Sequestration of supercritical carbon dioxide in deep subsurface formations through stable trapping and mineralization has received considerable attention in recent years. Once in place, potential exists for the separate phase supercritical liquid or water with dissolved carbon dioxide to leak through the capping layer to shallower formations with potential for serious environmental consequences. Comprehensive fundamental understanding of trapping and leakage processes is required to estimate subsurface carbon dioxide storage capacity and to assess ecological and other environmental risks from leakage. Given the complexity of the problem, developing a comprehensive process-based understanding, through developing modeling tools to design field applications, is only possible by analyzing the problem at multiple scales. This project will complete a set of innovative laboratory sand tank experiments at scales ranging from small (cms) to intermediate (meters) that explore the fundamental processes. The proposed research will use surrogate fluids and innovative testing and measuring methods to complete experiments applicable to deep formation conditions, including high pressures, under ambient laboratory conditions. Existing numerically based models that simulate the mechanisms of multiphase fluid flow, including entrapment, mass transfer, density driven mixing, and bubble migration, will be tested to evaluate their ability to capture the fundamental processes observed in the laboratory. Improvements to these models will be made using the new knowledge generated in this research. Methods to up-scale processes that are characterized in the laboratory systems will be developed and tested.
This research is of direct relevance to climate change because it will lead to improvements in carbon dioxide sequestration approaches intended to mitigate increasing atmospheric carbon dioxide. Although some aspects of carbon dioxide sequestration have been extensively studied, the fundamental mechanisms that control stable storage and leakage potential have not received adequate attention. Improvement of our fundamental understanding of these mechanisms through incorporating them into models will facilitate improved site selection and efficient design. Minimizing the risk of leakage will be beneficial to groundwater quality, ecosystems, and the environment. The unique laboratory data sets will be shared with investigators involved in carbon dioxide sequestration research, as well as in other multiphase phase flow problems in hydrology and environmental engineering. Significant educational impacts will be achieved by training students through participation in this research on processes critical to climate change mitigation.
Background and Objectives: Carbon Capture and Storage (CCS) represents a technology aimed to reduce atmospheric loading of CO2 by injecting it into deep geological formations, such as saline aquifers. CCS relies on the effectiveness of safely retaining the injected CO2 in supercritical fluid phase (scCO2) in the formation over the long term through structural, residual, dissolution, and mineral trapping. Numerically based simulation models of trapping in deep geologic formations are increasingly going to play an important role in site selection and assessment of carbon storage potential, design of injection strategies and assessment of permanence. It is known that the natural heterogeneity of typical sedimentary formations has a significant effect on the propagation of the scCO2 plume and eventually accumulating mobile phase underneath the caprock. However, knowledge gaps exist on how the heterogeneity of the formation affects trapping and how the mechanisms to be parameterized for modeling. The primary objective of this research was to fill the knowledge gaps in the understanding of how ScCO2 gets trapped trough processes associated with capillarity and dissolution in naturally heterogeneous, deep saline geologic formations. Filling of these knowledge gaps is expected to contribute toward better conceptual models, improved numerical models, more accurate assessment of storage capacities, and optimized placement strategies. The data generated in highly controlled laboratory settings helped to evaluate the ability of existing models to capture the fundamental processes that contribute to capillary and dissolution trapping. Approach: Since fully characterizing the geologic heterogeneity at all relevant scales and making observations on the spatial and temporal distribution of the migration and trapping of supercritical CO2 are not practical, studying these trapping processes at a fundamental level is not feasible in field settings. Laboratory experiments using scCO2 under ambient conditions are also not feasible as it is cost prohibitive to develop test systems with controlled high pressures to keep the scCO2 as a liquid. Hence, an innovative approach that used surrogate fluids in place of scCO2 and formation brine in multi-scale, multiple length scales in synthetic sand reservoirs was developed and implemented. These experiments were designed to provide insight into the impact of natural geologic heterogeneity on the flow and trapping of scCO2 under drainage and imbibition conditions. Basic to this approach was the use of surrogate for scCO2 and formation brines to simulate the interplay of governing forces that are expected under conditions in deep reservoirs. A series of immiscible displacement experiments was performed to observe and record the migration preferential entrapment of scCO2 in well-defined heterogeneous sand packing configurations. A second set of multi-scale experiments were conducted with another suite of surrogate fluids to study dissolution of the trapped scCO2 and the subsequent dissolution trapping through mechanisms of convective mixing and diffusion. Each of the capillary trapping experiment consisted of a drainage event simulating the injection of a non-wetting phase (scCO2) into a brine-wetted reservoir, followed by a gravity relaxation period characterized by spontaneous imbibition at the trailing edge of the plume. Spatial and temporal variations of non-wetting phase saturation represent important features to evaluate capillary trapping; for this reason x-ray attenuation analysis was used as a non-destructive technique that allows the precise measurement of phase saturations throughout the entire flow domain. The data on the dissolution trapping were obtained by extracting aqueous samples from ports on the test tanks walls and conducting laboratory analysis to determine dissolved concentrations. Modeling algorithms were developed to obtain addition insights through numerical simulations. Results and findings: Analysis of the experimental results complemented by numerical simulations suggests that intermediate-scale experiments with analog fluids can provide quantitative information about large-scale phenomena and therefore can be used to predict storage performance of natural reservoirs under a range of boundary conditions. Discoveries and success stories: Successfully developed experimental methods to use surrogate fluids for supercritical and brine to conduct experiments under ambient laboratory conditions. Using these methods, we were able to generate highly accurate data to evaluate the ability of multiphase codes used in carbon storage modeling to capture important fundamental processes. Obtained new insights into how geologic heterogeneity affects capillary and dissolution trapping. These insights have helped us to develop improved conceptual models that will lead to methods to optimize injection schemes and make improvements to currently used DOE codes. Demonstrated how heterogeneity could be used to enhance stable trapping, controlled by capillarity and dissolution coupled with diffusion into low permeability zones of the formation. Identified deficiencies and limitations of current multiphase models for their ability to capture some of the basic processes observed in the highly controlled experiments. Identified limitations in currently used constitutive models of fluid retention and relative permeability. New insights into mechanisms CO2 exsolution when brine leaking from deep formations containing the dissolved gas reaches the shallow subsurface, specifically under heterogeneous conditions.
