Geologic sequestration is the process of injecting captured carbon dioxide into rock formations far below the Earth’s surface for the purpose of long-term storage. Essentially, this entails putting carbon back underground into the kinds of rock formations from which it came. But how, exactly, does it work?
Created through geologic conditions underground, hydrocarbons such as oil and gas are a natural way for the planet to store energy. Once extracted by humans and burned, the energy within this hydrocarbon is released along with carbon dioxide. The accumulation of this carbon dioxide in the atmosphere causes the global warming challenge that is impacting us all.
Addressing this challenge will require a number of carbon reduction pathways, including geologic carbon sequestration. Geologic sequestration is integral to Carbon Capture, Utilization and Storage (CCUS) according to the IEA Clean Technology Scenario (CTS), CCUS is projected to contribute 13% of the cumulative emissions reduction through 2060 in order to meet the world’s climate goals.
Geologic sequestration allows us to permanently keep CO2 out of the atmosphere by capturing it and injecting it deep into the ground within the very types of rock formations from which it was extracted. These rock formations were able to trap carbon for millennia, naturally, and are therefore able to resume this role to help keep carbon dioxide out of the atmosphere. Occidental has safely conducted carbon sequestration for over 40 years.
How does the process work?
The general process of geologic sequestration has five steps, which include:
1. Sequestration Strategy
The first step involves determining the purpose of the sequestration, and how it will fit into an organization’s carbon action plan. Carbon dioxide can be sequestered as a way to enable a Carbon Neutral Production (CNP) operation that contributes to the production of carbon-neutral fuels. Or it can be a dedicated sequestration operation that stores emissions from a single company’s operations permanently underground without associated production.
There are other models as well, but the first step is to determine how sequestration can best fit into a specific organization’s CO2 emission reduction plan.
2. Reservoir Modeling & Simulations
This step resembles reservoir engineering for oil and gas wells, except in reverse. Instead of determining how best to extract hydrocarbons from the Earth, these engineers are doing the opposite—determining what will happen when carbon, liquified via compression for easier management, is injected back into a rock formation. This step gives engineers a plan for how the reservoir will work.
Understanding the impact of injecting CO2 into a rock formation far below the Earth’s surface requires specialized expertise. It demands a broad, multi-disciplinary approach that includes volume calculations, seismic analysis, depth, stratification and zone analysis, Equation of State (EOS) fluid property analyses, porosity and permeability evaluations and much more, often with the use of supercomputers.
3. Well Engineering
Once the team knows where the carbon should go, they need to design, engineer and develop a well that will deliver the carbon from the surface to the well’s target depth and location. To ensure that carbon does not escape, the well is designed using a number of safeguards including significant well depth, natural geologic barriers such as an impermeable “cap rock” and multiple casing layers on the well itself that create a cement-reinforced series of concentric pipes-within-pipes.
4. Secure Geologic Storage
When the well is ready, the supercritical fluid CO2 is then injected into the reservoir. Typically the target zones include structures with a proven record of trapping buoyant fluid and containing it securely for millions of years. These are generally depleted oil and gas reservoirs, or saline formations. In accordance with the engineer’s design, carbon is contained within their subsurface rock in a variety of ways such as:
CO2 dissolves into saline water or hydrocarbons in the reservoir, becoming a part of the reservoir fluids trapped under the cap rock
Supercritical CO2 becomes trapped in the pore spaces of the rock by the same forces that hold water in a sponge
CO2 reacts with existing minerals to form new minerals, permanently trapping the CO2 as a rock
5. Monitoring, Reporting & Verification
Carbon sequestration through Carbon Neutral Production (CNP) operations isn’t like simply burying something in the ground. This precision process is meticulously controlled through a program of strict Monitoring, Reporting and Verification (MRV). Occidental sequestration operations are monitored by the United States Environmental Protection Agency. In fact, Occidental developed the first two EPA-approved MRV programs in the nation.
The MRV program systems include:
Everything from super-sensitive downhole pressure sensors to rigorous preventive maintenance programs for surface equipment ensure the integrity of these systems.
Geologic sequestration has the potential to help deliver carbon reduction goals like few other technologies. And it’s an area of expertise in which we have unique knowledge to help accelerate program deployment. Now energy and industrial stakeholders must find new ways of working together to make the most of this proven process to take carbon out of commission for good.