Hands-on Activity Rock Solutions:
A Solid Underground CO2 Storage Engineering Challenge

Learning Objectives

After this activity, students should be able to:

• Describe how porous and impermeable materials affect fluid and gas movement.
• Explain the role of cap rock in trapping gases underground.
• Collect and analyze quantitative data from a physical model.
• Apply the engineering design process to improve system performance.
• Evaluate trade-offs between storage capacity and leakage.

Educational Standards

Each Teach Engineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in Teach Engineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

Materials List

Each group needs the following:

• 1 clear plastic cup
• 50 mL sand
• 50 mL gravel
• 50 mL clay
• 2–3 tablespoons baking soda
• 20 mL vinegar + 3 drops food coloring
• 1 plastic dropper/pipette
• 1 ruler (cm)
• 1 stopwatch/timer
• 1 spoon
• 1 Design Sheet per student
• 1 pair of nitrile gloves per student
• 1 pair of safety goggles per student
• paper towels

Worksheets and Attachments

Pre-Req Knowledge

Students should be familiar with:

• States of matter (solid, liquid, gas).
• Basic measurements (length, time).
• Simple data recording.

Introduction/Motivation

Let’s start with a big question: What happens to carbon dioxide (CO₂) after it’s released into the atmosphere? (Let students answer. Potential answers: It stays in the atmosphere; plants absorb some of it; oceans absorb some of it; it traps heat; it contributes to climate change.)

That’s right. While plants and oceans absorb some CO₂, much of it remains in the atmosphere, where it acts as a greenhouse gas. As CO₂ levels increase, more heat is trapped, contributing to climate change. Scientists and engineers are asking an important question: What if we could store some of that CO₂ somewhere else—like underground? Where do you think it could be stored? (Let students answer. Potential answers: In rocks; in underground rock layers; beneath Earth's surface.)

Exactly. This process is called carbon capture and storage (CCS). Engineers capture CO₂ before it enters the atmosphere and inject it deep underground into suitable rock formations. But here's the challenge: If we inject CO₂ underground, what might go wrong? (Let students answer. Potential answers: It could leak; it could escape to the surface; it could move into other rock layers.)

Yes, preventing leaks is critical. Whether CO₂ stays underground depends on the properties of the rocks. Some rocks are porous, meaning they contain tiny spaces, or pores, that can hold gases. If those pores are connected, the rock is permeable, allowing gases to move through it.

So, which type of rock is better for storing CO₂—porous or non-porous rock? (Let students answer. Answer: Porous rock, because it has spaces that can hold the gas.)

Correct—but a porous storage rock alone isn't enough. Engineers also need a cap rock, a layer of rock with very low permeability that traps the CO₂ and prevents it from escaping. Do you think a thicker cap rock is always better? (Let students offer answers. Potential answers: Maybe; it might reduce leaks; it could have trade-offs, such as reducing storage space or increasing cost.)

Exactly! Engineers must balance storage capacity, safety, and the potential for leakage when designing carbon storage systems.

Today, you'll act as an environmental engineer. You'll design a layered underground system using sand, gravel, and clay to model rock formations that store CO₂ while preventing leaks. You'll test your design, collect data, and improve it using the engineering design process. If your design leaks, is that a failure? (Let students answer. Answer: No—it provides data that can be used to improve the design.)

Let's start designing!

Procedure

Background 

Carbon Capture and Storage (CCS)

Carbon capture and storage (CCS) is a technology used to reduce carbon dioxide (CO₂) emissions that contribute to climate change. Instead of releasing CO₂ into the atmosphere, the gas is captured from sources such as power plants and industrial facilities, compressed, and injected deep underground into suitable rock formations for long-term storage.

For underground storage to be successful, engineers must identify rock formations that can safely contain the CO₂ for hundreds or even thousands of years. Selecting an appropriate storage site requires an understanding of geology, rock properties, and fluid movement underground.

Porosity and Permeability

Two important properties determine whether a rock is suitable for storing carbon dioxide:

• Porosity is the amount of empty space (pores) within a rock. Rocks with high porosity can store larger amounts of fluids or gases.
• Permeability describes how well those pore spaces are connected, allowing fluids or gases to move through the rock.

A rock can be highly porous but have low permeability if its pores are not well connected.

Reservoir Rocks and Cap Rocks

Successful carbon storage relies on two types of rock layers:

• Reservoir rocks are porous and permeable, allowing injected CO₂ to enter and be stored within the pore spaces. Sandstone is a common example.
• Cap rocks are dense, nonporous, and have very low permeability. They act as a seal that prevents CO₂ from escaping to the surface. Shale is a common natural cap rock.

In this activity:

• Sand and gravel model reservoir rocks because they contain connected spaces that allow fluids and gases to move.
• Clay models a cap rock because it is much less permeable and slows or prevents leakage.

