4.1 Preliminary concepts

4.1.1 Microstructure of geologic materials

The microstructure of rocks varies widely, from lumped crystals in igneous rocks to fossil carbonate skeletons in diatomite-rich chalk. We will discuss mostly sedimentary rocks (shown in Fig. 4.1). However, igneous rocks can also host hydrocarbons (really? how?) and constitute the basement of sedimentary basins. For example, induced seismicity from deep injection of produced-water mostly originates in basement igneous rocks. Sedimentary rocks include shales, sandstones, and carbonates among other types. The microstructure of rocks governs their failure properties and characteristics. For example, uncemented sands cannot hold tensile stresses (Fig. 4.1-a). At low mean effective stress (as in the sandboarding picture) rock failure happens through grain rotating and roll-over. Sandstone is formed by cemented grains (Fig. 4.1-b). At relatively high porosity, the strength of sandstones is dominated by the strength of cemented contacts (bonds). At failure, the bonds rather than the grains tend to break. Matrix-supported carbonates form a continuous mineral matrix (Fig. 4.1-c). Failure usually involves cracking of the solid matrix.

Figure 4.1: Influence of rock microstructure on failure mechanisms.

4.1.2 Length scales v.s. process zone size

Petroleum and subsurface engineering involves rock failure at many length scales, from the millimeter-scale to the kilometer-scale (Figure 4.2). The failure properties of rock (and many other properties too) depend on the length scale of analysis. Small-scale process zones engage the rock “matrix” properties. Rock cutting at the drill-bit scale and wellbore stability (in homogeneous and non-fractured rock) are two examples. The samples we test in the laboratory are at this small scale as well. Large-scale process zones involve fractures, multiple sedimentary layers, and faults. For example, hydraulic fracturing tends to reactivate neighboring fractures in shear and reservoir depletion can reactivate large faults in shear as well. Recognizing the appropriate length-scale is extremely important to use adequately the rock strength measured in the laboratory and simple mechanical formulations such as linear elasticity.

Figure 4.2: Rock failure properties are a function of process-zone size and length scale.

4.1.3 Overview of types of rock failure

Rock yield (plastic deformation) and failure can happen due to tensile stresses, shear stresses, compressive stresses, and a combination of the three. The following sections explore these types of rock damage separately.

Figure 4.3: Overview of rock failure modes: tension, shear, and compression.