Post-injection normal closure of fractures as a mechanism for induced seismicity

📝 Abstract

Understanding the controlling mechanisms underlying injection-induced seismicity is important for optimizing reservoir productivity and addressing seismicity-related concerns related to hydraulic stimulation in Enhanced Geothermal Systems. Hydraulic stimulation enhances permeability through elevated pressures, which cause normal deformations, and the shear slip of pre-existing fractures. Previous experiments indicate that fracture deformation in the normal direction reverses as the pressure decreases, e.g., at the end of stimulation. We hypothesize that this normal closure of fractures enhances pressure propagation away from the injection region and significantly increases the potential for post-injection seismicity. To test this hypothesis, hydraulic stimulation is modeled by numerically coupling fracture deformation, pressure diffusion and stress alterations for a synthetic geothermal reservoir in which the flow and mechanics are strongly affected by a complex three-dimensional fracture network. The role of the normal closure of fractures is verified by comparing simulations conducted with and without the normal closure effect.

💡 Analysis

Understanding the controlling mechanisms underlying injection-induced seismicity is important for optimizing reservoir productivity and addressing seismicity-related concerns related to hydraulic stimulation in Enhanced Geothermal Systems. Hydraulic stimulation enhances permeability through elevated pressures, which cause normal deformations, and the shear slip of pre-existing fractures. Previous experiments indicate that fracture deformation in the normal direction reverses as the pressure decreases, e.g., at the end of stimulation. We hypothesize that this normal closure of fractures enhances pressure propagation away from the injection region and significantly increases the potential for post-injection seismicity. To test this hypothesis, hydraulic stimulation is modeled by numerically coupling fracture deformation, pressure diffusion and stress alterations for a synthetic geothermal reservoir in which the flow and mechanics are strongly affected by a complex three-dimensional fracture network. The role of the normal closure of fractures is verified by comparing simulations conducted with and without the normal closure effect.

📄 Content

Post-injection normal closure of fractures as a mechanism for induced seismicity E. Ucar1, I. Berre1, 2, and E. Keilegavlen1
1 Department of Mathematics, University of Bergen, Bergen, Norway. 2 Christian Michelsen Research, Bergen, Norway. Corresponding author: Eren Ucar (eren.ucar@uib.no)
Key Points: • Normal closure of stimulated fractures after the termination of injection enhances post- injection seismicity. • Processes are strongly affected by the complex structure of three-dimensional fracture networks.
• Consistent with microseismic data analyses, our simulations show that seismic events occur at the rim of the stimulation region.

Abstract Understanding the controlling mechanisms underlying injection-induced seismicity is important for optimizing reservoir productivity and addressing seismicity-related concerns related to hydraulic stimulation in Enhanced Geothermal Systems. Hydraulic stimulation enhances permeability through elevated pressures, which cause normal deformations, and the shear slip of pre-existing fractures. Previous experiments indicate that fracture deformation in the normal direction reverses as the pressure decreases, e.g., at the end of stimulation. We hypothesize that this normal closure of fractures enhances pressure propagation away from the injection region and significantly increases the potential for post-injection seismicity. To test this hypothesis, hydraulic stimulation is modeled by numerically coupling fracture deformation, pressure diffusion and stress alterations for a synthetic geothermal reservoir in which the flow and mechanics are strongly affected by a complex three-dimensional fracture network. The role of the normal closure of fractures is verified by comparing simulations conducted with and without the normal closure effect. 1 Introduction Enhanced Geothermal System (EGS) technology is considered key to unlocking geothermal energy resources because it can allow for the production of geothermal energy resources that are less dependent on initial hydrogeological conditions. EGSs can be created by hydraulically stimulating the reservoir to enhance permeability and achieve commercial flow rates. The elevated pressures activate pre-existing fractures by decreasing the friction resistance, which may lead to shear failure of the fractures depending on the fracture orientation relative to the direction and strength of the background stress anisotropy. Following shearing, the dilation of the fractures occurs in the normal direction of the fracture surfaces, which results in a permanent increase in the fracture permeability. Such a treatment is called shear dilation stimulation (which is also known as shear stimulation, hydroshearing or low-pressure stimulation). As long as the elevated pressures are below the minimum principal stress, shear dilation remains a dominant mechanism for fracture opening [Pine and Batchelor, 1984]. Hydraulic stimulation is essential for geothermal development in low-permeability formations, such as crystalline basement rocks. However, excessive induced seismicity is a by- product observed in many EGS projects [Majer et al., 2007]. Concerns related to induced seismicity can lead to the termination of costly projects, which was observed for the Basel geothermal project [Häring et al., 2008], or cause unfavorable public perception [Majer et al., 2007]. These concerns provide additional motivation to identify and understand the controlling mechanisms underlying the stimulation process and incorporate these mechanisms in modeling approaches.
Fluid injection causes elevated pressures inside the fractures and reduces contact forces, resulting in decreased frictional resistance between the fracture surfaces. These changes facilitate shear slip in fractures that are favorably oriented in the anisotropic background stress field. This mechanism is known as effective stress reduction and has been identified as the major cause of induced seismicity in numerous modeling studies [Bruel, 2007; Kohl and Mégel, 2007; Rothert and Shapiro, 2003]. The importance of stress redistribution following the shearing of rock for injection-induced seismicity has been emphasized [Catalli et al., 2013; Catalli et al., 2016], and the stress alteration has been coupled with the fluid flow in several models [Baisch et al., 2010;

McClure and Horne, 2011]. As mentioned by Baisch et al. [2010], the stress change in the reservoir caused by slip must be included during numerical modeling because the shear stress of the slipped area decreases by the shear slip and the shear stress at the neighboring region is correspondingly increased; hence, slip avalanches can be initiated.
Particular geothermal fields, such as Soultz-sous-Forêts, Basel and Paralana, provide interesting examples of induced seismicity because large-magnitude events occurred in the shut- in period of the hydraulic stimulations, which is the

This content is AI-processed based on ArXiv data.