Old oil fields may be less susceptible to induced earthquakes – Eurasia Review

Subsurface carbon sequestration – storing carbon in rocks deep in the earth – provides a partial solution to removing carbon from the atmosphere. When used in conjunction with emissions reductions, geological carbon sequestration can help mitigate anthropogenic climate change. But like other underground operations, it carries risks – including earthquakes.

Geophysicists are still working to understand what can cause human-caused earthquakes, which have been documented since the 1960s. A new study published in the journal Geology explores why earthquakes occur in part of a heavily producing oil field in the United States, and some that do not. For the first time, the authors show that the impact of oil drilling in the past compresses faults in a way that makes fluid injection less likely to cause or launch earthquakes today.

The study focuses on the Delaware Basin, an oil and gas production field that straddles the border between West Texas and New Mexico. Drilling has been there since at least the 1970s, with more than 10,000 active wells spread across the area. There, Stanford University geophysicists Noam Dvory and Mark Zubak noticed an interesting pattern in seismic activity. Most recent shallow earthquakes have been in the southern half of the basin, while the northern half is seismically quiet, despite shallow wastewater pumping through the basin.

“The burning question, then, is why are all shallow earthquakes confined to one area and not more widespread?” ZiwiPeak says.

Earthquakes can be caused by injecting fluids such as sewage underground. When wastewater is injected into the rocks, the pressures increase, causing the rocks and any faults present to be placed under higher pressure. If these stresses and pressures rise high enough, an earthquake can occur.

Injection quakes in the southern Delaware Basin tend to be relatively shallow and low in magnitude, usually strong enough to rattle dishes, but not sufficient to cause damage. However, if deeper faults are activated, high-strength earthquakes can occur and cause damage. For example, in March 2020, a 4.6-magnitude earthquake occurred in Menton, Texas, likely due to deep injections that interacted with faults in crystalline basement rock five miles underground.

“The magnitude of an earthquake is limited by the size of the fault that slips,” Dvory explains. When the faults are shallow and small (only a few kilometers in size), earthquake degrees tend to be small. “You can still feel it, but it’s less serious.”

Reducing earthquake risk is a goal of any subsurface operation, whether it is oil and gas production or carbon sequestration. That made the Delaware Basin, with its strange pattern of earthquakes, a great target for Dvory and Zoback. It was a natural experiment in Earth mechanics, the “why” behind induced earthquakes.

To decipher the pattern, Dvory and Zoback first modeled the underground stresses required to cause trough faults to slide and correlate these values ​​with the estimated stress values. Once this baseline was established, they calculated pore pressures around the Delaware Basin. Their results showed a clear pattern: the geologic formations in the Northern Basin where hydrocarbons were previously produced had lower pore pressures than the “undisturbed” rocks, and there were no earthquakes. The southern basin, which had almost no previous production of the same formations, had higher initial stresses and seismicity.

“In some areas we have evidence of oil and gas development up to the 1950s,” Dvory says. “Where there has been significant hydrocarbon production, the pressure is depleting, and the formations have essentially become more stable.”

Now, when fluids are injected back into those “stable” rocks that were previously drilled, the starting pressure is lower than the first time it was drilled.

“In places where oil production has occurred previously, current injection results in significantly lower pressure so that earthquakes are less likely to occur,” Zubak explains. “It is not inconceivable that at some point, if you inject enough, it could potentially cause an earthquake. But here in the area we are studying, we are able to document that what happened previously strongly influences how current operational processes affect the probability of an earthquake “.

Targeting these sites that once produced oil, with lower seismic risk, could be a good carbon sequestration approach.

“We have a global challenge of storing massive amounts of carbon dioxide underground in the next 10 to 20 years,” Zuback says. “We need places to safely store massive amounts of CO2 for hundreds of years, which obviously includes not allowing pressure to build up to cause earthquakes. The importance of geoscience in meeting this challenge cannot be overstated. It is an enormous problem, but geosciences is the crucial place to start.” .

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