, it is possible to perform detailed wellbore stability analysis for a range of stress conditions, well trajectories and formation anisotropy. Consideration of the results provides beneficial information for drilling, such as operating mud weight window and predicted cuttings volume. In this study, Elfen wellbore software is used to provide detailed assessment of both wellbore deviation and formation anisotropy including bedding plane effects. A representative volume is calculated for each case that corresponds to both the deteriorated material around the wellbore and also undamaged cavings separated from the wellbore surface. The combination of such modelling and results assessment techniques available in Elfen wellbore, aims to enhance current wellbore stability assessments and limit the risks associated with drilling in increasingly difficult conditions.

1.INTRODUCTION

Instability of subsurface excavations is critical and can pose serious problems affecting the timing and success of a project. Prediction of such instabilities has long been recognised as a key factor in many industries. With the oil and gas industry dealing with increasingly challenging geological environments and with complex recovery techniques becoming standard, sufficient prediction and wellbore analysis, requires a wider framework, accounting for difficult geological conditions. Such conditions concern drilling in depleted formations, highly deviated wells or laminated formations. Similar geological environments are also encountered in other applications facing subsurface integrity issues and "reservoir containment geomechanics" (Schultz et al., 2016) such as geothermal fields (Moeck and Bakers, 2011; Ghassemi, 2012) and carbon dioxide sequestration (Streit and Hillis, 2004; Rutqvist, 2012; Zoback and Gorelick, 2012; Altman et al., 2014).

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