Geologic sequestration of carbon dioxide (CO2) is an important tool for curbing the human-induced climate change caused by our addiction to burning fossil fuels. But a new paper from researchers at Princeton suggests that the very process of getting those fossil fuels out of the ground could ruin our ability to put CO2 back in. The culprit? The controversial practice known as “fracking,” a.k.a. hydraulic fracturing.
Despite posing similar risks to groundwater, regulation of CO2 sequestration is vastly different than regulation of hydraulic fracturing. CO2 sequestration is regulated through the Underground Injection Control (UIC) program under the authority of the Safe Drinking Water Act (SDWA). Hydraulic fracturing, on the other hand, is exempt from the SDWA, meaning that EPA has no authority to regulate it*. Doing away with the loophole that exempts hydraulic fracturing from the SDWA and ensuring that oil and gas wells are subject to rigorous construction, operation and maintenance standards under the UIC program will not only provide uniform federal standards to protect groundwater, it will also allow EPA to coordinate regulation of fracking and geologic sequestration under the same program and help avoid potential conflicts between these two activities.
In order to successfully store CO2 underground it needs to be injected into a container with a lid on it - a porous and permeable rock formation to receive the CO2, capped by an impermeable rock formation to keep it from escaping. These “caprocks” are often shale formations, because shale is essentially impermeable to CO2 and subsurface fluids. The Princeton study found that in 60% of places where we have a CO2 container (a formation with sufficient porosity and permeability to accommodate large volumes of injected CO2), there is the potential for hydraulic fracturing being employed in the overlying lid. If such fracturing impaired the trapping function of the lid it could potentially reduce available storage capacity significantly.
Nature is, however, much more complex than the simple analogy of a container and a lid, and the Princeton authors state they were looking at a worst-case scenario. In reality, the formations that are suitable for storing CO2 are thousands of feet underground and are often overlain not by one, but by many layers of rock capable of providing a seal to keep the CO2 trapped. In addition, any particular caprock layer can itself be hundreds or thousands of feet thick. For example, the portion of the Marcellus Shale that is targeted by gas drillers is only 50-100 feet thick and above it are thousands more feet of shale and other low permeability rocks. In other words, even if a small section of a caprock is fractured, the larger formation could still serve as a seal, and will often have multiple seals above it in addition. This situation is common across the country (see example, below). The paper should therefore not be interpreted to mean that geologic sequestration of CO2 and hydraulic fracturing for unconventional oil and gas are necessarily incompatible.
There is another and potentially bigger conflict between fracking and geologic sequestration not addressed in the paper. Going back to the analogy of a container with a lid, not only might parts of the lid be fractured, the lid will also be punctured by a bunch of straws: the tens of thousands of wells that are needed to get unconventional oil and gas out of the ground. Studies show that the most likely pathway by which CO2 might escape from the subsurface at properly selected sites is through man-made wells, and not through geologic features. For this reason, geologic sequestration projects require wells to be rigorously designed, constructed and maintained to reduce this risk of CO2 and other fluids leaking through them. This includes using well construction materials that are resistant to CO2, which becomes corrosive when injected into subsurface formations containing water. On the other hand, the patchwork of state regulations that cover oil and gas wells are not anywhere near as rigorous as the regulations for geologic sequestration projects. The casing and cement used to construct oil and gas wells naturally degrade with time, diminishing the integrity of these wells, and contact with corrosive CO2 can speed this degradation.
Furthermore, well construction standards are just one example of where oil and gas well regulations fall short – there are regulatory gaps in every step of the oil and gas extraction process, from well siting to construction to operation to plugging and abandonment. If the formations that these oil and gas wells are drilled into eventually become the caprock for CO2 sequestration projects, these wells could be exposed to harsh conditions they haven’t been designed to withstand, potentially creating thousands of new pathways for CO2 to escape to the atmosphere.
Whether the lid to the CO2 container is being shattered or riddled with holes, the potential for conflict is real and must be addressed. As the authors rightly suggest, a logical solution is, “…a more comprehensive management strategy for subsurface resource utilization…” This also presents a challenge though, due to the differences in the way to two practices are regulated, as discussed above.
Federal regulation of hydraulic fracturing under the SDWA is needed to help reduce the potential for conflict between these two practices. The ability to store CO2 underground must not be compromised, especially by those practices that are helping put it into the atmosphere in the first place. Many places are too sensitive for any drilling at all, but with common-sense, smart regulation, fracking and geologic sequestration of CO2 can coexist where they do occur.
*Except when diesel is used in the frac fluid.