Large-scale geological storage of carbon dioxide (CO2) is widely considered as necessary to mitigate detrimental effects to the climate. Discussions of formation evaluation in carbon capture and storage (CCS) often focus on the central themes of capacity, injectivity, and containment. Chemical reactions induced by CO2 in rock-fluid systems are usually a secondary consideration, representing one of the less well-developed aspects of CO2 storage assessment. The impact of CO2-driven reactions can range from negligible to significant, depending on formation lithology, fluid composition, and petrophysical properties. Water evaporates into dry CO2, enabling the precipitation of salt from brine. Acidic CO2-bearing solutions can dissolve primary minerals and precipitate secondary minerals. These phenomena may drive positive or negative changes in porosity and permeability on different time scales. The significance varies for minerals of siliciclastic, carbonate, and igneous rocks, and their effects touch on all the key operational criteria. Formation evaluation should play an essential role in predicting the evolution of CO2-driven phenomena and in designing strategies to avoid problems. In this work, we develop the connection between downhole logs and CO2-driven chemical reactions at two important levels: early-stage petrophysical evaluation and more detailed reservoir simulations.
We first introduce analytical models for CO2 reactivity based on petrophysical logs and foundational expressions for chemical reaction rates. The kinetics of mineral dissolution (and precipitation) are used to estimate initial rates of chemical reactions. Geochemical logs from elemental spectroscopy provide essential information on mineralogy and lithology: We adapt and extend methods that are established from oil and gas reservoirs. Information on water composition is important: Insight can be derived from several means, including a chlorine measurement from elemental spectroscopy, the macroscopic cross section sigma, dielectric dispersion, resistivity, or local knowledge. The available surface area for reactions is driven by porosity, mineralogy, and pore-size distribution.
Numerical simulations provide a more comprehensive way to predict the rate and extent of CO2 reactions over long time scales. Several options exist for modeling reactive fluid transport, including commercial software and codes from US national laboratories and government agencies. We demonstrate results from codes for 3D multiphase reactive-transport simulation. Sensitivity studies illustrate the importance of accurate knowledge of formation properties. Global sensitivity analyses the variation of numerical results on CO2 saturation, plume extent, formation porosity, and fluid pH with respect to key parameters of formation evaluation. The results imply that correct information on mineralogy, pore structure, and fluid compositions are all essential. Examples are provided for CO2 injection simulations based on real-field CCS operations.
