Journal Article

Frictional Stability of Laumontite Under Hydrothermal Conditions and Implications for Injection‐Induced Seismicity in the Gonghe Geothermal Reservoir, Northwest China

Yao, Leihua, Zhang, Chongyuan, Elsworth, Derek, Zhang, Hao, He, Manchao, Zhang, Lei, Hu, Zijuan, Zhang, Linyou

2024

Overview

Laumontite is a common and potentially frictionally unstable hydrothermal alteration product present in deep faults of the Gonghe EGS reservoir. We characterize the friction‐stability characteristics of synthetic laumontite gouge under in situ reservoir conditions. The pure laumontite gouge is frictionally strong (μ = 0.73–0.98) and the quartz/laumontite mixture (1:1) is generally less strong (μ = 0.73–0.78) under experimental conditions (Pc = 95 MPa, T = 90–250°C, Pf = 0–90 MPa). The shear velocity was stepped between 6.1, 0.61, then 0.061 μm/s for our experiments. For both gouges, the friction coefficient is independent of temperature and increases with elevated pore pressures. The pure gouge and mixture are strongly velocity‐weakening over a broad range in temperatures (∼90–220°C) and excess pore pressures (0–90 MPa) relevant to the Gonghe stimulation. Microearthquakes (MEQs) observed during stimulation are confined to within the broad depth range of inferred frictional instability—although fluid overpressures are also limited to this region. The observation that laumontite mixtures are frictionally unstable over a broad range of pressures and especially temperatures representative of EGS reservoirs and insensitive to the presence of the coexisting mineral phase (quartz) suggests its presence is a strong indicator of potential seismic hazard. Plain Language Summary Laumontite is a very low‐grade altered mineral that can easily occur in fractures or faults in granite, basalt, or sandstone. Laumontite is widely developed in the Gonghe geothermal reservoir of Western China. Fluid injection into deep geothermal rock mass may reactivate subsurface faults containing altered minerals and cause earthquakes. Hence, we conducted laboratory experimental analysis on the frictional characteristics of simulated laumontite gouge to further understand the impact of fluid injection on the triggering of deep fault earthquakes. These experiments were performed at conditions reflecting the temperature and pressure of the water injection depth of the Gonghe geothermal reservoir. The results showed that the fault's strength and friction stability strongly depend on pore pressure and temperature. Our study emphasizes the significant role of the altered mineral laumontite in controlling fault strength and stability, as well as its potential for inducing earthquakes. A possible implication of this work is that when selecting geothermal resource targets that require fluid‐injection operations, it is best to avoid laumontite‐rich sites or reservoir sections. Key Points We report the first evaluations of frictional stability properties for laumontite gouge and mixtures under hydrothermal conditions Gouges are strongly velocity‐weakening over a broad range of temperatures and insensitive to pressures and relative proportions This instability field is coincident with P‐T conditions typical for EGS and hence an indicator of ubiquitous seismic hazard

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Basalt / Coefficient of friction / Earthquakes / Enhanced geothermal systems / Fault lines / Faults / Fluid injection / fluid‐induced seismicity / Fractures / Friction / frictional properties / Geological hazards / geothermal reservoir / Geothermal resources / Hydrothermal alteration / hydrothermal conditions / Laboratory experimentation / laumontite / Microearthquakes / Minerals / Mixtures / Pore pressure / Pore water pressure / Quartz / Reservoirs / Sandstone / Sedimentary rocks / Seismic activity / Seismic hazard / Seismic stability / Seismicity / Stability / Stimulation / Temperature / Velocity / Water depth / Water injection