In this chapter, we discuss wellbore stability during and after drilling. This includes basic mechanisms like shear failure and tensile failure around boreholes (lower and upper mud weight limits). Symptoms, reasons and consequences are described. Additional effects for shale stability are presented, like time effects, temperature effects, fluid effects, shale anisotropy and creep. Some practical issues related to drilling in deep water, hole cleaning, surge and swab are discussed. Stability issues in different lithologies (sandstone, carbonate, salt, coal) are described. Problems related to drilling in depleted reservoirs are presented together with some practical recommendations. The possibility of using creeping shale as a barrier for plug and abandonment is discussed.

In this Appendix, we give typical values for a set of relevant parameters, for some relevant rock types and materials. We also provide some empirical correlations for estimation of strength based on log data (sonic transit time and porosity).

Publisher Summary This chapter takes a look at some operational aspects related to solids production. When hydrocarbons are produced from a reservoir, solid particles sometimes follow the reservoir fluid into the well. This unintended byproduct of the hydrocarbon production is called solids production. Mechanisms that may cause sand production are then discussed in the chapter, followed by discussions on how to predict sand production. Chalk production is discussed more briefly, which reflects that the public literature is rather limited on this subject. Moreover, methods to control solids production involves additional costs and usually results in the reduced production, such that in later years a more relaxed tolerance for solids production is sometimes considered. When dealing with heavy oil, solids production is even deliberately provoked in some cases.

This chapter discusses basic processes related to hydraulic fracturing. Hydraulic fracturing in rocks takes place when the fluid pressure within the rock exceeds the smallest principal stress plus the tensile strength of the rock. This results in tensile failure or splitting of the rock. A hydraulic fracture may be initiated by natural, geological processes in the earth whereby the fluid pressure increases and/or the smallest principal stress decreases. Artificial or man-made hydraulic fractures in petroleum activities are normally initiated by increasing the fluid pressure in the borehole to the point where the smallest principal stress at the borehole becomes tensile. Hydraulic fracturing has been used commercially as a stimulation technique in the petroleum industry since the early fifties. Finally, this chapter concludes by exploring that, such fracturing jobs are designed to stimulate production from reservoirs with low permeability.

In this chapter, we present basic principles of hydraulic fracturing, starting with tensile failure around a borehole in an elastic formation. Criteria for fracture initiation are presented, both for an impermeable and a fully permeable borehole wall. Fracture orientation, growth, confinement and geometries are discussed. The fracture closure process is of special interest, since the smallest stress in a formation can be determined from this process. This is discussed in relation to shut-in/decline tests and flowback tests. Some aspects of thermal fracturing are described. Finally, a brief introduction is given to mechanics and monitoring of fracturing in unconventional reservoirs.

This chapter discusses failure mechanics. This chapter discusses the most elementary and well known models for rock failure. It is important to note, that these are only simplified descriptions of real rock behavior. This chapter illustrates that rock failure is a complex process, which is still not fully understood. Moreover, much of the framework used to handle rock failure is therefore based on convenient mathematical descriptions of observed behavior, rather than derivations from basic laws of physics. The chapter assumes that rocks are homogeneous and isotropic. This chapter discusses anisotropy, while some consequences of inhomogeneities are briefly addresses in the chapter. It also takes a look at a couple of the most important tests used to measure rock strength, the uniaxial and triaxial tests, in order to illustrate the complexity of rock failure, and to introduce some basic concepts.

Publisher Summary This chapter discusses the rock mechanical core analysis. Cores provide a possibility for direct measurement of, for example, rock strength parameters and static elastic properties. This chapter starts by looking at the rocks to be tested: the cores in the laboratory may not be fully representative of the formation one intends to study. They may also have been altered during coring and subsequent handling. The chapter also discusses the laboratory equipment and measurement techniques applied in the rock mechanical testing, including also acoustic measurements. This chapter illustrates that in some cases it may be useful to perform a set of simpler measurements, which can be used as indicators of rock strength and stiffness. This can provide information at low cost and low material consumption. If performed at the rig-site, results may be available closer to real-time. Finally, this chapter concludes by describing some of these so-called index tests.

In this chapter, we describe the stress and pore pressure conditions around a circular hole in a porous, linearly elastic formation. Next, we establish borehole failure criteria by combining the effective stresses at the borehole wall with shear and tensile failure criteria. Borehole stability beyond failure initiation. We also include descriptions of elliptical holes and spherical cavities, and anisotropic and nonlinear formation properties.

This chapter deals with the mechanical properties and in situ stresses from field data. The chapter looks at methods for estimating mechanical properties, and in situ stresses from field tests and field data. The methods can be grouped into three main categories: measurements while drilling, wireline logging methods, and well tests. The in situ stresses are given by the three principal stresses and three parameters giving the orientation of the principal stresses. The stress level determines whether a rock is critically loaded or not. Additionally, the in situ stresses influence both the elastic parameters and the strength parameters. This chapter shows that the strength of a material is dependent on the stress level, and the failure criteria that describe actual data normally have at least 2–3 adjustable parameters. Finally, this chapter is devoted to various types of acoustic measurements, primarily from downhole measurements, but also from rig measurements. The principles of acoustic wave propagation around boreholes are also addressed in the chapter.

The focus of this chapter is P- and S-wave propagation in porous rocks. Starting from the wave equation for isotropic solids, we extend the description to poroelastic and anisotropic rocks, including dispersion and attenuation mechanisms. We emphasise the important link between static rock mechanics and dynamic rock acoustics. We outline the basics of seismic and sonic borehole measurements, and give a brief background of acoustic emission mechanisms with relevance to microseismicity.

In this appendix, we list often-used formulae that would burden the main text too much. Various expressions for stress invariants are listed, including their relation to the principal stresses. Compact expressions for isotropic linear thermoporoelasticity are given, followed by useful formulas for transverse isotropic elastic materials. Some details on acoustic wave propagation and stresses in the vicinity of a crack tip are given, followed by formulas for solids production and subsidence. Finally, the expressions for common vector operators in cylindrical coordinates are summarised.