Aquifers vs. Enhanced Oil Recovery

Various industries such as cement and steel production, hydrogen production from fossil fuels, waste incineration, and power generation utilize on-site carbon capture and storage in Enhanced Oil Recovery (EOR). This process involves intercepting the carbon dioxide before it is released into the atmosphere.

The process involves compressing carbon dioxide to a pressure exceeding 100 atmospheres and actively injecting it into porous rock layers located at least one kilometer below the Earth’s surface. These rocks, capable of retaining the carbon dioxide for several millennia, are positioned beneath the surface.

Geological formations filled with brine, otherwise known as aquifers, are at odds with enhanced oil recovery (EOR) techniques when it comes to on-site carbon sequestration. By injecting carbon dioxide at pressure, EOR can increase the recovery of oil and gas. The purpose of this article is to explore both aquifers and EOR as methods of carbon dioxide sequestration. 

Aquifers

Aquifers are subterranean reservoirs of briny water that can be found beneath a layer of impermeable ‘caprock’ in sedimentary rock. Representing a major carbon capture and storage (CCS) option, these formations are located at depths of more than 1 km and are widespread across the globe. When injected into the rock, carbon dioxide is compressed to a density of between 200-800 kg/m3 and displaces the brine, resulting in a plume emanating from the injection point that typically heads toward the top of the aquifer.

When carbon dioxide meets brine, it will dissolve to a certain extent (1-2% solubility) and some water from the brine will mix with the carbon dioxide plume, leading to an increase in acidity which in turn can alter the chemical composition and ecology of the aquifer. If left undisturbed for tens of thousands or millions of years, the carbon dioxide will mineralize and turn into rock. To ensure that the aquifers have enough sequestration capacity, careful reservoir engineering is necessary to evaluate the rock properties before any investments are made into the top-level infrastructure.

The rate at which carbon dioxide is injected and the overall capacity of the aquifer are actively determined by the geology and pressure limits of the aquifer.  To maintain the containment of carbon dioxide within the plume or brine, it is crucial to actively control the pressure at a certain level. This determination is influenced by the rate of carbon dioxide injection and the speed at which the brine penetrates the rock. When injection ceases, the pressure will steadily diminish over centuries as the carbon dioxide continues to dissolve and mineralize. Additionally, the acidity can cause the dissolution of the caprock/seal, based on the characteristics of the rock. 

Aquifer-based Carbon Sequestration: Safety and Policy Considerations

Carbon dioxide leaks into drinking water or soil can negatively impact the preservation of the reservoir. Monitoring is essential to identify the possibility of leakage and prevent it from happening.

To ensure safety, experts recommend using seismic and other technologies to survey the area during and after injection. Additionally, when abandoning the site, it is actively recommended to give active consideration to closing the injection point. This measure helps prevent potential acidification of the metals and concrete used.

Aquifers currently serve as massive-scale storage due to potential fines associated with emissions from these operations. Estimates indicate that aquifers actively possess the capacity to store more than one trillion tonnes of carbon dioxide. The injection process has a low cost, less than $30/te, excluding expenses of collection, transportation, and pressurization. To stimulate greater utilization of aquifers, it is important to take the right policy actions.

Active acknowledgment is necessary to recognize the magnitude and expense associated with rapidly developing the industry. This development involves billions of tonnes of carbon dioxide and trillions of US dollars. Additionally, there needs to be a harmonization of national and international frameworks that oversee rights to subsurface resources. It is essential to ensure that laws do not impede the usage of aquifers. Laws should protect aquifer users while preventing contamination of drinking water aquifers and other potential negative consequences. Simultaneously, we should actively consider financial and legal conditions in the event of any leakage situation.

Enhanced Oil Recovery

Enhanced Oil Recovery (EOR) encompasses a range of techniques actively designed to enhance the production of oil and gas. One technique used in the process is to inject carbon dioxide at a pressure greater than 700 meters.

At this pressure level, carbon dioxide enters a supercritical state, acting as a solvent. It actively facilitates the release of oil and gas from the rock strata and transports them to the wellhead. Moreover, it is possible to inject both carbon dioxide and water into the well. Since its initial testing in 1972, this technology has become a frequently used method for mature oil and gas wells. The injected carbon dioxide serves as a secondary driving force, actively pushing out any residual hydrocarbons from the reservoir. Of all the EOR approaches, it appears that carbon dioxide injection technology is the most popular.

The process obtains the carbon dioxide used from the most cost-effective local sources, primarily consisting of naturally occurring emissions. EOR retains carbon dioxide in the underground reservoir, offering an attractive aspect of the technique. Many decades of retention of carbon dioxide in the underground reservoir are possible even after the field has been depleted. In certain cases, the reservoir, including any aquifers, can actively store the carbon dioxide generated when burning subsequent oil production.

Promoting Carbon Dioxide EOR: Policy Incentives and Collaboration

For carbon dioxide EOR to be competitive, it must compare favorably to other oil extraction methods. The methods that carbon dioxide EOR must compare favorably to include thermal EOR. Thermal EOR involves using steam to heat the oil and reduce its viscosity. Another method is chemical EOR, which employs acids or alkalis to loosen hydrocarbons. Lastly, there is polymer EOR, which utilizes polymers to increase the viscosity of the flushing water for hydrocarbon extraction.

The effectiveness of carbon dioxide EOR is contingent on the appropriateness of the reservoir, the necessary payback period caused by the more expensive capital expenses, the local cost of carbon dioxide, and the availability of technical skills to do the work. According to a 2015 study by the International Energy Agency, the capacity for storing carbon through enhanced oil recovery is between 50 and 350 Gt. Onshore oil fields have the most noteworthy potential for carbon dioxide EOR globally, albeit some advantageous offshore facilities exist. Data from Rystad Energy found that of all the oil-producing fields that have the possibility for carbon storage, more than 80% are onshore. 

To boost the usage of advanced oil recovery, policymakers could urge the oil and gas sector to implement carbon dioxide EOR.

Establishing a credit system based on the eventual sequestration of carbon dioxide when closing a well or selling hydrocarbons obtained through carbon dioxide EOR can prove beneficial. Furthermore, it would be advantageous to incentivize the capture of carbon dioxide from human-induced sources and stimulate collaboration between industrial sources of carbon dioxide and EOR users.  

Future directions

Researchers have extensively conducted studies over a long period of time on carbon sequestration using aquifers and enhanced oil recovery techniques. These studies have actively demonstrated the highly effective nature of these methods in removing carbon dioxide from the atmosphere, as they possess the capability to store significant volumes of it beneath the surface. Nonetheless, more research is necessary to progress the two procedures. 

We should actively undertake efforts to enhance the attractiveness of carbon dioxide enhanced oil recovery for the oil and gas sector and reduce the relative expense of carbon dioxide EOR compared to other oil recovery strategies. Additionally, we should actively explore new methods to increase carbon dioxide sequestration beyond the amount required for EOR.

It is also important to construct the necessary infrastructure to address the problems regarding different locations of carbon dioxide sources and aquifers. Additionally, a collaboration between countries will be essential to take advantage of unused capacity. To ensure that the technology is secure and to raise the required funding to carry out geological investigations and to scale up to hundreds of millions of tonnes/yr, it is important to increase the understanding of the technology and gain public approval.