Modeling Carbon Storage

Students construct a simplified model of an underground storage system using layers of sand, gravel, and clay. Colored vinegar represents injected CO₂, while the baking soda produces carbon dioxide gas during the chemical reaction. Although this model does not perfectly replicate underground conditions, it allows students to observe how different layer arrangements affect fluid and gas movement and the ability of a cap rock to contain the simulated CO₂.

Remember that this investigation is a model of underground carbon storage and has several limitations.

• The materials do not perfectly represent real rocks.
• The reaction between vinegar and baking soda produces carbon dioxide quickly, unlike industrial injection processes.
• Real underground storage occurs thousands of feet below Earth's surface under high pressure and temperature.
• Engineers use extensive geological surveys, computer modeling, and long-term monitoring before selecting a storage site.

Despite these limitations, the model demonstrates the importance of reservoir rocks, cap rocks, and engineering design in reducing the risk of CO₂ leakage.

Before the Activity

• Gather the materials needed by each group.
• Make copies of the Design Sheet (1 per student).

During the Activity

Part 1: Ask, Research, Imagine, Plan (25 minutes)

1. Divide students into teams of 2-4.
2. Present the design challenge: Your team will design a layered underground system using sand, gravel, and clay to model rock formations that can store carbon dioxide (CO₂) while preventing leaks.
3. Introduce the available materials.
4. Explain the design constraints:

• All three materials must be used.
• The cap rock layer must be 1–3 cm.
• The maximum injection volume is 20 mL.
• The completed model must fit within the provided cup.

5. Explain how models will be tested:

• Students will inject a measured volume of colored vinegar into the baking soda at the bottom of their model while timing the reaction.
• As the reaction occurs, students will observe how the liquid and gas move through the model and record when and where any leaks occur.

6. Describe the measurements to be recorded:

• Leak Time (s): Time until the first visible leak occurs.
• Leak Height (cm): Vertical distance the liquid or gas travels from the bottom of the model before leaking.
• Spread Width (cm): Horizontal distance the leaked liquid spreads.

7. Allow students to ask clarifying questions about the design challenge, testing procedure, and measurements.
8. Optional: Review the characteristics of the rock layers used in carbon storage (e.g., porous layers, cap rock, sand, gravel, clay, etc.).
9. Distribute one Design Sheet to each student.
10. Give students 10 minutes to complete Part 1 of the Design Sheet by answering the questions and sketching a layered model design.
11. Remind students that their sketches should:

• Identify the materials to be used.
• Label the porous layer(s) (sand and/or gravel).
• Label the cap rock layer (clay is recommended).
• Record the planned cap rock thickness (1-3 cm).
• Record the planned injection volume (20 mL or less).

12. Have students complete the Design Decision section of their Design Sheet.

Part 2: Build and Test (25 minutes)

13. Distribute materials to each group, or have groups collect the materials they need from a common area.
14. Have each group add a 1–2 cm layer of baking soda to the bottom of their cup. (See Image 1.)

15. Ensure each group positions the dropper tip so that it reaches the baking soda layer.
16. Instruct teams to build their models according to the sketches they completed in Part 1.
17. Once each team has completed their model, have them measure and record the actual cap rock thickness for Trial 1 in Part 2 of the Design Sheet.
18. Optional: Review the testing procedure. (See Part 1.)
19. Optional: Review the measurements to be recorded. (See Part 1.)
20. Give teams time to test their model by:

• Filling their dropper with colored vinegar (record volume).
• Starting the timer when injection begins.
• Slowly injecting vinegar into the baking soda.
• Observing the movement of liquid and gas.

21. Have students record their Trial 1 results in Part 2 of the Design Sheet.

Part 3: Analyze (10 minutes)

22. Have each team review the results from Trial 1 recorded on their Design Sheet.
23. Ask teams to analyze the performance of their model by considering questions such as the following:

• Did the model successfully contain the liquid and gas?
• Where did leaks occur?
• Which layer(s) performed well?
• Which layer(s) may have contributed to the leak?

24. Explain that they will answer the questions in Part 3 of the Design Sheet.

Part 4: Redesign (15 minutes)

25. Instruct each team to identify one aspect of their design to improve. Explain that, to conduct a fair test, teams may modify only one design variable while keeping all other variables the same.
26. Have each team select one of the following variables to modify:

• Cap thickness
• Material type
• Layer arrangement
• Injection volume (20 mL or less)

27. Have students record their planned design change and explain why they believe it will improve their model in Part 4 of the Design Sheet.
28. Instruct teams to rebuild their model using the revised design.
29. Have students measure and record the actual cap rock thickness for Trial 2.
30. Give teams time to test their redesigned model using the same testing procedure described in Part 2.
31. Have students record their Trial 2 results in Part 2 of the Design Sheet.
32. Optional: If time allows, let teams analyze their redesigned results and complete additional redesign iterations.

