Class VI Well Considerations: Storage Capacity

Injection Wells and Sustainable Energy Systems Carbon Capture Utilization and Storage Technology Class VI Well Considerations: Storage Capacity

How do geologists and engineers determine that a potential storage site is suitable for large-scale CO₂ injection and storage? Important factors are the geological characteristics of the subsurface and how they affect storage capacity. Beneficial characteristics include:

An extensive rock formation with high porosity and permeability to maximize storage space and accessibility
Depth below 800m¹ to ensure that the CO₂ remains in a supercritical state
A stable geological environment to prevent unplanned movement of CO₂

In this section, let’s take a look at the reservoir extent and porosity, both characteristics that impact the capacity of CO₂ that can be held for permanent storage.

Reservoir Extent

Suitable storage formations occur in both onshore and offshore settings, and within different types of geologic formations. Saline formations, filled with salty water (brine), span large volumes deep underground across large sedimentary basins. These formations maximize reservoir extent and are porous. Thus, saline formations have the largest potential volume for storing CO₂ globally. A number of federal agencies have examined the potential of saline formations in large sedimentary basins across the United States.²

Porosity

The area below the surface does not contain large empty pockets like caves that can have liquid or gas freely pumped into them; instead, the rock formations contain trillions of tiny spaces between the rock grains called pores that act together like a giant, rigid sponge.³

Rock formations contain trillions of tiny spaces between the rock grains called pores.

The porosity of a given rock is the percentage of the rock’s total volume that consists of these empty spaces, and can range from practically zero to 50%. For example, a limestone formation with 50% porosity can theoretically hold 10 times more CO₂ than a sandstone formation with 5% porosity. Excellent porosities make many depleted oil and gas fields suitable for CO₂ storage. Once the oil and natural gas are extracted, a porous and permeable reservoir remains that can be filled with CO₂.⁴

Because additional factors like capillary pressure and low permeability can significantly affect the ability of CO₂ to spread or migrate into available pore spaces, it is critical to select storage reservoirs to maximize porosity. Because of other factors, the amount of available pore space filled with CO₂ by the completion of injection is usually estimated at only a few percent.⁵

Let’s look at how porosity impacts two important spatial components of a CO₂ storage project: the CO₂ plume and the Area of Review.

CO₂ Plume

The CO2 plume refers to the area of a formation occupied by flowing supercritical CO₂ during and after injection. Because of the way surface rights and mineral rights operate under U.S. law, a plan for an injection site must accurately model vertical and lateral expansion of the CO₂ plume. In general, we want to maximize vertical expansion and minimize lateral expansion. This is a delicate balancing act between a number of physical and geological factors.

Capillary pressure needs to be low enough to economically inject large amounts of CO₂, but high enough so that the plume extends vertically before it extends laterally. The geometry of the injection reservoir is also an important factor. In a thick formation with CO₂ injected near the base, the plume will expand radially for a short distance, then start to move upwards until it encounters the caprock. Conversely, in a thin zone the vertical extent of the formation will quickly be saturated and the plume will migrate outwards laterally, increasing the footprint of the plume as well as exerting pressure on the caprock.

We also want to assess the ability of the plume to reach the available porous areas of rock. Again, this comes down to measuring the buoyancy of the supercritical CO₂ against the existing capillary forces in the rock. As we will discuss later, capillary pressure is an important factor in the long-term storage of CO₂. But if the existing capillary pressure is too high from the start, it is unsafe to inject CO₂ at the required injection pressure to establish the plume, and uneconomical, as the majority of the available pores will be inaccessible the further from the injection point the plume travels.⁶

Area of Review

Federal regulations require extensive modeling of the reservoir area of a storage project prior to CO₂ injection and regular modeling during and after injection. These two requirements are closely linked, because modeling the characteristics of the injection reservoir will result in determining the Area of Review (AoR), or the extent of the area to which the CO₂ plume will migrate.⁷

The area of review is the region surrounding the geologic sequestration project where USDWs may be endangered by the injection activity. The area of review is delineated using computational modeling that accounts for the physical and chemical properties of all phases of the injected carbon dioxide stream and is based on available site characterization, monitoring, and operational data.
40 CFR § 146.84

The AoR essentially comprises the geological and geographic extent where regular monitoring for leaks will occur over the life of the injection project. Additionally, reservoir modeling experts are required to periodically reevaluate the AoR throughout the project duration, updating models and adjusting monitoring practices accordingly.

Images: “Wine cellar” by hxdbzxy via Shutterstock.com

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Class VI Well Considerations: Storage Capacity

Reservoir Extent

Porosity

CO2 Plume

Area of Review

CO₂ Plume