Fracture growth under reactive subsurface conditions: Processes, Mechanisms, and Significance for Geoenergy

Fractures significantly control subsurface heat and fluid transport and the mechanical properties of rock formations. Natural and stimulated fracture growth processes are thus essential for production of oil and gas in conventional and unconventional reservoirs, caprock integrity, underground storage of carbon dioxide, hydrogen, or wastewater, and for geothermal energy production in systems that require fracture stimulation or that depend on natural fractures for heat extraction. While the formation of fractures is conventionally seen as a purely mechanical process, chemical processes can decrease or increase the propensity for fracture growth as a function of stress conditions, fluid chemical and physical environment, rock composition, and rate of change of fracture driving loading conditions. The influence of chemical reactions on rock fracture processes and their implications for subsurface energy resources is thus increasingly recognized.In combination with field structural observations of fractures in a variety of natural settings, we conduct double torsion fracture mechanics tests for sandstone, shale, and polycrystalline halite to quantify effects of fluid chemical environment on fracture mechanical properties. Tests are conducted under a range of fluid compositional and environmental conditions that are relevant to subsurface hydrogen and CO2 storage and geothermal energy production. Double torsion fracture mechanics tests measure fracture toughness and subcritical fracture index. Fracture toughness quantifies the loading stress for critical fracture growth, and subcritical fracture index the rate of fracture propagation under subcritical loading conditions. Tests are conducted under ambient room conditions, in dry N2, CO2, or H2 gas environments, and partially or completely saturated aqueous conditions. Some materials are also reacted in an autoclave under elevated temperature and pressure conditions in the presence of H2 and N2 gas prior to fracture testing.Both fracture toughness and subcritical fracture index are influenced by the chemical environment to varying degree dependent on rock mineral composition, fluid composition, and environmental conditions. For all rock types except polycrystalline halite, samples tested under dry conditions have higher toughness and subcritical index values compared to partially or fully water-saturated samples. This can be beneficial for caprock integrity of CO2 or H2 storage reservoirs where injected gas would dry out the formation reducing the tendency for fracture-controlled leakage of top seals. Aqueous chemical reactions triggered by H2 or CO2 gas injection in porous reservoirs can both impede and enhance mechanical fracture processes depending on the combined effects of mineral dissolution and concurrent precipitation of newly formed minerals. With increasing temperature, the effects of aqueous mineral reactions on fracture properties are generally more pronounced, demonstrating the significance of reactive fracture processes in conventional and enhanced geothermal reservoirs. It is envisioned that chemical effects of fracture growth can be utilized to reduce undesired fracture growth or to optimize stimulated fracture growth to obtain desired fracture geometries that benefit subsurface energy operations.