Reactive viscous fluid fracturing of volcanic rocks: Chemical weakening offsets viscosity-induced pressure increase

Hydraulic stimulation of volcanic reservoirs can be hindered by strong fluid infiltration into pores, vesicles, and microfractures, which suppresses pressure build-up and limits the formation of conductive fractures. Increasing fluid viscosity can reduce fluid loss, but typically at the cost of higher fracturing pressure. Here, we tested whether this viscosity–pressure trade-off can be mitigated by combining viscosity control with chemical weakening of the rock matrix. Triaxial fracturing experiments were conducted at 180 °C on volcanic rocks using water-like fluids, non-reactive viscous fluids, and reactive viscous fluids composed of the biodegradable chelating agent N,N -bis(carboxymethyl)- L -glutamic acid (GLDA) and the biopolymer scleroglucan. Water-like fluids showed strong matrix infiltration and were ineffective at generating large-aperture fractures. Increasing viscosity suppressed infiltration and promoted wider fractures, but increased fracturing pressure. When GLDA was added to the viscous fluid, fracture aperture and fracture-network complexity increased further, while fracturing pressure in the benchmark andesitic tuff experiments remained comparable to that during water injection despite the much higher fluid viscosity. The results are consistent with chemically induced weakening offsetting the pressure increase normally associated with viscous fluids. Additional experiments on andesitic lava and rhyolitic reservoir rock suggest that reactive viscous fluids can also propagate pre-existing fractures and promote branching in volcanic rocks with contrasting pore structures. These findings highlight the potential of reactive viscous fluid fracturing as a low-pressure stimulation strategy for geothermal reservoirs, with possible relevance to proppant-less reservoir stimulation.