Grantee Research Project Results
Final Report: Understanding and Managing Risks Posed by Brines Containing Dissolved Carbon Dioxide
EPA Grant Number: R834383Title: Understanding and Managing Risks Posed by Brines Containing Dissolved Carbon Dioxide
Investigators: Falta, Ronald W. , Murdoch, Lawrence C. , Benson, Sally M.
Institution: Clemson University , Stanford University
EPA Project Officer: Aja, Hayley
Project Period: November 1, 2009 through October 31, 2012 (Extended to October 31, 2014)
Project Amount: $891,342
RFA: Integrated Design, Modeling, and Monitoring of Geologic Sequestration of Anthropogenic Carbon Dioxide to Safeguard Sources of Drinking Water (2009) RFA Text | Recipients Lists
Research Category: Targeted Research , Water
Objective:
Geologic disposal of supercritical carbon dioxide in saline aquifers and depleted oil and gas fields will cause large volumes of brine to become saturated with dissolved CO2 at concentrations of 30 g/L or more. As CO2 dissolves in brine, the brine density increases slightly. This property favors the long-term storage security of the CO2 because the denser brine is less likely to move upwards towards shallower depths. In fact, one proposed strategy for reducing risk from CO2 injection activities involves pre-dissolving the CO2 into brine at the surface, and injecting this brine into the disposal formation. While dissolved phase CO2 poses less of a threat to the security of shallower drinking water supplies, the risk is not zero. There are plausible mechanisms by which the CO2 laden brine could be transported to a shallower depth, where the CO2 would come out of solution (exsolve), forming a mobile CO2 gas phase. This significant mechanism for drinking water contamination has received little attention, and there are basic science and reservoir engineering questions that need to be addressed in order to reduce risks to underground drinking water supplies.
This research was organized into six tasks, which included both laboratory experimentation and theoretical analysis using mathematical modeling. The modeling work ranged in scale from the laboratory core scale, through the scale of a sedimentary basin.
Summary/Accomplishments (Outputs/Outcomes):
Task 1. Laboratory Experiments at the Core Scale
This work focused on the core-scale behavior of dissolved and supercritical CO2 in sandstones under realistic reservoir conditions. The first part of this effort, described in Zuo et al. (2012) involved experiments where water was saturated with dissolved CO2 at reservoir temperature and pressure (124 bar and 50 oC) and injected into the sandstone cores. At this high pressure, CO2 has a solubility of more than 50 g/L. The experiments involved gradually depressurizing the cores to a final pressure of 28 bar, and observing the characteristics of supercritical or gas phase CO2 exsolution. During each depressurization step, the CO2 phase distribution within the cores was imaged using a medical X-ray CT scan, and the outflow of water and CO2 was continuously measured. It was found that the CO2 exsolved uniformly through the cores during depressurization, and no preferential pathways were formed. The measured relative permeability of the exsolved CO2 phase was found to be extremely low, even at CO2 phase saturations of 40%. This observation is in direct contrast to the much higher relative permeability of CO2 measured during a conventional core-flood experiment.
The second part of this work examined the history-dependent trapping of supercritical CO2 in sandstone cores. This work is described in Ruprecht et al. (2014) and it consisted of CO2 and water injection into a sandstone core at 50 oC and 90 bar. Each experiment consisted of cycling the injected CO2 fractional flow to higher levels in order to observe the resulting CO2 trapping as a function of the maximum CO2 phase saturation. As in the earlier experiments, the CO2 saturation was imaged using X-ray CT, and the flows and pressure gradient were continuously monitored. A key finding was that the amount of CO2 that became trapped at each step was a linear function of the maximum CO2 saturation achieved. Relative permeability curves for the CO2 phase and water were measured, and it was shown that the simple hysteretic relative permeability model of Patterson and Falta (2012; 2014) developed in Task 3 provided an excellent fit to the experimental data.
Task 2. Pore-Scale Behavior
We completed experimental studies of CO2 exsolution in small micromodels (Zuo et al., 2013). These models include individual pore structures, and by using a microscope, we were able to visualize the exsolution process as water saturated with CO2 was depressurized. As with the core experiments, these experiments were performed at reservoir temperature and pressure (90 bar and 45 oC). It was found that the exsolution process results in nearly uniform distributions of a dispersed CO2 phase that has very low mobility, an observation that is similar to the behavior of the exsolved CO2 in the laboratory cores. In contrast, CO2 injection in the same micromodel resulted in much higher CO2 phase mobility at lower saturations. During CO2 injection, the injected CO2 is able to find preferential pathways in the larger pore structures, thereby bypassing much of the porous media. This leads to relatively high CO2 mobility at low saturations.
