Static friction-coefficient increase in chelating-agent-treated granite saw-cut fractures associated with selective mineral dissolution and void formation

Abstract

Induced seismicity remains a critical challenge for enhanced geothermal systems (EGS), motivating stimulation strategies that enhance permeability while maintaining fracture stability. Chemical stimulation has been widely studied as a complement to hydraulic stimulation, yet its mechanical effects on fracture friction and slip behavior remain poorly constrained. In particular, chemical stimulation is often implicitly assumed to weaken fractures and promote instability. Here, we investigate the shear-slip behavior of granite fractures chemically treated by a biodegradable chelating agent, N,N -bis(carboxymethyl)- L -glutamic acid tetrasodium salt (GLDA-Na 4 ). A 45° saw-cut fracture in granite was treated in a batch experiment with a 20 wt.% GLDA-Na 4 solution (pH 4) at 150 °C, followed by triaxial shear-slip experiments conducted at an axial stress of 55 MPa and a confining pressure of 30 MPa. Static friction coefficients and shear-slip velocities were quantified before and after chemical treatment. Chemical stimulation selectively dissolved biotite, a mineral with a relatively low friction coefficient, and generated dissolution-induced voids on the fracture surface. As a result, the static friction coefficient increased from 0.48 ± 0.03 to 0.62 ± 0.01. Following rapid slip onset, the treated surfaces gradually decelerated into a prolonged lower-apparent-velocity phase. The reported shear displacement and velocity are termed apparent because they were inferred from axial deformation. Microstructural analyses using XCT, μXRF, and SEM suggested that the increase in frictional resistance was associated with low-friction mineral removal and dissolution-induced void formation, possibly accompanied by rim spalling. These results show that chemical treatment does not necessarily reduce fracture frictional resistance. When selective mineral dissolution produces this type of mineralogical and topographical surface alteration, static friction can increase under the present dry, stress-controlled laboratory conditions, providing a basis for future shear-slip experiments involving fluid flow.