Depleted oil and gas reservoir (such as those used in enhanced oil recovery) have been proposed as a storage method for captured atmospheric CO₂ due to the large amount of infrastructure already existing on site
Summary
Depleted oil and gas reservoirs are being considered for CO₂ storage due to existing infrastructure from previous oil operations and favorable geological characteristics. These reservoirs typically feature porous rock, an impermeable caprock, and depths over 1000 meters. CO₂ is trapped through multiple mechanisms including structural containment under caprock, residual trapping in pore spaces, dissolution in brine, and mineral conversion. While offering economic benefits through reuse of infrastructure and enhanced oil recovery potential, this method raises concerns about perpetuating fossil fuel use and potential environmental risks like water contamination from residual hydrocarbons.
Depleted oil and gas reservoirs are often considered as a storage method for CO₂, as the technology has been developed and tested for enhanced oil recovery (EOR) by the oil industry, and much of the infrastructure required already exists on site.
Characteristics of depleted oil and gas reservoirs
The characteristics of depleted oil and gas reservoirs are very similar to saline aquifers, which contain similar rock types and properties. Depleted oil and gas reservoirs also often contain large amounts of briny water, which was utilized in the initial conventional stimulation of the oil reservoir 1. Some of these characteristics include:
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High porosity and permeability: Oil and gas reservoirs are typically composed of porous rock formations.
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Impermeable caprock: Without the impermeable caprock, much of the initial oil and gas reservoir would have escaped before they were tapped by humans 1.
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Depth: Many depleted oil and gas reservoirs are located at depths of more than 1000 meters below the surface 1.
Trapping Mechanisms
Similar to the characteristics of depleted oil and gas reservoirs, the trapping mechanisms are also similar to saline aquifers and contain one or more of the following:
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Structural/stratigraphic trapping: Structural trapping is the dominant trapping mechanism for depleted oil and gas reservoirs. Structural trapping is simply the trapping of CO₂ under the impermeable caprock. As the density of the CO₂ is lower than the density of the brine, the CO₂ will naturally rise to the surface and be trapped under the caprock 1. This is analogous to taking an empty cup, placing it upside down, and then submerging it in water. If the cup is impermeable, the air will be trapped inside.
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Residual trapping: Residual trapping is the trapping of CO₂ in the remaining pore space after the CO₂ has been injected into the depleted oil and gas reservoir. This occurs when the CO₂ is injected into the aquifer and displaces the brine or water. When the displaced brine or water starts to return to the rock, the CO₂ will be trapped in the remaining pore space.
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Solubility trapping: Solubility trapping is the trapping of CO₂ into the brine as the CO₂ dissolves 1.
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Mineral trapping: Mineral trapping is the trapping of CO₂ via the precipitation of carbonate minerals. This is achieved through the formation of carbonic acid (H₂CO₃) in the brine, which reacts with other minerals present in the rock formation such as calcium (CaCO₃) and magnesium (MgCO₃).
Advantages
The main advantage of utilizing depleted oil and gas reservoirs for CO₂ storage is the large amount of ready-to-use infrastructure already existing on site. Additionally, the feasibility of the site has already been evaluated in the initial oil prospecting process, which can lead to greater confidence in the integrity of the caprock. The possibility for EOR also makes this storage method significantly more economically favorable as it can generate revenue from the stored CO₂.
Disadvantages
The main disadvantage of utilizing depleted oil and gas reservoirs for CO₂ storage is that they assist in continuing the production and usage of fossil fuels, which in turn counteracts the purpose of CO₂ storage. There are also additional environmental concerns associated with EOR, such as the contamination of water due to the presence of toxic compounds that are present in the unrefined oil, such as benzenes 1.