Numerical Modeling of Wellbore Instability (Tensile Failure) Using Fracture Mechanics Approach

Numerical Modeling of Wellbore Instability (Tensile Failure) Using Fracture Mechanics Approach.

ABSTRACT

When a well is drilled, the equilibrium in-situ stress is changed. In order to support the stress relief induced by the drilling and to prevent hydrocarbon influx into the cavity, the borehole is filled with a fluid.

These operations create new stress configurations. The main point in wellbore projects is the definition of the drilling fluid density to keep the wellbore stable.

The lower bound to the fluid density is the collapse stress that is the limit to shearing. The upper bound is the fracture stress that limits the tensile failure. The fluid densities between these limits is named safe mud weight window.

Conventional wellbore stability analysis usually considers the effects of shear or tensile failure using failure criteria that are modeled based on the strength of the formation.

This thesis uses numerical finite element method techniques to simulate cracking phenomena that can lead to instability of well configurations within and between shale formations that are relevant to oil wells under pressure.

The range of critical conditions associated with possible crack lengths are established by equating the computed crack driving forces to the ranges of published fracture toughness data reported in earlier studies.

The ranges of pressures associated with upper mud weight drilling pressure are thus established and compared with the prediction from empirical theories.

INTRODUCTION

Wellbore stability is a serious drilling problem that cost the oil and gas industry over $500 -$1000 million each year. It is also reported that shale account for 75% of all formations drilled by the oil and gas sector, and 90% of wellbore stability problem occur mainly in the shale formations (Lal et al,1999).

Wellbore instability has become an increasing concern for horizontal and extended reach wells, especially with the move towards completely open hole lateral section, and in some cases, open hole build-up section through shale cap rocks (Tan et al, 2004).

More recent drilling innovations such as underbalanced drilling techniques, high-pressure jet drilling, re-entry horizontal wells and multiple laterals from a single vertical or horizontal well often give rise to challenging wellbore stability questions (Kristiansen, 2004).

Over there years models have been developed to solve the problems associated with shale instability though limited, the models do not capture the varying mechanical properties over the depths of the wells. At present, the mechanical property measurements are made on core samples that are expensive to extract and test using the convectional mechanical testing approach.

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