Part 5: Analysis and Discussion (10 minutes)

33. Give students time to complete the reflection questions in Part 5 of the Design Sheet.
34. Facilitate a brief discussion by asking teams to compare the results of Trial 1 and Trial 2 and determine whether their design modification improved the model's ability to contain the simulated CO₂. Guide students to analyze:

• Which design had the longest leak time?
• Which design minimized leakage?
• How did cap thickness affect results?
• Which materials worked best?

Vocabulary/Definitions

cap rock: An impermeable layer that traps fluids underground.

leakage: Refers to the escape of stored fluids from a designated underground geological formation through permeable rocks.

permeability: The ability of a rock to allow fluids to pass through.

porosity: The measure of empty spaces in a rock that can hold fluids.

Assessment

Pre-Activity Assessment

Predict and Discuss: Students offer answers to the questions in the Introduction/Motivation section:

• What happens to carbon dioxide (CO₂) after it’s released into the atmosphere? (Potential answers: It stays in the atmosphere; plants absorb some of it; oceans absorb some of it; it traps heat; it contributes to climate change.)
• What if we could store some of that CO₂ somewhere else—like underground? Where do you think it could be stored? (Let students answer. Potential answers: In rocks; in underground rock layers; beneath Earth's surface.)
• If we inject CO₂ underground, what might go wrong? (Potential answers: It could leak; it could escape to the surface; it could move into other rock layers.)
• So, which type of rock is better for storing CO₂—porous or non-porous rock? (Answer: Porous rock, because it has spaces that can hold the gas.)
• Do you think a thicker cap rock is always better? (Potential answers: Maybe; it might reduce leaks; it could have trade-offs, such as reducing storage space or increasing cost.)

Activity Embedded (Formative) Assessment

Imagine and Plan: Students imagine, plan, and then sketch a layered model design in their Design Sheet. Check sketches to ensure they all contain the following:

• Bottom: baking soda (gas source)
• Middle: porous layer (sand/gravel)
• Top: cap rock (clay)
• Injection point reaching bottom layer

Data Collection: Students collect data from their model and record it in Part 2 of their Design Sheet. Upon completion, have them compare their data with another group.

Post-Activity (Summative) Assessment

Reflection Questions: Students answer reflection questions in their Design Sheet.

• What trade-offs exist between storage capacity and leak prevention? (Potential answers: More porous materials like gravel allowed for more gas storage, but it also caused faster and wider leakage. Thicker or less permeable caps slowed leakage but limited how quickly the gas could be injected or distributed.)
• What are two limitations of this model compared to real systems? (Potential answers: The model does not simulate the high pressures and temperatures found deep underground; the model cannot replicate long-term behavior such as rock fractures, chemical reactions, or supercritical CO₂ flow.)
• How might engineers monitor underground leaks? (Answer: Engineers could monitor leaks using pressure sensors, soil or air gas detectors, seismic monitoring, or satellite and remote sensing technologies to detect gas escaping from storage sites.)

Class discussion: Students participate in a class discussion analyzing the following questions:

• Which design had the longest leak time?
• Which design minimized leakage?
• How did cap thickness affect results?
• Which materials worked best?

Activity Extensions

Investigating Cap Rock Thickness
Challenge students to investigate how cap rock thickness affects carbon dioxide containment. Have each team build multiple models that differ only in cap rock thickness (e.g., 1 cm, 2 cm, and 3 cm) while keeping all other design variables constant. Students should test each model, record the leak time, leak height, and spread width, and compare the results.

Activity Scaling

For younger students:

• Provide a partially completed Design Sheet with guiding questions.
• Demonstrate how to build one example model before students begin.
• Conduct the first trial as a whole-class demonstration.
• Limit redesign choices to only cap rock thickness.
• Focus discussion on identifying leaks rather than quantitative measurements.

For older students:

• Allow students to determine their own design constraints.
• Require students to justify their design decisions using principles of porosity, permeability, and cap rock integrity.
• Introduce concepts such as reservoir pressure, geologic formations, fault zones, and carbon sequestration.
• Have students go beyond basic observations by:
• Calculating averages across multiple trials.
• Graphing relationships (e.g., leak time vs. cap rock thickness or injection volume vs. spread width).
• Identifying trends in the data and using evidence to justify their conclusions.
• Have students calculate the percentage improvement between Trial 1 and Trial 2.
• Require students to prepare an engineering report or presentation summarizing their design process, data analysis, redesign decisions, and conclusions.

Copyright

Contributors

Supporting Program

Acknowledgements

This curriculum was developed under National Science Foundation - The Nanotechnology Research Experience for Teachers at Rice University Grant Number NSF EEC-2302127. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.