Laboratory Demonstration of Fracture Caging in a Fault with Calibrated Microseismic Monitoring

Abstract

ABSTRACT: Induced seismicity is a major unsolved challenge that stands in the way of harnessing geothermal energy to its fullest extent. To confront this problem, fracture caging was proposed; where pre-drilled boundary wells are placed around an injection zone to contain fluid and limit seismic risk. Prior work has shown that tensile fractures can be caged, but seismicity is primarily caused by shear fractures rather than tensile fractures. This paper investigated the validity of fracture caging in shear faults loaded with anisotropic stresses. In our experiments to simulate what we call ‘Caged Geothermal Systems’ (CGS), we inject fluid into a repeatably constructed assembly of two aluminum wedges bonded by plaster while the embedded fault is held at a critical stress condition for shear-slip. Meanwhile, fluid production is enabled using a five-spot pattern of pre-drilled boundary wells (i.e., a fracture cage). Post-mortem fluid patterns, decreasing microseismicity over time, calibrated seismic magnitude, high recovery of the injected fluid, and the stability of the fault system during high-rate high-pressure fluid injection prove that boundary wells can cage shear in faults. Thus, CGS retains promise to solve the challenge of injection induced seismicity and thereby pave the path forward for geothermal energy technologies. 1. INTRODUCTION Geothermal resources could play a key role in the looming energy transition from fossil-fuels to alternative energy sources. Supplied from deep underground and independent on the weather conditions, geothermal offers a clean and low-carbon energy source with high baseload power potential. However, several barriers exist, including induced seismicity, in the way of harnessing geothermal energy to its fullest extent. Deep fluid injection during operation of an Enhanced Geothermal Systems (EGS) triggers seismic events of different extents with a worst scenario being damaging earthquakes (Elsworth, 2013). This seismic risk is of serious public concern and can cause irreversible damage. For example, Pohang (South Korea) and Basel (Switzerland) are two notable EGS sites where Mw 5.4 and Mw 3.4 earthquakes led to $75.8 million and $9 million damage to buildings and other structures, respectively (Kim et al., 2022; Edwards et al., 2015). Both sites were shut down due to the injection induced seismicity threat. These examples highlight the adverse role of induced seismicity in EGS development and the need for a reliable solution. To confront this problem, fracture caging was proposed; where pre-drilled boundary wells are placed around an injection zone to contain fluid and limit seismic risk (Frash et al., 2021). Prior laboratory experiments on acrylic, concrete, and granite blocks as well as numerical models have shown promising results for fracture caging to control induced seismicity. Particularly, we observed: (1) ceased hydraulic fracture growth with boundary wells, (2) contained high-rate high-pressure fluid flow within a cage, and (3) prevention of fluid leak-off and high recovery rates from production wells (Frash et al., 2015, 2019, 2021, 2023; Hu & Ghassemi, 2018; Fu et al., 2013; Settgast et al., 2017). The maximum event magnitude and total counts of acoustic emission (AE) were hypothesized to be limited by the cage's size (Frash et al., 2015). However, the prior work was focused on tensile fracture caging despite seismicity originating from shear fractures. Pre-existing shear faults under critical stress present the most challenging scenario for large-magnitude injection-induced seismicity. In this paper, we present our ongoing effort to investigate fracture caging with a new focus on critically loaded shear faults. Here, we anticipate that shear fracture caging should be indicated by a steady increase of microseismic events with fluid injection that is followed by a significant and sustained AE reduction once the fracture cage is established by boundary wells, despite continuing high-rate high-pressure fluid injection. Our preliminary results indicate that caged geothermal systems (CGS) retain promise to solve the challenge of injection induced seismicity and thereby pave the path forward for safer and more sustainable geothermal energy production.