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Depleted Oil & Gas Reserves vs. Saline Aquifers

Structural and stratigraphic trapping of CO₂ in geologic formations is a highly researched strategy to store injected CO₂ permanently. Depleted oil and gas reservoirs and saline aquifers are the two main sequestration options over which large-scale commercial CO₂ storage is being developed in the US onshore.

The most prominent and efficacious strategy to sequester CO₂ has been the enhanced recovery of oil and gas reservoirs (EOR/EGR) – depleted reservoirs. EOR technology has been developing for over 50 years and provides an option to produce hydrocarbons with a neutral or negative carbon impact.

Currently, depleted reservoirs offer a few advantages over saline reservoirs. The main advantage relates to the copious amount of research gathered in the exploration and production of the depleted reservoirs. Hundreds of thousands of wells in the US have been drilled into geologic formations known to contain reservoirs that trapped hydrocarbons for millions of years, proving trapping integrity. Additionally, the reservoir's pressure perturbations and induced stress changes are small and well documented in depleted reservoirs. Finally, using existing equipment and modifying existing wellbores allows additional cost savings and a swift start-up.

Saline aquifers can store large amounts of CO₂ and can be found in many more locations than depleted reservoirs. The IEA estimates saline aquifers have the capacity to store over 10,000 Gt of CO₂, enough room to hold over 100 years' worth of emissions from all global stationary sources. However, much less data on saline aquifers has been recorded as these reservoirs have not been a target of exploration and research until recently. This lack of data makes starting a new project costly as the risk of leakage needs to be clearly defined before large-scale sequestration can commence. Unlike depleted reservoirs that can offset the cost through hydrocarbon production, saline reservoirs currently offer investors little economic benefit. The largest investors in saline aquifer development are sponsored by government carbon offset and credit programs.

Tools such as Carbon AXIOM are providing key insight into the viable options for CO₂ emitting companies investigating potential carbon sequestration.

Categorize a Reservoir

Understanding the geologic reservoir is the most important aspect of any long-term CO2 storage project. Mapping the connectivity of porous formations, documenting pressure and temperature fluctuations throughout the reservoir, and understanding the micro to macro stress limitations of the strata is essential to determining the feasibility of long-term CO2 storage.

Depleted reservoirs have abundant wireline, pressure, mechanical and seismic data to easily mitigate the risk of injecting CO2 into a small (generally less than 5 km laterally) defined storage reservoir. On the other hand, Saline aquifers can be vast, more than 50km laterally, making it much more difficult to accurately map the entire formation. Additionally, new saline projects will need an extensive exploration program to provide stratigraphic wells to retrieve cores and test mechanical failure and porosity availability.

The ability of the caprock to withstand the pressure derived during CO2 injection will be different for saline and depleted reservoirs. In Saline reservoirs, the injection process will cause overpressure that may result in caprock failure or fault reactivation as the saline displaces to accommodate the CO2. Depleted reservoirs, in contrast, can be repressurized as CO2 fills depleted pore space.

Big Data for Big Success

Data is the most critical aspect of building a safe and reliable CO2 storage facility. To establish a functional CO2 storage site, the location will have to be proximal to current emitters, and a transportation method will need to be in place. This data, combined with data concerning the geology, physics, and eventual reservoir monitoring, is essential to start and maintain a successful site.

Carbon AXIOM, a subsurface analytics tool for CO2 storage and Enhanced Oil Recovery connects customers to the vast TGS database where information concerning emissions producers, storage licenses, transportation data, and detailed subsurface information can be analyzed to monitor emerging markets and technologies. TGS has amassed a significant geology and geophysics data library containing wireline, seismic, basin temperature models. This has been incorporated into Carbon AXIOM to give unparalleled insight.

Carbon AXIOM A valuable prospecting and screening tool for potential carbon storage sites. Learn more >>>

A Gateway to the Future of Energy

Insight into the potential of carbon storage continues to grow. As technology progresses, other storage options will become viable. Saline reservoirs offer huge future potential, and these reservoirs are currently being studied to reduce risk. Mineral trapping in basalt as a storage option is the latest development to see traction, as are strategies to incorporate coal beds and deep-sea sediments as reservoirs for CO2.

There is no doubt that the role carbon storage plays in the energy transition and the mechanism with which it is employed will evolve over time. New technologies will no doubt be developed to achieve the ideal solution for direct capture and storage. In the meantime, governments and large industries are pushing forward with depleted reservoir technology as a proven reliable and safe option while investing in research to promote future development.