The 15th International Virtual Conference on Greenhouse Gas Control Technologies will take place from March 15th to 18th, 2021. GCCC will have a large presence there with researchers giving nine presentations over three days.
Time in UAE (For Full Session)
2F-Environmental Impacts & Remediation
Assuring long-term storage of captured CO2:
In 2009, with the pending Federal legislation to limit greenhouse gas emissions (Waxman-Markey Bill), the Texas Legislature used SB 1387 to advance CCUS in Texas.
One result was a joint report by the Railroad Commission (RRC-oil and gas regulation), Texas Commission on Environmental Quality (TCEQ), General Land Office (GLO), and Bureau of Economic Geology (BEG) entitled: Injection and Geologic Storage Regulation of Anthropogenic Carbon Dioxide. A lot has changed in the decade since, but this report still provides information useful for understanding CCS in Texas and makes 9 recommendations covering various aspects of the topic. A primary goal of this report is to clarify some of the technical and regulatory issues surrounding geologic storage of CO2.
Once injected, CO2 does not stay in one place. CO2 storage depends on a variety of trapping mechanisms to ensure long-term storage. Once in the reservoir, CO2 interacts with the resident brine water, minerals, and other chemicals in the subsurface, determining its ultimate destination. Predicting the movement of CO2 is a crucial step in the monitoring of carbon capture and storage projects which the Gulf Coast Carbon Center (GCCC) specializes in.
The primary trapping mechanism for a CO2
storage project is the overlying nonporous caprock that restricts the vertical,
buoyant flow of CO2. A secondary trapping mechanism called residual
trapping is a major contributor to the volumes of CO2 that can be
Residual trapping can occur when two immiscible fluids flow together in porous media. The porous media is usually “wetting” to one fluid and “nonwetting” to another. Wetting is the ability of water to spread out and cover a substrate, often in water-loving or “hydrophilic” settings. Typically, water is the wetting medium and CO2 is the nonwetting. When both flow through the porous media, water surrounds the nonwetting fluid (CO2) and restricts its movement. This causes trapping at the individual rock grain level, so that CO2 does not have enough driving force to get out of the microscopic spaces between grains.
GCCC’s newest post-doctoral student, Hailun Ni,
studies two different types of residual trapping: pore-scale and mesoscale
capillary heterogeneity trapping at carbon storage sites. Mesoscale captures
rock at the millimeter scale and pore scale looks at rock within the micron
range and smaller. At the mesoscale, rock heterogeneities such as lamination
can act like tiny caprocks, restricting the flow of CO2 and trapping
it securely underneath.
Ni’s research, under the guidance of Tip Meckel, is a
unique addition to our research consortium because we have scientists working
on the pore-scale and reservoir trapping, but not in between. By constraining
the movement of CO2 at these different scales, we have a more
accurate prediction of CO2 movement in the subsurface, therefore
Before joining GCCC, Ni earned her Ph.D. from Stanford University where she studied Energy Resources Engineering under Sally Benson. On Tuesday, Ni gave a talk during the weekly GCCC staff meeting regarding her Ph.D. research.
As part of her Ph.D. research, Ni conducted coreflooding
experiments and CT imaging to characterize and quantify the impact of
small-scale heterogeneity on CO2-water capillary flow and residual
trapping with sandstone cores. During her talk titled, “Coreflooding
experiments and prediction of CO2 residual trapping,” Ni covered the
first half of her Ph.D. thesis.
Many studies have been conducted using microCT scans which do not capture the mesoscale heterogeneity of rocks, which can have a significant influence on flow. No previous studies have used real rocks to quantify the relative contribution of the two different CO2 residual trapping mechanisms for different sandstone types.
In order to find residual trapping rates, she used an experimental apparatus that injects CO2 in nine representative sandstone cores with varying petrophysical properties. For most of the cores, the flow rates of CO2 remained the same during injection throughout the experiment. The core was initially soaked with water, and the injection fluid was initially composed of half CO2 and half water. Gradually, the ratio of CO2 was increased in the injectate until it was 100 percent CO2 which was plotted as the initial CO2 saturation value. Then, the CO2 ration was decreased until the injectate was 100 percent water, which was the final value for residual saturation.
The results of the experiment help distinguish the
relative importance of the two residual trapping mechanisms. Experimental
results demonstrate that CO2 residual trapping ability decreases
with porosity and increases with the degree of heterogeneity. In order to quantify
the level of heterogeneity in a sample, Ni and colleagues also evaluated a
number of metrics that act as proxies for heterogeneity. Ni found that the two
best predictors for a sandstone core’s CO2 residual trapping ability
are porosity and the maximum standard deviation in the drainage CO2
saturation field. As stated in the paper, additional results indicate that “pore-scale
trapping mechanisms account for 46–97% of the residually trapped CO2
and the mesoscale capillary heterogeneity trapping mechanism accounts for 3–54%
of the residually trapped CO2 for the nine sandstone samples tested.”
Ni’s next steps are to ask geological questions: What
simple parameters can be used to infer more difficult parameters in order to
characterize the regional-scale geology? During the preliminary discussion, one
of our researchers suggested that cementation, minerology, and depositional
facies can derive other parameters, and could be a launching point for Ni’s
future work with the GCCC.
As for her work with the GCCC, Ni will continue to
build on the work of previous Ph.D. student Prasanna Krishnamurthy to build
sand tanks with realistic sedimentary structures and conduct
capillary-dominated, gravity-driven multiphase flow experiments with the light
transmission visualization method to study how different types of heterogeneity
affect CO2 migration and capillary heterogeneity trapping (local
capillary trapping) at a larger scale.