2.5 Fluid Pressure and Hydraulic Fractures
Hydraulic fracturing, a mechanism used in the field of unconventional oil and gas extraction, uses the same principles as the phenomenon of natural hydraulic fracturing. These principles are explained in this section.
Positive fluid pressure (p) is present in a rock when it is saturated, which is when all its pores are filled by a fluid (or fluids). This fluid can be water and/or oil, the latter formed by the degradation of organic matter. When the flow of these fluids is not impeded by any type of confinement, a hydrostatic fluid pressure is present. However, at depth, fluid is often confined and this leads to overpressure or abnormal fluid pressure (e.g., Ramsay & Hubber, 1987; Fossen, 2016). Fluid pressure is isotropic and reduces each one of the main stresses equally, resulting in the following effective stresses: σ1 − p, σ2 − p and σ3 − p. This implies that the Mohr circle, in the Mohr diagram, undergoes a shift to the left (Figure 20), and the effective normal stress on any plane is also reduced by the fluid pressure. The shift does not affect the shear stress.
Figure 20 – Overpressure—or abnormal fluid pressure (p)—may generate fractures under otherwise stable conditions. Traditionally only joints are called hydraulic fractures. a) The differential stress is large and the isotropic reduction in stress, by the fluid pressure p, causes the displacement of the circle to the left until it touches the failure envelope (i.e., shear stress exceeds the shear failure criterion). b) The differential stress is smaller, so that with displacement to the left, the circle touches the Griffith failure envelope, and hybrid fractures are formed. c) The differential stress is even smaller and, with displacement to the left, the Mohr circle touches the Griffith envelope at point T, causing the generation of joints (modified from Cosgrove, 1998). The scale of the axes is constant for ease of reference when comparing the Mohr circle sizes.
The stress field in the crust tends to be compressive and, in some cases, can be lithostatic, that is, the principal stresses are derived solely from the weight of the overlying rock column at a given depth. Under lithostatic conditions, the differential stress is not great enough to cause brittle failure. However, even under these conditions, fractures can be generated when the fluid pressure is high enough. Overpressure explains the formation of joints even at great depths (5 to 10 km or even more), where the stresses are strongly compressive (Davis et al., 2011). Depending on both the magnitude of p and the mechanical properties of the affected rocks, the shift of the Mohr circle to the left can be large enough to cause the circle to reach either the shear envelope, generating shear fractures, or the point T, generating joints; it can also reach the Griffith’s failure envelope generating hybrid fractures (Figure 20). Cosgrove (1998) and Cosgrove & Hudson (2016) state that all these types of fracturing are the expression of hydraulic fracturing. However, traditionally, only joints formed due to overpressure under compressive stresses are called hydraulic fractures (e.g., Engelder, 1987; Davis et al., 2011; Brenner & Gudmundsson, 2004). The fracture orientation of the hydraulic fractures, as is the case for any fracture, depends on the tectonic regime, which is discussed in Section 3.