Rock core samples may be tested by destructive methods to determine the material’s mechanical strength. These measurements may however introduce further uncertainties, which often evolve around how to apply the laboratory derived data to subsurface conditions. Indeed, strength discrepancies have been reported between cylindrical plugs and hollow cylinders, which suggest a geometry issue. A scaling concern also presents itself when using small laboratory samples to represent an operationally exposed rock volume. In addition, matching the correct failure model to the lab measurements is often ambiguous. This article reflects on these issues from a wellbore stability perspective. A case is presented where testing datasets (including both triaxial and hollow cylinder tests) are used to characterise the material’s strength suite. Implementing these findings to a wellbore stability calculation shows how an incomplete or poorly interpreted dataset may be misleading, thereby highlighting the inherent uncertainties and difficulties associated with borehole stability modelling.

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).

Various laboratory experiments have shown that matching classical weak plane models to anisotropic rock strength test results can be challenging. Discrepancies between the applied models and laboratory observations are often explained by heterogeneities or local variations in both the intrinsic material and bedding planes, without physically addressing possible modeling flaws. Recent developments have however challenged this approach by introducing the patchy weakness concept. Here, local shear sliding is allowed to occur prior to macroscopic failure, thereby reducing strength at lower loading angles than predicted by the classical weak plane theory. In this paper the original 2D formulation is extended to describe failure under a fully 3D stress state. The updated criterion is applied to a borehole geometry, and based on laboratory tests on core plugs the full strength parameter suite of a caprock shale is derived. This allows for improved predictive modeling of wellbore stability in rock with anisotropic strength.

A working methodology to minimize wellbore stability problems has been established through the use of unique laboratory tests, an experimental database for fluid-rock interaction and physical properties of shales, and an integrated modeling approach utilizing different types of experimental and field data. The model simulations provide output accounting for a wide range of input parameters such as well inclination, mud chemistry, rock mechanical properties, field stresses and pressures, formation anisotropy, and shale mineralogy. The model output can subsequently be used to diagnose field drilling problems or to design drilling operations. As an example, data from a high pressure/high temperature (HP/HT) field offshore Mid-Norway as well as a field in the overpressured shales in the southern part of the Norwegian North Sea have been analyzed and compared.

Abstract Several challenges are experienced with wellbores when drilling in shale above and within reservoirs. These are especially prevalent in deviated wells and wells through depleted intervals caused by some years of production. Drilling the well to total depth of the wellbore section might be easy. However, in some cases depending upon formation properties and drilling conditions it is a big challenge to come out of the hole and subsequently run casing or liner. It is of great importance to take the right actions regarding recommended downhole parameters like mud circulation rate, RPM on drillstring and pulling speed. Some of the wellbore aspects to discuss are: unstable hole, small margins between mudweight and collapse pressure, what is the right technique to use with the drillstring, flow rates and finally oil based mud versus water based mud. In case of a marginal downhole situation, it would be beneficial to test out some effects in a laboratory beforehand. Such questions were discussed and became an issue within a joint industry project on borehole stability in shales run at SINTEF Petroleum Research (SPR). A modified test cell was designed and used with the purpose of systematically testing these effects in the laboratory. This paper describes how this equipment was designed and tested in order to achieve reliable laboratory results as well as results from initial work on outcrop shale. The main objective was to a certain extent to visualize some effects in a laboratory and conclude with some good recommendations regarding drilling procedures on the rig offshore. Thus for testing some of the down hole effects, a long period of qualification tests was gone through. This was important before real tests could be done on shale at simulated downhole condition. Testing was performed on hollow cylinder samples. The initial results show that it is possible to run such laboratory simulations. It is possible to a certain extent to visualize some of the effects with varying flowrates and RPM on a drillstring with stabilizer. The laboratory tests so far suggest that high RPM on a drillstring can have destabilizing effects on a wellbore through a section with shale that has been exposed to very close to collapse conditions, calling for optimized drilling procedures in situ.

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.

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.

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.