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.