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Reactive Fracture Mechanics: Processes, Mechanisms, and Significance for the Energy Transition

Session: Faults, Fractures, and Geomechanics for the Energy Transition

Presenting Author:

Peter Eichhubl

Authors:

Eichhubl, Peter¹, Gajda, Dawid², Chen, Xiaofeng³, Espinoza, D. Nicolas⁴

(1) The University of Texas at Austin, Bureau of Economic Geology, Austin, TX, USA, (2) The University of Texas at Austin, Bureau of Economic Geology, Austin, TX, USA, (3) Oklahoma State University, Boone Pickens School of Geology, Stillwater, OK, USA, (4) The University of Texas at Austin, Hildebrand Department of Petroleum and Geosystems Engineering, Austin, TX, USA,

Abstract:

Fractures control flow and mechanical properties of rock formations. Fracture processes are thus essential for production of oil and gas in unconventional shale and sandstone formations, for caprock integrity in underground carbon and hydrogen storage, and for geothermal production in systems that require fracture stimulation or that depend on natural fractures for flow. While the formation of fractures is conventionally seen as a purely mechanical process, the influence of chemical reactions on rock fracture processes is increasingly recognized.

To quantify effects of fluid chemical environment on fracture mechanical properties, we conduct double torsion fracture mechanics tests for sandstone, shale, polycrystalline halite, and wellbore cement under a range of fluid compositional 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 mixed H₂-N₂ gas environments, and partially or completely saturated aqueous conditions. Sandstone, shale, and wellbore cement are also reacted in an autoclave under aqueous conditions in the presence of H₂and N₂ gas prior to fracture testing.

For all rock types, 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 H₂storage 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. Chemical effects on fracture mechanical properties are also observed in double torsion tests conducted under reactive conditions without prior exposure to these reactive conditions. These results suggest that reactions can affect reservoir or caprock performance during wellbore completion and during early injection or production operations.