Earth Sciences Division Berkeley Lab
Geologic Carbon Sequestration Program
Research Summaries 2006-2007
MODELING GEOLOGIC STORAGE OF CARBON DIOXIDE: COMPARISON OF NONHYSTERETIC AND HYSTERETIC CHARACTERISTIC CURVES Christine Doughty Contact: Christine Doughty, 510/486-6453, cadoughty@lbl.gov
RESEARCH OBJECTIVES nonhysteretic formulation, in which capillary pressure and re l aGeologic storage of carbon dioxide (CO2) in brine-bearing fortive permeability depend only on the current value of the gridmations has been proposed as a means of reducing the atmosblock saturation, would be adequate. However, for a more re a l i spheric load of greenhouse gases. In the subsurface, supercritical tic problem that includes both injection of CO2 (a drainage CO2 forms an immiscible gas-like phase and partially dissolves in the brine. Numerical models of process) and its subsequent postinjection CO2/brine systems use characteristic evolution (a combination of drainage and wetting), hysteretic characteristic curves to represent the interactions of curves are required to correctly capture nonwetting-phase supercritical CO2 and the behavior of the CO2 plume. This findwetting-phase brine. The simplest characteristic curves are nonhysteretic—the ing is a consequence of the fact that residcapillary pressure and relative permeual gas saturation Sgr (the saturation abilities depend only on the local saturabelow which free-phase CO2 is trapped) tion at the current time. A more sophistiis strongly process-dependent, with a cated approach is a hysteretic formulazero value during drainage and a potention, in which capillary pressure and reltially large value during imbibition, a ative permeabilities depend not only on value that increases with the maximum local historical value of gas saturation. the current value of the local saturation, but on the history of the local saturation SIGNIFICANCE OF FINDINGS and the process that is occurring: drainage (replacement of wetting phase During post-injection periods, the with nonwetting phase) or wetting leading edge of the CO2 plume undergoes (replacement of nonwetting phase with drainage (Sgr=0), while the trailing edge wetting phase, also known as imbibiundergoes wetting (large Sgr). Thus, the tion). The objective of this research is to plume is more mobile at the leading edge investigate the impact of using nonhysand less mobile at the trailing edge, so it teretic or hysteretic characteristic curves elongates. In addition to trapping signifiFigure 1. Free-phase CO2 distributions at a series to model various aspects of CO2 geologi- of times for hysteretic (top) and nonhysteretic cant quantities of free-phase CO2 at the cal storage. trailing edge of the plume, this elongation (middle and bottom) formulations for characterisenables more CO2 to dissolve in the brine tic curves. CO2 injection at 1,000 m depth lasts for APPROACH one month. Neither nonhysteretic case can model and greater interaction with rock minerals the post-injection period in which the CO2 plume to occur, further immobilizing CO2. It is Two example problems involving is more mobile at the top than at the bottom and a geologic CO2 storage are simulated with impossible to correctly model these small amount of CO2 reaches the surface while the p rocesses using a nonhysteretic formulaTOUGH2, a multiphase, multicomponent bulk of the plume remains trapped at depth. tion for characteristic curves. code developed at Berkeley Lab for flow and transport through geologic media. TOUGH2 considers all RELATED PUBLICATION flow and transport processes relevant for a two-phase (liquid-gas), three-component (CO2, water, dissolved NaCl) system. Both a Doughty, C., Modeling geologic storage of carbon dioxide: nonhysteretic and a newly developed hysteretic formulation are comparison of non-hysteretic and hysteretic characteristic employed to represent characteristic curves, to illustrate the applicurves. Energy Conversion and Management 48(6), cability and limitations of nonhysteretic methods. The first applica1768–1781, 2007. tion considers leakage of CO2 from the storage formation to the Related web site: http://www-esd.lbl.gov/GEOSEQ/index.html ground surface, while the second examines the role of heterogeneACKNOWLDGMENTS ity within the storage formation. This work was conducted as part of the Geologic Carbon ACCOMPLISHMENTS Sequestration (GeoSeq) and Zero Emissions Research and Technology (ZERT) projects, supported by the Assistant Secretary for Fossil Simulation results show drastically different behavior for the Energy, Office of Sequestration, Hydrogen, and Clean Coal Fuels, long-term evolution of CO2 plumes for nonhysteretic and hysthrough the National Energy Technology Laboratory, U.S. teretic formulations. For an idealized problem Department of Energy, under Contract No. DE-AC02-05CH11231. that involves only drainage or only wetting, a
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