Oil and gas extraction may stabilize shallow faults, making depleted reservoirs a potentially safe candidate for CO2 storage.
By Fionna M. D. Samuels, Ph.D. student and Temblor extern
Citation: Samuels, F. M. D., 2021, Injecting CO2 into depleted oil fields may not cause quakes, Temblor, http://doi.org/10.32858/temblor.190
The impacts of climate change are already evident this year, from record setting temperatures in the Pacific Northwest and British Columbia to extreme flooding and infrastructure collapse in New York City. Carbon dioxide contributes to the warming of the planet. The current level of atmospheric carbon dioxide, or CO2, is 415 parts per million — 48 percent more than pre-industrial levels in 1850. To combat climate change, international negotiations aim to reduce future emissions of this greenhouse gas. Yet, even with reduced emissions, the planet will face sustained, high levels of carbon dioxide for many lifetimes; the gas decomposes slowly and lasts in the atmosphere between 300 and 1,000 years. To address this problem, policymakers in the United States have begun to invest in carbon capture and storage technologies, many of which rely on injecting carbon dioxide into the ground. This practice can induce earthquakes raising concerns about its safety. However, a recent publication in Geology suggests a potentially safe place for captured carbon — depleted oil and gas reservoirs.
Noam Dvory, a geophysical researcher at Stanford University and lead author of the new Geology paper, says the idea of storing carbon dioxide in depleted oil and gas fields started when he and his colleagues noticed that in the Delaware Basin of western Texas and southeastern New Mexico, induced seismicity largely occurs only in the southern part of the basin even though oil and gas production occurred throughout.
In underground oil and gas reservoirs, fluids exert pressure on the rocks. This pore pressure increases when fluids are injected. Oil and gas production in this region uses hydraulic fracturing, in which liquids are injected into the ground to fracture rocks to allow oil and gas to be pumped to the surface.
“One of the main causes of injection-induced earthquakes is that changing pore pressure can help trigger the earthquakes,” says Megan Brown, a geoscientist at Northern Illinois University who is unaffiliated with this study. Conversely, depleting oil and gas reservoirs decreases pore pressure.
The geographical difference in observed induced seismicity piqued Dvory’s interest because the hydrocarbon production should lead to similar rates of earthquakes throughout the basin. He and his colleagues set out to explore why the northern end of the basin behaved differently.
Old wells stabilize new injections
Using data from the TexNet Earthquake Catalog, Dvory and his colleagues determined the locations of past earthquake epicenters, and calculated the pore pressure needed to cause an earthquake at each epicenter. Dvory says that through this analysis, the team found that when oil and gas reservoirs are depleted — meaning oil and gas have been removed — the shallow faults in the basin become more stable. New drilling sites at the southern end induced earthquakes, whereas the depleted wells on the north side stabilized surrounding faults.
Brown says it’s one of those studies where you read it and think, “Of course, that makes sense.” Because pore pressure decreases as oil and gas reservoirs are depleted, she says, the pore pressure would have to be increased to its original state, and then beyond, to induce an earthquake on shallow faults after production.
Different regions, different sweet spots
“Let’s call it ‘sweet spots’ in which you can inject more into the reservoir than in other places,” Dvory says. With the global climate crisis, these sweet spots in depleted reservoirs may be places where captured carbon could be stored with relatively little resulting seismic activity, he says.
As more technologies facilitate injecting carbon dioxide into the ground, Dvory notes that understanding the stress state of a region’s faults is fundamental to mitigating induced earthquake occurrence. “You need to understand the stress state; you need a very good and reliable geomechanical model of the [region],” he says. Such a model would require drilling data, reservoir data, and data on past earthquake behaviors to accurately model how the region will behave with added pore pressure.
Brown says that the Delaware Basin has been studied extensively by previous workers, which meant that the researchers had a large amount of data to work with to determine pore pressure, which may not be the case at other sites. Although every depleted reservoir may not be a safe place to store carbon dioxide by using predictive geomechanical models, Brown says, “I think you can have a good amount of confidence without a huge dataset.” However, she says, “it’s going to be very site specific.” She adds that the inherent infrastructure of depleted reservoirs makes them good candidates for carbon storage.
A team effort
Present injection technologies limit using depleted oil and gas reservoirs to store carbon dioxide. For example, the Illinois Industrial Carbon Capture & Storage Project, which focuses on studying what happens when carbon dioxide is stored underground, plans to inject 1 million tons of carbon dioxide annually into the Illinois Basin. However, in 2019, the United States emitted 5.1 billion metric tons of carbon dioxide. At the current rate of injection at the test site, it would take more than 5,000 years of injection to store one year of U.S. emissions.
There is no silver bullet to climate change. But collaborations among researchers, policymakers and the public can help mitigate its effects and minimize harm. And injection of carbon dioxide into the same reservoirs from which we extracted much of that carbon in the first place might be one piece of the puzzle.
Dvory, N. Z., & Zoback, M. D. (2021). Prior oil and gas production can limit the occurrence of injection-induced seismicity: A case study in the Delaware Basin of western Texas and southeastern New Mexico, USA. Geology.
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