Task 3. CO2 Phase Relative Permeability Functions for Multiphase Flow Models
We proposed new hysteretic CO2 phase relative permeability and capillary pressure curves that account for the variable trapping that occurs as a function of the CO2 phase saturation history at a given location. This work is described in the MS thesis of Chris Patterson (Patterson, 2011) and in Patterson and Falta (2012, 2014). These functions are based on the observation that the amount of a nonwetting phase that becomes trapped in a porous media is more or less a linear function of the maximum historical saturation of that phase at that location. Therefore, if a location has not seen any CO2 phase, and only a small amount invades that zone, then only a small amount will be trapped. Conversely, if the CO2 saturation becomes high, then a large amount of CO2 will be trapped. The new relative permeability and capillary pressure curves use conventional models that contain a nonwettng phase residual saturation. The modification involves changing the nowetting phase residual saturation from a static value, to one that depends on the nonwetting phase saturation history in each gridblock.
This relatively simple modification was shown to provide a good match with a laboratory experiment involving n-decane (a nonaqueous phase liquid) and water. Simulation results for field scale CO2 migration are similar to those obtained using much more complicated hysteretic modeling approaches. The new relative permeability model also provided an excellent fit to the experimental CO2 trapping data measured by Ruprecht et al. (2014) described in Task 1.
Task 4. Regional-Scale Variable Density Groundwater Modeling
Injection of CO2 in deep saline aquifers is expected to increase the pressure in these deep aquifers. One potential consequence of pressurization is an increase in the upward flux of saline water. Saline groundwater occurs naturally at shallow depths in many sedimentary basins, so an upward flux of solutes could degrade the quality of freshwater aquifers and threaten aquatic ecosystems. One problem could occur where saline water flowed upward along preferential paths, like faults or improperly abandoned wells. Diffuse upward flow through the natural stratigraphy could also occur in response to basin pressurization. This process would be slower, but diffuse upward flow could affect larger areas than flow through preferential paths, and this motivated us to evaluate this process.
In a recent MS thesis (Xie, 2014) and journal manuscript (Murdoch et al., 2014), we analyzed idealized 2D and 3D geometries representing the essential details of a shallow, freshwater aquifer underlain by saline ground water in a sedimentary basin. The analysis was conducted in two stages, one that simulated the development of a freshwater aquifer by flushing out saline water, and another that simulated the effect of a pulse-like increase in the upward flux from the basin. The results showed that increasing the upward flux from a basin increased the salt concentration and mass loading of salt to streams, and decreased the depth to the fresh/salt transition. The magnitude of these effects varied widely, however, from a small, slow process that would be challenging to detect, to a large, rapid response that could be an environmental catastrophe. The magnitude of the increased flux and the initial depth to the fresh/salt transition in groundwater controlled the severity of the response. We identified risk categories for salt concentration, mass loading, and freshwater aquifer thickness, and we used these categories to characterize the severity of the response. This showed that risks would likely be minor if the upward flux was smaller than a few tenths of the magnitude of recharge, according to the 2D analyses. The 3D analyses indicated that even larger fluxes could occur without a significant increase in the risk categories. The major contribution of this work is that it shows how a large increase in diffuse upward flux from a basin would cause significant problems, but a small increase in upward flux may occur without significantly affecting risks to the shallow freshwater flow system.
Task 5. Multiphase Flow Simulation of Field Scale Effects
This task focused on two main areas: the flow of dissolved CO2 and brine up abandoned wells (Ellison, 2011), and dissolved CO2 and brine migration in permeable fault zones (Falta et al., 2013).
Abandoned wells that penetrate a CO2 storage formation represent a clear hazard that has received much attention. Most of the work in this area has focused on the flow of supercritical CO2 or brine up an abandoned wellbore. However, most or all of the injected CO2 will eventually dissolve. Under specific circumstances with formation overpressure or overlying aquifer drawdown, CO2 saturated brine can flow up improperly abandoned wells where it can potentially enter and contaminate drinking water aquifers. The possibility that depressurization in the wellbore may cause CO2 exsolution from brine to form a separate buoyant gas phase is of primary concern. Analytical as well as numerical models were used to evaluate these effects in wellbores as well as to examine the effects of system parameters on brine leakage rates through wellbores (Ellison, 2011).
A simple analytical model for uniform density flow was used to evaluate the effects of physical parameters on fluid leakage. It is a useful screening tool for estimating leading order effects of system parameters on leakage of CO2 laden brine. A multiphase flow simulator was also used to evaluate wellbore leakage of dissolved CO2 considering gas exsolution due to pressure, temperature, phase, and salinity changes.
