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 stored.

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 decreasing risk.

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.

Read more about her results and study design in her publication, “Predicting CO₂ residual trapping ability based on experimental petrophysical properties for different sandstone types.”

 

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. 

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This post was originally published by the Bureau of Economic Geology

Each year, the Gulf Coast Carbon Center (GCCC) holds two mid-year meetings for their industrial affiliates: one in January and one in August. These meetings cover the latest developments in carbon capture and storage research completed by the center’s scientists. The most recent meeting took place on August 25 and was attended by nearly 50 representatives from the GCCC’s sponsors.

To give a different look and feel to the poster session this summer, researchers used Prezi, an online presentation software, to create an interactive environment in place of the usual static poster content. During each of three time slots, grouped into three themes, researchers gave a technical brief followed by lengthy discussion. You can read an overview of each theme below.

Following the poster sessions, the researchers conducted a short feedback poll to glean insights for the next sponsor meeting. Researchers that serve as point liaisons with the various sponsor companies will follow up after the meeting to schedule more individualized meetings to further discuss research and future directions to pursue. GCCC researchers are looking forward to applying lessons learned to the next meeting.

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BIGFOOT

BIGFOOT refers to GCCC research into storage that has a large footprint. Enhanced tax credits for storage, combined with falling oil prices and public concerns over climate change, have spurred interest in near-term, moderate-scale storage projects on the Gulf Coast and longer-term interest in large-scale storage hubs capable of storing several megatons or more per year.

Work to date has demonstrated the Gulf Coast’s capacity for storing large volumes of CO₂ at industrial rates, but the process of optimizing site selection for individual projects is still an open question, particularly for large-scale projects. BIGFOOT aims to address that gap by developing both a Gulf Coast prospect inventory and a set of broadly applicable characterization workflows that can reduce cost and increase confidence in siting very large-volume storage.

During the BIGFOOT presentation, Alex Bump gave an overview of the GCCC’s recent work on large-scale storage projects. He shared the center’s progress on adapting petroleum exploration workflows for carbon capture and storage (CCS) using layered risk maps calibrated with historic gas production to predict regional variations in storage performance of the Gulf Coast Oligocene and Miocene sections. He also presented promising storage play concepts and more detailed, site-specific characterizations.

Modeling and Monitoring

CO₂ storage regulations require that storage operations are rigorously monitored to provide assurance of long-term storage integrity. To evaluate injection scenarios and estimate storage capacity and security, an accurate understanding of the subsurface migration of CO₂ plume and its trapping mechanisms is essential. Through pore-scale studies, researchers have shown how dissolution of CO₂ in brine contributes to the trapping of the advancing plume during the injection stage.

During the Modeling and Monitoring presentation, Sahar Bakhshian covered two topics: prediction of a CO₂ plume migration and trapping using numerical models, and advanced field-scale monitoring techniques. Among other models and simulations, she presented an analytical solution to a basin-scale theoretical model for the migration and capillary trapping of CO₂ plume in a sloping aquifer during the post-injection stage.

She also reviewed the recently deployed process-based technique for soil-gas monitoring at geologic CO₂ storage sites that her team is working on, and demonstrated her team’s machine learning techniques for facilitating prediction of plume migration and their data-driven deep learning approach for anomaly detection in streaming environmental sensor data. 

Ecosystem

The Ecosystem theme covered outreach and networking done by the GCCC for the energy ecosystem outside and within the field of carbon capture and storage. During the Ecosystem presentation, Katherine Romanak covered research and outreach relevant to the ever-changing CCS ecosystem. To reiterate the importance of stakeholder-oriented research, Romanak remarked:

“The ecosystem includes expanding our connections with new stakeholders such as the Gulf Coast Carbon Collaborative and the Carbon Utilization Research Council. We also have continued our previously successful activities, such as implementing the 4th International Workshop on Offshore CO₂ storage and continuing our involvements with ISO standards. We are also embarking on targeted studies, including looking at the potential for recommissioning offshore infrastructure and providing screening of storage sites looking at fetch-trap pairs. 

“Looking to the future, we are developing an online CCS course for petroleum professionals who may need to retool their skills for application in CCS industry and applying our environmental monitoring techniques to satisfy the technically challenging requirements of the low carbon fuel standard CCS protocol.”

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GCCC’s next sponsor meeting is scheduled for January 2021.

To become a sponsor member of the Gulf Coast Carbon Center, please contact Research Program Coordinator Emily Moskal.

Read more about the GCCC on the center’s website.

Current GCCC sponsor companies include Air Products, BHP, BP, Chevron, ExxonMobil, Petra Nova Parish Holdings, Shell, and Total.