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.

In this chapter, we explore the composition and fabric of unconventional reservoir rocks in order to understand the variations in elastic properties and anisotropy. First, we survey the range of compositions in unconventional basins and develop a simple classification for the various lithofacies. We then discuss each of the constituents of the rock matrix and describe their role in forming the rock fabric at various scales. The next section covers laboratory measurements of elastic properties and anisotropy, beginning with a brief review of methods for obtaining static, dynamic, anisotropic elastic properties. We discuss the variations of elastic properties with composition and fabric in the context of theoretical bounds from simple rock physics models of layered media to develop a physical understanding of microstructural controls on the stiffness of the rock matrix. The final section covers how elastic properties are estimated from geophysical well logs and reservoir-scale seismic studies. We compare field- and lab-derived elastic properties and discuss their applications for understanding the in situ physical properties of unconventional reservoir rocks.

The goal of this book is to address a range of topics that affect the recovery of hydrocarbons from extremely low-permeability unconventional oil and gas reservoirs. While there are various definitions of unconventional reservoirs, in this book we consider oil- and gas-bearing formations with permeabilities so low that economically meaningful production can only be realized through horizontal drilling and multi-stage hydraulic fracturing. These reservoirs have permeabilities measured in nanodarcies, not millidarcies – in other words, a million times lower than conventional reservoirs. Despite their ultra-low permeability, there is no question about the scale and impact of production from unconventional oil and reservoirs in the US and Canada over the past decade.

In the previous chapter we presented examples of earthquakes triggered during hydraulic fracturing, injection of flowback water after hydraulic fracturing and injection of produced water. In this chapter we address steps that can be taken to minimize the occurrence of such events. Of course, one of the most obvious ways to avoid injection-induced earthquakes is to minimize injection volumes. It’s not a coincidence that areas in Pennsylvania where nearly all the hydraulic fracturing flowback water is recycled have very few injection-induced earthquakes. In the sections that follow, we first discuss the issue of avoiding injection into potentially active faults.

In this chapter, we continue to explore the mechanical properties of unconventional reservoir rocks by considering deformation mechanisms active at various stress and strain conditions. Specifically, we will focus on rock strength – the stress required for brittle failure of intact rock – and ductility – the time-dependent (viscous) strain response as a function of stress.

Production from unconventional reservoirs requires hydraulic fracturing and stimulation of pre-existing faults in order to access more reservoir surface area. Diffusion of fluid pressure from hydraulic fractures induces shear slip on faults by lowering the effective normal stress (Chapter 10). Induced fault slip increases formation permeability through inelastic damage in the surrounding rock and creates a network of relative permeability flow paths that increase access to the ultra-low permeability rock matrix. Slip on pre-existing faults is documented as microseismic events that cluster around hydraulic fractures and are thought to define the stimulated rock volume from which hydrocarbons are produced (Chapter 12). While this paradigm is widely accepted, multiple lines of evidence indicate that the deformation associated with microseismicity can only account for a fraction of production. To understand the relationship between hydraulic stimulation and production, it is important to consider under what conditions faults will slip and whether or not fault slip will cause microseismic events.