Injection-Driven Shear Slip and the Coupled Permeability Evolution of Granite Fractures for EGS Stimulation

Ye, Zhi (The University of Oklahoma) | Janis, Michael (The University of Oklahoma) | Ghassemi, Ahmad (The University of Oklahoma)

OnePetro 

ABSTRACT: Permeability enhancement through shear slip has been considered as standard treatment of engineered geothermal systems (EGS). The process reactivates pre-existing fractures, making them slip and dilate using fluid pressures below the minimum principal stress resulting in increased permeability. It can also cause fracture propagation in the shear and tensile modes creating secondary cracks. Control and optimization of shear stimulation can be achieved by studying how fracture permeability evolves with shear slip and dilation. However, most experimental studies that have considered fracture slip and permeability evolution have used force-driven shear tests or have manually displaced the specimens to represent fracture slip. A few studies have considered fluid injection-driven slip but only using saw-cut smooth joints. In this work, we have conducted shear slip test by water injection on rough fractures. Water was injected into a granite sample containing a single tensile rough fracture to induce shear slip under triaxial conditions. Flow rate during shear slip was measured to investigate fractures’ permeability evolution. In addition, the effects of confining pressure, differential stress, and injection pressure on stress-dependent permeability of the granite fractures were characterized. We tested three separate samples using different methods. Non-shear flow tests were conducted on a fractured Sierra White granite sample (SW #1) under both hydrostatic and triaxial conditions to characterize stress-dependent fracture permeability. We observed a linear relationship between flow rate and injection pressure, and an exponential relationship between flow rate and confining pressure. In addition, fluid injection-driven shear tests were performed on fractured samples SW #2 and SW #3 using constant stress mode and constant displacement mode, respectively. Shear rates observed during the constant stress test were ˜10−3 m/s and yielded up to 3 orders of magnitude increases in flow rate while the constant displacement mode caused ˜10−5 m/s sliding rate and 20 times increase in flow rate through the fracture. Furthermore, permeability evolution during injection-driven shearing tends to linearly evolve with the shear slip and dilation. The irreversible behavior of shear slip was found to explain the permeability hysteresis during shear sliding.

1. INTRODUCTION

Enhanced Geothermal Systems (EGS) represent enormous renewable energy reserves (Tester et al., 2006; Brown et al., 2012). Field experiments have been carried out at sites such as Newberry, Desert Peak, Fenton Hill, Soultz-sous-Foretz and Rosemanowes. The primary objective of these field demonstrations has been to create flow paths through basement rock to facilitate heat transfer. Various stimulation mechanisms have been tested at these sites depending on the geology of the host rock, fault structures, natural fractures etc. Shear slip (Pine and Batchelor, 1984; Willis-Richards et al., 1996; Baria et al., 1999; Rahman et al., 2002; Nygren and Ghassemi, 2005; Cheng and Ghassemi, 2016) and the propagation of natural fractures are important mechanisms of permeability enhancement in EGS reservoirs (Min et al., 2010; Ghassemi, 2011; Huang et al., 2013; Jung, 2013; McClure and Horne, 2013; Kamali and Ghassemi, 2016). Often, crystalline basement rocks have pre-existing sealed fracture networks that are activated prior to the creation of new fractures since the sealing material is usually much weaker than the surrounding rock. The objective of shear stimulation is to create a stimulated volume with increased permeability to circulate large volumes of water. This is carried out at pressures below the minimum principle stress to avoid excessive fracture growth and large surface area. Fractures that are sheared tend to self-prop due to asperities.