Micromechanical testing for the evaluation of chemo-mechanical alteration of CO₂ storage rocks

This thesis investigates the relationship between the chemically and mechanically coupled alteration of CO2-storage rocks during CO2 geological storage and the ensuing changes in rock properties. I analyzed how the scratch toughness and hardness varied with alteration by CO2-fluid mixtures by employing indentation and scratch test methodologies. Rock samples were selected from the Crystal Geyser site near Green River Utah, where a natural seepage of CO2 altered outcrops of the Entrada sandstone and Summerville siltstone formations near faults over tens of thousands of years. Results from tests on Entrada sandstone and Summerville siltstone from the Crystal Geyser site show that mechanical parameters measured with indentation (indentation hardness, Young’s modulus and contact creep compliance rate) and scratching (scratch hardness and scratch toughness) consistently indicated weakening of the rock after CO2-induced alteration. Decreases of measured parameters vary from 14% to 87%. In order to investigate the time scales of variation of mechanical and petrophysical properties differing to those before exposure, I conducted autoclave reaction experiments with Entrada sandstone and Summerville siltstone exposed to either de-ionized water or synthetic brine under reservoir pressure (9 10 MPa) and temperature (80°C) conditions for up to two weeks. I designed and constructed a scratch testing apparatus to conduct scratches on the laboratory altered rock samples. Scratch toughness and hardness show decreases of up to 60% in the case of Entrada sandstone and 92% in the case of Summerville siltstone after CO2-induced alteration in the laboratory. To understand chemical reactions during the laboratory alteration experiments, I conducted parallel experiments using powdered samples of Entrada sandstone and Summerville siltstone. I quantified aqueous ion concentrations for fluid samples collected from these autoclave experiments using analytical geochemistry. Dissolution of calcite and silicate cements are the primary reactions identified for both samples during the laboratory experiments. Recognizing the susceptibility of rock facies to CO2-related alteration at target CO2 geological storage formations is critical to ensuring the long-term mechanical stability and security of CO2 trapping.