Simulations identified the conditions under which a separate gas phase exsolves in a wellbore during CO2 laden brine leakage. Up to 20% CO2 in the dissolved brine was found to exsolve in the numerical simulations. This gas accumulates along the top of a drinking water aquifer as a buoyant phase. Simulations also showed that the degree of leakage is constrained by the properties of the well, with the permeability of the well being of chief importance. However, at high well permeabilities, simulations show that the geologic formations provide more resistance to flow than the well and constrained leakage rates. Additional analyses were performed to see how dissolved CO2 may leak from a wellbore in a geologic system of stratified permeable layers. It is found that the presence of stratigraphy limits the possibility of upward migration of dissolved CO2, whether through overpressure of drawdown.
The study related to dissolved CO2 and brine flow in permeable faults (Falta et al., 2013) employed the CO2 relative permeability data measured in Task 1 by Zuo et al. (2012). When traditional coreflood relative permeabilities were used, simulations showed that an upward flow of a CO2 saturated brine lead to exsolution and the development of a highly mobile CO2 gas phase. However, when exsolution relative permeabilities were used, the tendency for the exsolved CO2 to migrate as a separate phase was greatly reduced, and the exsolved CO2 can partially block brine flow through the open fault. This type of reduced flow could also be expected in abandoned well bores that were filled with sand or some other porous material.
One important observation from both of these studies was that the upward flow of brines containing dissolved CO2 stops when the external driving force is removed, and no runaway instability is seen.
Task 6. Remediation Designs and Alternative Injection Schemes to Reduce Risk
Our work in this area has focused on a comparison of dissolved CO2 injection compared to the more conventional supercritical CO2 injection and on different CO2 injection schemes for economically minimizing CO2 mobility in the storage formation. This work is presented in the Ph.D. thesis by Catherine Ruprecht (Ruprecht, 2014), and is currently being prepared as two journal manuscripts.
In comparing dissolved CO2 injection with supercritical CO2 injection, it was found that dissolved CO2 injection was favorable in terms of storage security in all cases as it resulted in smaller areal extents on the caprock and the CO2 did not migrate appreciably beyond the time of the injection period. However, the distribution of dissolved CO2 was more strongly influenced by formation heterogeneities than supercritical CO2. In heterogeneous cases with high permeability zones, the storage efficiency of dissolved CO2 was lower than for supercritical CO2. An interesting observation in this work was that predicted storage efficiencies for dissolved CO2 injection tended to be higher than for supercritical CO2 injection in homogeneous or simple layered formations, but the reverse was true in more realistic heterogeneous systems.
The comparison of CO2 injection schemes involves an approach to optimizing CO2 storage design by determining economically efficient injection strategies that increase storage security through enhanced secondary trapping mechanisms. The simulations considered five different water/CO2 co-injection strategies, which were compared to a base case of standard supercritical CO2 injection. The results suggested that simultaneous water and gas injection, water alternating with gas injection and water flushing strategies could reduce overall costs and increase the degree of secondary CO2 trapping. Dissolved CO2 injection was found to have a substantially higher cost, but traps the most CO2 of any method considered.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 16 publications | 5 publications in selected types | All 5 journal articles |
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Falta RW, Zuo L, Benson SM. Migration of exsolved CO2 following depressurization of saturated brines. Greenhouse Gases: Science and Technology 2013;3(6):503-515. |
R834383 (Final) |
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Murdoch LC, Xie S, Falta RW, Ruprecht C. Effects of increased upward flux of dissolved salts caused by CO2 storage or other factors. Journal of Hydrology 2015;527:776-787. |
R834383 (Final) |
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Ruprecht C, Pini R, Falta R, Benson S, Murdoch L. Hysteretic trapping and relative permeability of CO2 in sandstone at reservoir conditions. International Journal of Greenhouse Gas Control 2014;27:15-27. |
R834383 (2013) R834383 (Final) |
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Zuo L, Krevor S, Falta RW, Benson SM. An experimental study of CO2 exsolution and relative permeability measurements during CO2 saturated water depressurization. Transport in Porous Media 2012;91(2):459-478. |
R834383 (2011) R834383 (2012) R834383 (2013) R834383 (Final) |
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Zuo L, Zhang C, Falta RW, Benson SM. Micromodel investigations of CO2 exsolution from carbonated water in sedimentary rocks. Advances in Water Resources 2013;53:188-197. |
R834383 (2012) R834383 (2013) R834383 (Final) |
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Supplemental Keywords:
Dissolved carbon dioxide, exsolution, carbon dioxide sequestrationProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.
Project Research Results
- 2013 Progress Report
- 2012 Progress Report
- 2011 Progress Report
- 2010 Progress Report
- Original Abstract
5 journal articles for this project