Single fluid-driven crack propagation in analogue rock assisted by chemical environment

Abstract

During the operation of hydraulic fracturing as used in many geo-energy and geo-environment applications, chemical stimulation is often incorporated for cracking enhancement in low-permeability geological formations for the purpose of an optimization of energy recovery. The mechanism of subcritical crack propagation in a chemically reactive environment is essential for understanding of the involved coupled chemo-mechanical process and a better control of acid-assisted hydraulic fracturing. It has been postulated that the rate of crack propagation under environmental loads is inherited from the chemical processes involved including the reaction and the diffusive transport. However, laboratory explorations focusing on the evolving interplay between the propagation of a fluid-pressurizing individual crack and the environment it is subject to via a variable chemical intensity imposed have been rare. Here we present an experimental investigation on a single tensile crack propagation in alginate hydrogel as an analogue material for brittle rocks, driven by the injection of a chemically reactive fluid kept at constant pressure using a Hele-Shaw cell setup. We show that an intensified chemical environment can accelerate tensile crack propagation in both subcritical crack growth and fracturing regimes, while leading to the Region III fracturing of less brittle characteristics. During the experiment, crack-tip blunting upon injection of reactive solutions was observed, suggesting a competing mechanism between the crack-tip geometry induced toughening and the chemically induced softening within the process zone as the crack advances. Our results provide quantitative insights into how a chemically reactive environment facilitates the growth of a single macroscopic crack of mode I opening in a low-permeability matrix through coupled chemo-mechanical feedback. The imposed chemical environment promotes crack propagation while alleviating the stress concentration at the advancing crack tip, suggesting a more energy-efficient method compared to pure water fracturing. We anticipate our experimental investigation presented here to be a starting point of sound laboratory support for future studies towards a more controllable technique of chemical stimulation in geomaterials as well as complementing the ongoing modeling efforts in reactive chemo-mechanics.