Laboratory investigation of coupled electrical interaction of fracturing rock with gases

Abstract

Abstract In the coupled electric interaction of rock fractures and gas invasion, that is, when gases interact with newly created crack surfaces, the unpaired electrons within the rock crystal defects are thermally stimulated, released into the crack due to the temperature rise at the crack tip via plastic work, and attached to ambient gas molecules to electrify them in a negative state. Using a working hypothesis that this mechanism is the source mechanism of seismo-electromagnetic phenomena, we conducted laboratory experiments in which rocks were fractured with pressurized N2, CO2, CH4, and hot water vapour. Fractures were induced by a flat-ended indenter equipped with a flow channel, which was loaded against blocks of quartz diorite, gabbro, basalt, and granite. Fracture-induced negatively electrified gas currents at ~ 25 °C and ~ 160 °C were successfully measured for ~ ≥ 100 μs after full development of the crack. The peak electric currents were as high as 0.05–3 μA, depending on the rock species and interaction area of fractured rock and gas and to a lesser extent on the gas species and temperature. The peak current from fracturing granite, which showed higher γ-ray activity, was at least 10 times higher than that from fracturing gabbro, quartz diorite, and basalt. The results supported the validity of the present working hypothesis, that coupled interaction of fracturing rock with deep Earth gases during quasi-static rupture of rocks in the focal zone of a fault might play an important role in the generation of pre- and co-seismic electromagnetic phenomena.

Keywords

• Seismo-electromagnetics
• Fault valve
• Rock fracture
• Deep Earth gas
• Exoelectron emission