Micromechanical Tests and Geochemical Modeling to Evaluate Evolution of Rock Alteration by CO2-Water Mixtures

Abstract:

Injection of large volumes of CO₂ into geologic formations can help reduce the atmospheric CO₂ concentration and lower the impact of burning fossil fuels. However, the injection of CO₂ into the subsurface shifts the chemical equilibrium between the mineral assemblage and the pore fluid. This shift will situationally facilitate dissolution and reprecipitation of mineral phases, in particular intergranular cements, and can potentially affect the long term mechanical stability of the host formation. The study of these coupled chemical-mechanical reservoir rock responses can help identify and control unexpected emergent behavior associated with geological CO₂ storage.

Experiments show that micro-mechanical methods are useful in capturing a variety of mechanical parameters, including Young’s modulus, hardness and fracture toughness. In particular, micro-mechanical measurements are well-suited for examining thin altered layers on the surfaces of rock specimens, as well as capturing variability on the scale of lithofacies.

We performed indentation and scratching tests on sandstone and siltstone rocks altered in natural CO₂-brine environments, as well as on analogous samples altered under high pressure, temperature, and dissolved CO₂ conditions in a controlled laboratory experiment. We performed geochemical modeling to support the experimental observations, in particular to gain the insight into mineral dissolution/precipitation as a result of the rock-water-CO₂reactions.

The comparison of scratch measurements performed on specimens both unaltered and altered by CO₂ over geologic time scales results in statistically different values for fracture toughness and scratch hardness, indicating that long term exposure to CO₂ caused mechanical degradation of the reservoir rock. Geochemical modeling indicates that major geochemical change caused by CO₂ invasion of Entrada sandstone is dissolution of hematite cement, and its replacement with siderite and dolomite during the alteration process.