Surface complexation reactions can modify structure, charge, and zeta potential of carbonate surfaces. Carbonates, including calcite (CaCO3), are pervasive mineral phases in the Earth’s crust. Carbonate phases often “cement” other grains together in sedimentary rocks and are susceptible to subcritical chemically-assisted fracturing. We observed that chemical complexation reactions at the crack tip of CaCO3 control subcritical fracturing. We demonstrate this chemical complexation mechanism for individual cracks in CaCO3 single crystals, and for core-scale consolidation experiments on CaCO3 grain packs. Subcritical fracture growth rate in CaCO3 single crystals was quantified on pre-cracked (100) surfaces. Surfaces were indented at 400 mN using a Vickers tip prior to exposure to aqueous fluids. Indentation produced micron-scale fractures with identical lengths, and these fractures grew when exposed to aqueous fluids. De-ionized H2O (with low HCO3- concentration), dilute acids (HCl and H2SO4), and synthetic hydrofracturing fluid were tested. The rate of subcritical crack growth ranged from 1.6×10-8 m s-1 to 2.4×10-10 m s-1 and correlated with the complexation constant (kβ) for the calcium-ligand complex with the ligand present in the fluid. Crack growth rate was fastest for the weakly complexing HCO3-, and the slowest for the strongly complexing SO42- ligands. Crack growth rate, however, did not correlate with the measured CaCO3 dissolution rate or changes in the zeta-potential. To determine whether the same chemical complexation mechanism affects fracturing behavior in core-scale samples, granular CaCO3 assemblages were saturated with target fluid chemistries (0.5M NaCl, NaHCO3, Na2SO4, and Na3C6H5O7) and then compacted using a hydrostatic consolidation stress path. The measured consolidation envelopes and observed grain fracturing support the proposed chemical complexation mechanism. The observed deformation and crack densities were highest in the presence of weakly complexing anion (HCO3-), and lowest in the presence of strongly complexing anion (SO42-). The subcritical crack densities correlate with chemical complexation reactions at the CaCO3 crack tips as well as the diffusion rate of the anion. This new knowledge can enable novel approaches for sustaining the integrity of subsurface reservoirs and explain chemically-induced deformation of sedimentary rocks.

Subcritical fracturing of calcite is defined by the surface complexation reactions inside the crack tip

ACS Fall 2021

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