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.
CHAPTER ONE
1.1 Background and 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 question (Kristiansen, 2004).
Over there years model 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 convectional
mechanical testing approach. Interlaminar fracture in the shale
formations is also difficult to model using available strength-based
models.
Fracture mechanics approach can be used in determining mechanical
properties of rocks such as Compressive Strength, Young‟s moduli and
fracture toughness using cuttings that are obtain during convectional
drilling operations. These approaches give room for measurement of rock
mechanical properties across layers that are relevant for predicting
wellbore stability and Interlaminar failure. Results obtained from using
this approach can be incorporated into modeling software such as ABAQUS
CAE 6.12 (teaching Edition) for predicting wellbore instability under
different drilling conditions.
This work will focus on mechanical wellbore stability under
conditions that result in failure in the rock formation due to fracture.
Conditions such as mixed mode (axial versus shear) loading, mud window
weight will be studied to understand their effect on wellbore
instability.