Carbon sequestration involves capturing, securing, and storing atmospheric carbon dioxide in soils, plants, the ocean, and other geologic formations. The whole idea is to minimize carbon dioxide in the atmosphere, reducing global climate change. Carbon sequestration occurs both naturally and because of anthropogenic activities. Therefore, we can see considerable interest in the possibilities of escalating the rate of carbon sequestration through geoengineering techniques such as carbon capture and storage, land use, and forestry. There usually are two types of carbon sequestration: geological carbon sequestration and biological carbon sequestration. 

Geologic vs Biologic Carbon Sequestration? 

The difference between Geologic and Biologic Carbon Sequestration is that: 

Geologic carbon sequestration involves separating, capturing, and storing carbon dioxide (CO2) in underground geological formations or rocks. Typically, CO2 gets captured from industrial sources like cement or steel production or energy sources such as natural gas processing facilities or power plants. Then, injection into porous rocks takes place for long-term use. 

On the other hand, biological carbon sequestration involves removing carbon dioxide from the atmosphere by micro-organisms or plants and storing it in woody plants, soils, vegetation, and aquatic environments. Biologic sequestration is promoted through the growth of plants-specifically larger plants such as trees, which help remove CO2 from the atmosphere.

The Concept of Geologic Carbon Sequestration

The general process of Geologic Carbon Sequestration involves five main techniques. These processes may vary in different geologic carbon sequestration plants.

The processes include:

1. Sequestration Strategy
2. Reservoir Modeling and Simulations
3. Well Engineering
4. Secure Geologic Storage
5. Monitoring, Reporting, and Verification

Sequestration Strategy

Sequestration Strategy is the first stage of geologic carbon sequestration. The primary purpose of this stage is to determine the basis of CO2 removal and how it fits into the organization’s carbon action plan. Firstly, CO2 removal allows for Carbon Neutral Production (CNP) operation. CNP contributes to the production of carbon-neutral fuels. Secondly, it can be a dedicated sequestration operation.

Finally, the procedure involves permanently storing all the CO2 emissions of a company underground, without any associated production. Therefore, as the first process, you must first establish how sequestration will fit into a particular organization’s carbon dioxide reduction plan. 

Reservoir Modeling & Simulations

The Reservoir Modeling and Simulations step is generally a reservoir engineering process that works in reverse. In this stage, engineers can establish a plan on how the reservoir will work. From here, engineers conduct a reverse approach to determine what will happen when carbon is liquified using a compressor and then injected back into the rock formation.

Understanding the effect of injecting carbon dioxide far into the earth’s surface requires specialized expertise. The process demands a broad and multidisciplinary approach that includes seismic analysis, zone analysis, permeability evaluations, volume calculations, Equation of State (EOS) fluid property analyses, stratification, depth, and much more.

Well Engineering

Once the engineers have isolated the carbon dioxide, they must design a road map to the target depth and location. Then, the engineers will develop a well transporting carbon from the surface to the target well. Several control measures are put in place to ensure the carbon does not escape during this process. The safeguards include delivering a significant well depth and building multiple casing layers on the well and natural geologic barriers. 

Secure Geologic Storage

Once the engineered wells are ready, the supercritical fluid carbon dioxide gets directed and injected into the reservoir. In most cases, the target reserve zones are well-structured, and record-proven to trap buoyant fluids and contain them securely for millions of years. As a result, the reservoirs are often oil and gas depleted reservoirs, or saline formations. Containment of CO2 within the subsurface rock takes place in three different states. The states include:

• Solution Trapping: In the Solution Trapping state, CO2 dissolves into hydrocarbons or saline water in the reservoir, where it becomes part and parcel of the reservoir fluid in the caprock. 
• Residual Trapping: In the Residual Trapping state, the supercritical fluid, carbon dioxide, is trapped in the porous space of the subsurface rock by the same forces as those that hold water in a sponge. 
• Mineral Trapping: In Mineral Trapping, the CO2 solution reacts with the minerals present in the rocks to form new minerals. The procedure ensures the permanent trapping of CO2 in the subsurface.

Monitoring, Reporting, and Verification

Geologic Carbon Sequestration is not as easy as simply burying something in the ground. Instead, these programs require strict Monitoring, Reporting, and Verification. To uphold the integrity of these programs, everything from the rigorous preventive maintenance programs to the super-sensitive downhole pressure sensors is all monitored, reported, and verified. 

Concerns About Geologic Carbon Sequestration

Although the current procedure plays a crucial role in minimizing CO2 in the atmosphere, it is still relatively experimental, and as a result, controversial. There are several unanswered questions about geological carbon sequestration. Some of the critical unanswered questions include:

• For how long does the sequestered CO2 remain captured?
• What are the risks and consequences to be considered?
• Will this procedure minimize our fossil fuels consumption?

Biological Carbon Sequestration

Biological sequestration occurs through increased photosynthesis. Therefore, sustainable forest management, reforestation, and genetic engineering play a significant role in land-use practices. Biological carbon sequestration occurs in several ways, as discussed below:

Soils

Soil contains more carbon dioxide as compared to what is available in the vegetation and the atmosphere combined. Sequestration of carbon in the ground takes place through the process of photosynthesis. The carbon storage in the earth is in the form of organic carbon (SOC) or carbonates. Carbonates are formed over millions of years when carbon dioxide dissolves in water percolating the soil. The factors that enhance biological carbon sequestration in soil include:

• Crop rotation: Crop rotation minimizes the loss of carbon from the ground. Crop rotation combined with additions such as phosphorus or manure will add carbon to the ground.
• Cover cropping: Cover cropping enhances carbon sequestration by improving the soil structure and adding organic matter.
• Conservation tillage: The procedure enhances water use efficiency, reduces soil erosion, and increases carbon concentrations in the topsoil.

Oceans

Research indicates that oceans absorb approximately 25 percent of the CO2 emitted from human activities, with colder parts of the ocean absorbing more CO2 than the warmer parts. There usually are two methods of ocean carbon sequestration (OCS). The methods include:

• OCS – direct injection: The injected CO2 dissolves in water, forming bicarbonates. The process minimizes the amount of CO2 present in the atmosphere.
• Ocean fertilization: The process involves promoting photosynthetic fixation of carbon dioxide by ocean organisms. The procedure minimizes a certain percentage of CO2 in the atmosphere. 

Forests

Approximately 25 percent of CO2 emitted globally goes to plant-rich landscapes like grasslands, forests, and rangelands. Plants absorb CO2 from the atmosphere, which they use to make their food through photosynthesis. In addition, forests capture CO2 from the atmosphere and transform it into biomass via the photosynthesis process. The sequestered CO2 accumulates in the form of forest soil, litter, biomass, and deadwood. 

With the current global health crisis, there has been a temporary decline in the daily global CO2 emissions. The decrease is significant because of the fall in industrial outputs and coal consumption. Because of the pandemic, most of the world’s largest CO2 emitters stopped their operations.

In addition to that, government policies as a result of the pandemic have affected the energy demand patterns around the world. Several international borders were closed, transport and consumption patterns changed. During the forced confinement periods, CO2 emissions drastically decreased. 

Contact Melzer Consulting

Are you looking for a certified professional in matters related to CO2 Enhanced Oil Recovery (EOR) industry? Melzer Consulting is here to help you. We have a highly experienced team with over 30 years of assisting clients. In addition to keeping a close watch of your CO2 market dynamics, we can consult several governmental and commercial organizations on business and technical aspects in the field of CO2 EOR.