Summary

In the northwest shelf, a region of growing importance for oil and gas production in Australia, drilling delays and well suspension before reaching targets have occurred frequently because of wellbore instability. As a result, the economic benefits from new drilling technologies are significantly reduced because of high additional drilling costs. To improve drilling efficiency and minimize drilling costs, it is crucial to develop and apply a practical approach for optimizing wellbore instability management.

This paper initially presents guidelines, in the form of design charts, for efficient wellbore stability analysis and wellbore profile design. A comprehensive, practical approach that uses the design charts for wellbore profile design is described subsequently. The application of the design charts is demonstrated through field case studies for a range of in-situ stress regimes.

Introduction

Drilling delays and suspension of wells before reaching the targets have occurred1,2 frequently in the northwest shelf because of wellbore instability. A practical approach to optimize wellbore instability management is developed and applied in this paper to improve drilling efficiency and minimize costs.

This approach is presented in three parts: presentation of guidelines in the form of design charts, description of the design charts' use, and their application to field case studies for a range of in-situ stress regimes.

The study shows that in-situ stress regime, material strength property, strength anisotropy, and, to a lesser extent, material poroelastic anisotropy and induced pore pressure are critical to wellbore stability. The design charts enable assessments to be made readily on wellbore drillability, optimum wellbore profile, and the influence of various critical factors on wellbore stability; they consolidate the factors into a manageable, pragmatic framework. Understanding the influence of these factors will enable the determination of an efficient approach to wellbore stability analysis. Applying the charts to field case studies demonstrates the practicality and feasibility of the approach in wellbore profile design for efficient wellbore instability management.

Wellbore Failure and Prevention

To better understand the design charts, it is necessary to briefly describe mechanical (stress-induced) wellbore failure mechanisms and their prevention.

Drilling a well in a formation changes the initial stress state and causes stress redistribution within the rock surrounding the wellbore. The redistributed stress state may exceed either the tensile or shear strength of the formation, which leads to failure. The stress state around the wellbore comprises contributions from in-situ stresses and from the wellbore pressure imposed by the mud column. Fig. 1 depicts different failure modes in relation to the principal stresses imposed on the material. When the mud pressure is not sufficiently high to support the wellbore, breakout, toric shear failures, and exfoliation tensile failure can occur. However, when the mud pressure is excessively high, it can cause helical and elongated shear failures and hydraulic fracture.

In addition, because of the presence of bedding planes in shale, wellbore failure may be initiated by shear or tensile failure of the planes of weakness. Therefore, wellbore stability analysis should incorporate bedding-plane failure.

Ideally, a mud-weight program should provide a mud pressure that can prevent failure of any type. Fig. 2 shows an example of the critical mud weights required to prevent both tensile and shear failures of intact rock and bedding planes. The highest lower bound and the lowest upper bound values define the safe mud-weight window to prevent wellbore instability.

However, there can be no safe mud-weight window for certain wellbores and stress regimes (i.e., one or more types of failure cannot be avoided). For these wellbores, the adopted mud weight should provide a pressure that can either prevent the most severe failure or reduce the failure scale of any type. Hydraulic fracture and breakout are the most commonly experienced failure types in the field and often have more severe consequences than other types of failure. Hydraulic fracturing may lead to severe mud losses, while breakout failure may lead to pack-off or hole collapse. When there is no mud weight to prevent both simultaneously, priority is normally given to preventing hydraulic fracture propagation while minimizing breakout as much as possible.

Design Charts

Results from previous works have shown that the factors critical to wellbore stability include orientation and magnitude of in-situ stresses, wellbore trajectory, material poroelastic and strength properties, bedding planes, induced pore pressure, and mud pressure. 3–7 To design a mud-weight program, a wellbore stability assessment to produce critical mud-weight plots, such as those shown in Fig. 2, needs to be conducted. Such analysis requires knowledge of all the factors. The extent to which these factors influence wellbore stability varies dramatically and must be assessed. This knowledge will also assist in deciding which parameters need to be determined accurately.

A series of parametric studies has been conducted to investigate the effects of each factor in detail.8–10 The axis conventions of in-situ stress regime, wellbore trajectory, and material anisotropy used in the analyses are shown in Fig. 3. In these studies, the material was assumed to behave in accordance with poroelasticity theory, and a range of input parameter values was used. The design charts developed from the parametric studies are based on all in-situ stress regimes that may be encountered in drilling operations (see Fig. 4). The range of poroelastic and strength parameters used in these analyses are given in Tables 1 through 4. The detailed description of the analysis conditions and the determination of the input data is presented in previous publications.8–10

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