Abstract:

This study presents a numerical approach to account for the non-linear clay plasticity on wellbore stability analysis for deep water drilling. The numerical model considers the stress path dependent (anisotropic) strain-hardening/post peak softening behavior, which is typically observed during undrained laboratory testing of clay materials, using the NGI-ADP soil model. A linear-elastic assumption estimated almost no drilling window that did not fit with the field drilling experience. The analysis including the hardening behavior results in a smaller failure zone around the wellbore wall than the linear-elastic perfectly plastic model. When the collapse criterion is defined as an extension of the failure zone that does not trigger a drastic reduction in the shear strength around the well, the proposed approach estimates a possible drilling window for a horizontal well, which was originally estimated as having no drilling window. This study indicates that proper modelling of clay plasticity can provide an efficient solution for deep water drilling of shallow reservoirs.

Introduction

Shallow clay sections in deep water settings are a complex challenge regarding wellbore stability analysis. Often, clay strength is low compared to pore pressures and the drilled formation behaves under an undrained condition due to relatively low permeability compared to typical drilling times. Consequently, collapse gradients, estimated using an idealized linear elastic or linear elastic perfectly plastic behavior, are commonly estimated as too high [1].

As illustrated in Figure 1, clay materials show significant non-linear plastic ductility, more than mudstones or stiff shales. When the idealized elastic model is used for the clay material, importance of nonlinear plasticity is ignored. Neglecting plasticity may result in inaccurate wellbore analyses. It is common in the field to observe that drilling through clays or soft shale formations are possible even beyond their elastic limit or the peak failure strain due to non-linear plasticity [2, 3]. Therefore, for an optimum well design in deep-water drilling scenarios, the non-linear plastic behavior of clay around the drilled wellbore should be properly taken into account.

This study investigates the effect of non-linear clay plasticity on wellbore stability by a numerical approach.

The numerical model considers anisotropic strain-hardening and softening behavior of clay, which is typically observed during undrained laboratory testing of clay material, using NGI-ADP soil model [4]. First, a typical plastic behavior of clay while drilling is discussed and compared to laboratory tests. The model is then applied to a deep water drilling design, which initially estimated almost no drilling window using a linear-elastic assumption; a mismatch with a field drilling experience. In the end, applicability of calculated plasticity on the collapse gradient is discussed.

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