Surface Chemistry Controls on Crack Growth in Plagioclase Feldspars

Laboratory experiments were used to investigate the effects of surface chemistry on subcritical crack growth in granular orthoclase. Plagioclase feldspars are principal components in crustal rocks as both skeletal and granular components. Utilization of subsurface reservoirs for water usage, energy production/storage, and carbon sequestration can modify in situ fluid chemistry within stressed crack tips. Combined with changes in pore pressure, fluid chemistry changes could lead to chemically-assisted cracks and result in enhanced damage to the reservoir system, altering hydrologic properties, storage capacity, and surface installations. Greater understanding of chemical controls on fracture is necessary to protect and mitigate unwanted subsurface damage.

Experiments were conducted on nano- to cm-scales to investigate fluid chemistry effects on reduced modulus, surface hardness, and critical compaction pressure by employing both nanoindentation on single crystals of orthoclase as well as consolidation experiments on grain packs of granular orthoclase to induce a multitude of cracks. Both sets of experiments were conducted in the presence of fluids that can modify crack behavior due to complexation reactions between Ca(II) in the lattice and fluid anions (Cl-, SO42-, and HPO42-). Similar to our previous work in calcite, we observe that affinity between aqueous anions and surface Ca(II) ions control crack growth in single crystals and ultimately larger scale deformations of granular assemblages of orthoclase. Samples deformed in deionized water had the largest observed volume strain, and samples deformed in Na brines deformed less due to complexation reactions at the crack tips. Results presented here demonstrate mechanisms for fluid-crack interactions that could lead to better predictive capabilities for in situ applications as well as potential mitigation strategies to preserve reservoir behavior.

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