The Potential for Low‐Grade Metamorphism to Facilitate Fault Instability in a Geothermal Reservoir. Issue 11 (8th June 2021)

• Record Type: Journal Article
• Authors: An, Mengke; Zhang, Fengshou; Min, Ki‐Bok; Elsworth, Derek; Marone, Chris; He, Changrong
• Is Part Of: Geophysical research letters. Volume 48:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-08
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL093552
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Native and fluid circulation‐induced metamorphic products may affect the stability of faults in geothermal reservoirs–particularly epidote (Ep) and chlorite (Cl). Our laboratory experiments conducted at hydrothermal conditions show that these two minerals, when precipitated on natural faults, may promote fault instability or failure under conditions typifying a geothermal reservoir. Shear experiments on Ep‐rich fault gouges indicate potentially unstable frictional behavior—more pronounced at elevated temperatures and pore fluid pressures—and indicate increased potential for the nucleation of slip instability relative to that of the host granodiorite gouge. Experimental results indicate in particular that Ep can increase the potential for induced seismicity on existing faults. Increased proportions of Cl in gouges stabilize the faults but apparently lower the frictional strength. Our results reveal that the metamorphic minerals exert a subtle but potentially significant control on fault strength and stability in geothermal reservoirs. Plain Language Summary: Fluid injection into EGS (enhanced geothermal systems) reservoirs can reactivate subsurface faults and trigger earthquakes. To understand the influence of fluid injection to earthquake triggering on deep faults we analyzed the frictional properties of simulated faults containing the alteration minerals epidote (Ep) and chlorite (Cl). Many geothermal reservoirs worldwide contain these minerals both natively and as a natural result of water circulation into the hot and reactive reservoir. Our measurements of shear strength and frictional stability at in‐situ conditions of such geothermal reservoirs show that the simulated Ep fault gouge may destabilize fault slip at both in‐situ conditions and at higher temperatures and pore fluid pressures. The addition of Cl shows the opposite effect, stabilizing the fault and lowering the frictional strength; hence indicating a strong dependence of fault strength and stability on fault zone composition. Our results highlight the importance of natural metamorphism and mineral transformation in controlling fault strength and stability and the potential for earthquake triggering. Key Points: Epidote (Ep) and chlorite (Cl) are two low‐grade metamorphic minerals typically present in geothermal reservoirs Fault instability is promoted by Ep‐rich gouges at hydrothermal conditions but minimized with the addition of Cl Interplay of Ep/Cl mixtures exerts a detectable control on fault strength and stability for geothermal reservoirs