ABSTRACT:

This paper examines how the predicted mechanisms of horizontal wellbore stability are related to the constitutive model describing the mechanical behavior of the geologic formation around the wellbores. Two critical state soil models are used: Modified Cam Clay [1] and MIT-E3 [2]. The horizontal wellbores are modeled by a 2D plane strain model with horizontal and vertical axes of symmetry using non-linear finite elements. The paper considers wellbores drilled in cross-anisotropic saturated porous formation of unlithified hard clay/soft shale. Short-term wellbore instability during drilling in low permeability formations is analyzed assuming undrained conditions. The paper compares the MCC and MIT-E3 predictions of critical mud pressures at which failure occurs; and deformations and shear strains around the wellbore at a reference mud pressure. For the horizontal wellbore, the MIT-E3 model predicts failure due to a local increase in inward deformations at the cavity wall and higher critical mud pressure than the underbalanced drilling limit. The MIT-E3 model predicts lower magnitudes of shear strains around the wellbore but higher critical mud pressure at failure than that predicted by the MCC model. The more sophisticated MIT-E3 with an asymmetric yield surface gives a more accurate prediction of failure in horizontal wellbores.

1. INTRODUCTION

Efforts to expand oil resources and explore new territories are vital to sustain hydrocarbon production in the next few decades. Part of these efforts includes oil production from very shallow oil reservoirs, located at depths less than 1,000m in both deepwater environments and onshore prospects. Effective exploitation of such reservoirs relies on a small number of surface drilling locations, with highly deviated wells drilled with complex directional trajectories. The formations encountered at such shallow depths are often poorlylithified and are more properly classed as unconsolidated rocks or stiff clays.

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