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.
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.