This chapter lays out a simplified roadmap to the main aspects of fracture mechanics that are important for geologic structural discontinuities. Fracture mechanics is a branch of engineering that nevertheless provides tools, concepts, and a mechanical context for interpreting the major characteristics and patterns of geologic discontinuities that are seen in the field (e.g., Rudnicki, 1980; Pollard and Segall, 1987; Engelder et al., 1993; Pollard and Fletcher, 2005; Segall, 2010; Gudmundsson, 2011).

In this chapter we illustrate several ways in which geomechanical issues affect the success of particular well completion strategies. As it is impossible to cover the wide range of possibilities for each of the active unconventional plays, we attempt to illustrate several fundamental principles affecting completions that are related to the other topics we consider in this book. We present several instructive case studies and review modeling results that illuminate several ways in which seemingly subtle geomechanical principles have a significant effect on hydraulic fracture growth and distribution of proppant in hydraulic fractures. While the periods of trial-and-error testing used to inform optimal well completion parameters are critically important, it is equally important that the testing be done in a context in which the maximum amount of information as possible is obtained.

In this chapter we first discuss a variety of aspects of triggered slip on pre-existing fractures and faults during multi-stage hydraulic fracturing. The importance of inducing shear slip on pre-existing faults during multi-stage hydraulic fracturing was introduced in Chapters 1, 7 and 8. After briefly motivating this topic, we discuss how, in the context of Coulomb faulting theory introduced in Chapter 4, the high pore pressure perturbation associated with multi-stage hydraulic fracturing is capable of triggering slip on pre-existing fractures and faults in the formations surrounding the hydraulic fractures.

As previously discussed, microseismic events are generated when pore pressure reaches a pre-existing fracture plane and induces slip. This process is described in more detail in Chapter 10 as well as how microseismic data can be used to better understand the stimulation process.

To use the microseismic data properly, it is important to understand what can be determined and the limitations of such information. To this end, the topics considered in this chapter briefly consider how microseismic monitoring is carried out, how we know the events reflect shear slip on pre-existing faults, how accurately we know the locations of the seismic events, what can be determined about the seismic sources in terms of the size of the faults that slip (and the distribution of fault sizes) and the geometry of slip as defined by focal plane mechanisms (first introduced in Chapter 7).

Understanding the flow and sorption properties of unconventional reservoir rocks is essential for predicting the movement of water and hydrocarbons during stimulation (injection), production and depletion. Unconventional reservoir rocks are unified (and essentially defined) by their ultra-low matrix permeability, which is approximately a million to a billion times less than that of conventional reservoir rocks. For this reason, hydraulic fracturing and shear stimulation on pre-existing faults (Chapters 8 and 10) are necessary to expose more surface area of the matrix for production. Although the initial flow behavior during injection and production is controlled by the properties of the fracture network, the long-term flow behavior is controlled by the ultra-low permeability matrix.

In this chapter we first provide a brief overview of some of the environmental impacts associated with large-scale development of unconventional oil and gas reservoirs. While it is beyond the scope of this book to address all of the potential environmental issues that could arise, there are several which are related to the topics considered elsewhere in the book. We then focus on the topic of induced seismicity associated with unconventional reservoir development – a significant, if somewhat unexpected, environmental impact.

The pore networks of the rock matrix, and the pore fluids contained within it, determine the flow properties of unconventional reservoir rocks. Before examining the mechanisms and timescales of flow in Chapter 6, we will explore the characteristics of matrix pore networks, as well as the occurrence of in situ pore fluids and the flow properties of multiphase systems.

We first review the length scales relevant to pore networks and pore fluids in unconventional reservoirs, and discuss the sources of porosity in the rock matrix. We then address the issue of how to characterize and quantify matrix porosity and pore characteristics (size, shape and orientation). Through a detailed review of characterization methods, we explore how different methods may be validated by each other and/or combined for more complete coverage of length scales.

There are a number of interrelated topics presented in this chapter that define the geomechanical state of unconventional reservoirs. As alluded to in Chapter 1 (and expanded upon in Chapters 10–12) the process of hydraulic fracturing and stimulating slip on pre-existing fractures and faults is critical to the success of production from unconventional formations with extremely low permeability. This entire process depends on the interplay between the stress field, pre-existing fractures and faults, pore pressure and the perturbation of pore pressure that occurs during hydraulic fracturing. Chapter 8 discusses how this kind of comprehensive geomechanical characterization affects hydraulic fracturing and Chapter 11 discusses how the geomechanical characteristics of underlying and overlying formations affect vertical hydraulic fracture growth.

As introduced in Chapter 1, production rates from unconventional wells decline very rapidly during the first 2–3 years of production. In this chapter we first demonstrate that production rates (and cumulative production) are dominated by linear flow from the almost impermeable matrix into much more permeable fracture planes. The permeable fracture planes consist of the hydraulic fractures themselves and the pre-existing fractures and faults that have slipped in shear during stimulation. As we show, the rapid decrease in production rates is a natural consequence of depletion in these extremely low permeability formations. We argue that the cumulative area of permeable fracture planes created during stimulation is a key factor influencing ultimate resource recovery.

In this chapter we review several key aspects of horizontal drilling and multi-stage hydraulic fracturing. While this is not an engineering text, it is necessary to briefly cover several operational procedures associated with horizontal drilling and hydraulic fracturing to provide readers with a basic understanding of what is being typically done in the field, and why. More detailed information about the topics in this chapter is available from Economides & Nolte (2000), Ahmed & Meehan (2016), Smith & Montgomery (2015) and other sources. Detournay (2016) offers a comprehensive review of the mechanics of hydraulic fracturing from the perspective of theoretical fracture mechanics.