Researchers at the University of Texas at Austin (UT) are conducting simulations and experiments to establish proof-of-feasibility of a novel concept for assessing capillary trapping in storage formation, as well as to confirm that storage formations have characteristic, spatially correlated distributions of transport properties. Local capillary trapping is an important mechanism for immobilization of CO2 in the subsurface. It occurs at scales from centimeters to tens-of-meters during buoyancy-driven movement of CO2 through heterogeneous storage formations. The distribution of transport properties may vary according to the geologic characteristics of each formation. Certain geologic structures within the formation can become local capillary traps for rising buoyant fluid (Figure 1). Project researchers are using numerical simulation and laboratory experiments to analyze the extent to which local traps fill with stored CO2, and are systematically determining the geologic controls on potential capillary trapping structures. This requires detailed characterization of the structure in a storage formation to assess its heterogeneity and complexity. UT Austin researchers are gathering key formation property data from technical literature and using these data in a suite of geostatistical models. Potential local capillary traps will be identified in the models from maps of their capillary entry pressures. Using this method to identify potential traps, UT will study the influence of the geologic setting (dip angle, maximum height of CO2 column) on potential trapping structures and establish a protocol for conducting buoyancy-driven fluid displacements with supercritical CO2 in heterogeneous, bench-scale porous media (Figure 2).
Once potential traps are identified, the researchers will attempt to quantify and upscale local capillary trapping (Figure 2) by (1) conduct bench-scale experiments in which the overlying seal is breached, demonstrating and potentially validating the buoyant phase fluid’s ability to escape from the local capillary traps; (2) determine the influence of CO2 injection operating conditions on local trap filling; and (3) examine the limitations of the filling of capillary traps by buoyancy dominated displacement. UT researchers will then simulate structural filling when CO2 emplacement occurs at a range of gravity numbers, corresponding to a range of injection rates, and will repeat these simulations with various volumes of CO2 added to the storage formation. After researchers establish the extent of capillary trap filling, they will quantify the extent of trapping capacity that persists after the overlying seal fails.
The overall goal of the Department of Energy’s (DOE) Carbon Storage Program is to develop and advance technologies that will significantly improve the effectiveness of geologic carbon storage, reduce the cost of implementation, and prepare for widespread commercial deployment between 2020 and 2030. Research conducted to develop these technologies will ensure safe and permanent storage of carbon dioxide (CO2) to reduce greenhouse gas (GHG) emissions without adversely affecting energy use or hindering economic growth.
Geologic carbon storage involves the injection of CO2 into underground formations that have the ability to securely contain the CO2 permanently. Technologies being developed for geologic carbon storage are focused on five storage types: oil and gas reservoirs, saline formations, unmineable coal seams, basalts, and organic-rich shales. Technologies being developed will work towards meeting carbon storage programmatic goals of (1) estimating CO2 storage capacity +/- 30 percent in geologic formations; (2) ensuring 99 percent storage permanence; (3) improving efficiency of storage operations; and (4) developing Best Practices Manuals. These technologies will lead to future CO2 management for coal-based electric power generating facilities and other industrial ization of CO2 in all storage types. CO2 emitters by enabling the storage and utilization of CO2 in all storage types.
The DOE Carbon Storage Program encompasses five Technology Areas: (1) Geologic Storage and Simulation and Risk Assessment (GSRA), (2) Monitoring, Verification, Accounting (MVA) and Assessment, (3) CO2 Use and Re-Use, (4) Regional Carbon Sequestration Partnerships (RCSP), and (5) Focus Area for Sequestration Science. The first three Technology Areas comprise the Core Research and Development (R&D) that includes studies ranging from applied laboratory to pilot-scale research focused on developing new technologies and systems for GHG mitigation through carbon storage. This project is part of the Core R&D GSRA Technology Area and works to develop technologies and simulation tools to ensure secure geologic storage of CO2. It is critical that these technologies are available to aid in characterizing geologic formations before CO2-injection takes place in order to predict the CO2 storage resource and develop CO2 injection techniques that achieve optimal use of the pore space in the reservoir and avoid fracturing the confining zone (caprock). The program’s R&D strategy includes adapting and applying existing technologies that can be utilized in the next five years, while concurrently developing innovative and advanced technologies that will be deployed in the decade beyond. For this project, researchers are conducting computer simulations and laboratory experiments to establish the feasibility of a new technique for assessing capillary trapping in geologic formations.
The proposed simulation and experiments will systematically establish proof-of-feasibility of a novel concept, namely the long-term security of CO2 that fills local (small-scale) capillary traps in heterogeneous storage formations. The outcome will be a geologically grounded method for quantifying the extent of such trapping. The method can be implemented with simulation capabilities already being used to predict storage security. The impact will be a potential reduction in risks associated with long-term storage security, achieved simply by considering the physical implications of geologic heterogeneity. This research supports the Carbon Storage Programmatic goal of demonstrating 99 percent storage permanence.
The overall goal of this project is to obtain a high-quality assessment of the amount and extent of local capillary trapping expected to occur in typical geologic storage formations. The goal is being accomplished by addressing several key project objectives that include: (1) quantifying the influence of key geologic and petrophysical parameters on the structure of local capillary barriers in a heterogeneous formation, and hence on the potential number and volume of local capillary traps; (2) determining what fraction of these traps is filled during prototypical CO2 emplacement operations (injection followed by buoyancy-driven migration) by simulation and laboratory experimentation; and (3) simulation and experimentation to quantify what fraction of the filled local capillary traps retains CO2 if the top seal of the storage formation loses integrity and allows CO2 to leak. In order to complete the overall project objectives, researchers will conduct the following research efforts:
Characterizing petrophysical and geologic controls on the number and volume of potential local capillary traps.
Determining the degree to which potential local capillary traps are filled in anticipated geologic storage schemes.
Quantifying the extent of immobilization persisting after loss of integrity of the seal overlying the geologic storage formation.
Incorporating the results into a functional form which can be easily integrated into existing reservoir simulation packages.
Conducting lab-scale experiments to validate simulation modeling.
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