Storing carbon inn rocks underground: Old oil fields may be less prone to induced earthquakes

Geophysicists are still working to understand what can trigger human-induced earthquakes, which have been documented since the 1960s. A new study, published in Geology on Thursday, explores why part of a heavily produced oilfield in the U.S. has earthquakes, and part of it doesn't. For the first time, the authors demonstrate that the influence of past oil drilling changes stresses on faults in such a way that injecting fluids is less likely to induce, or trigger, earthquakes today.

The study focuses on the Delaware Basin, an oil- and gas-producing field spanning the border between West Texas and New Mexico. Drilling there has taken place since at least the 1970s, with over 10,000 active individual wells dotting the region. There, Stanford geophysicists No'am Dvory and Mark Zoback noticed an interesting pattern in seismic activity. Recent shallow earthquakes were mostly located in the southern half of the basin, while the northern half is seismically quiet, despite shallow wastewater injection occurring across the basin.

"The compelling question, then, is why are all the shallow earthquakes limited to one area and not more widespread?" Zoback says.

Earthquakes can be induced by injecting fluids like wastewater underground. When wastewater is injected into the rocks, pressures increase, putting the rocks and any faults that are present under higher stress. If those pressures and stresses get high enough, an earthquake can happen.

Earthquakes from injection in the southern Delaware Basin tend to be shallow and relatively low-magnitude, typically strong enough to rattle the dishes, but not enough to cause damage. However, if deeper faults are activated, higher-magnitude earthquakes can occur and cause damage. For example, in March 2020, a magnitude 4.6 earthquake rumbled in Mentone, Texas, likely due to deep injection that interacted with faults in the crystalline basement rock around five miles belowground.

"The size of an earthquake is limited by the size of the fault that slips," Dvory explains. Where faults are shallow and small (just a few kilometers in size), quake magnitudes tend to be small. "You can still feel it, but it's less dangerous."

Minimizing the risk of earthquakes is a goal for any subsurface operation, whether it's oil and gas production or carbon sequestration. That made the Delaware Basin, with its odd pattern of earthquakes, a great target for Dvory and Zoback. It was a natural experiment in geomechanics, the "why" behind induced earthquakes.

To decipher the pattern, Dvory and Zoback first modeled the underground pressures needed to cause faults in the basin to slip and connected those values to estimated stress values. Once they had established that baseline, they calculated the pore pressures around the Delaware Basin. Their results showed a clear pattern: geologic formations in the northern basin where hydrocarbons had previously been produced had lower pore pressures than in "unperturbed" rock, and there were no earthquakes. The southern basin, which had almost no previous production from the same formations, had higher initial pressures and earthquakes.

"In some areas we have evidence of oil and gas development from even the 1950s," Dvory says. "Where there was significant hydrocarbon production, pressure was depleted, and the formations essentially became more stable."

Now, when fluids are injected back into those 'stable,' previously drilled rocks, the starting pressure is lower than the first time they were drilled.

"So where oil production occurred previously, current injection results in significantly lower pressure such that it's much less likely to trigger earthquakes," Zoback explains. "It's not inconceivable that at some point, if you injected enough, you could probably cause an earthquake. But here in the area we study, we are able to document that what happened previously strongly affects how current operational processes affect the likelihood of earthquake triggering."

Targeting these sites of past oil production, with their lower earthquake risk, could be a good approach for carbon sequestration.

"We have a global challenge to store enormous volumes of carbon dioxide in the subsurface in the next ten to twenty years," Zoback says. "We need places to safely store massive volumes of carbon dioxide for hundreds of years, which obviously includes not allowing pressure increases to trigger earthquakes. The importance of geoscience in meeting this challenge can't be overstated. It's an enormous problem, but geoscience is the critical place to start."