Earthquakes from oil field wastewater

In the nation’s biggest oil field, faults that lay dormant for millennia are now being activated by injection of oil field wastewater into shallow disposal wells, according to a pair of Stanford University studies.

The research details how fluid pumped into wells just a mile or two below ground has caused hundreds of earthquakes since 2014 in a seismic hotspot of West Texas’ Permian Basin oil field. It also suggests continued injections longer term could damage wells that cross the once-stable faults.

Every barrel of oil pumped up from the Permian’s shale rock brings with it several barrels of extremely salty water that can contain toxic chemicals. All that wastewater – billions of gallons of it every year nationwide – has to go somewhere. The most common option is to send it back underground.

“As long as wastewater disposal continues, these faults are likely to continue to slip and make the occasional earthquake,” said geophysicist Bill Ellsworth, who co-directs the Stanford Center for Induced and Triggered Seismicity (SCITS) and is a senior author on both studies. Careful monitoring of fault movements, pressure, and stress in the rock layers where disposal wells are drilled could help operators identify when a fault might be approaching failure and avoid the zones where wastewater disposal is most likely to trigger quakes, he added.

Published in the Journal of Geophysical Research: Solid Earth and The Seismic Record, the research comes as shale companies in Texas face heightened restrictions on deeper disposal wells and an expedited approval process for shallow wells amid a spike in the frequency and magnitude of seismicity in the Permian.

It also arrives as Russia’s invasion of Ukraine has disrupted the global energy market and reignited debates over fracking. The United Kingdom, which suspended fracking in 2019 after a project caused tremors felt miles away, now says it will review the moratorium as Europe seeks alternatives to natural gas from Russia.

Earthquakes and wastewater

Oil and gas operators’ disposal of wastewater by deep underground injection has for years been linked to unnatural surges in seismicity in Texas, Oklahoma, Kansas, and a few other states. “Wastewater is known to have activated deeper faults located several miles below the surface, which can lead to bigger earthquakes,” Ellsworth said. But scientists have debated the mechanisms by which shallower fluid injections may lead to fault failures, and why seismicity occurs around some disposal wells and not others.

Using satellite-based measurements of ground deformation and artificial intelligence methods that detect previously imperceptible microquakes, the two new Stanford papers help to disentangle those mechanisms and identify earthquake triggers in the Delaware Basin, which forms the western wing of the Permian.

Sprawling across some 8,500 square miles of Texas and New Mexico, the Delaware Basin is one of the largest oil and gas fields in the world. In 2020, its daily output averaged more than 4 million of the nation’s 11 million barrels of oil per day, mostly from shale rock prepared for production through hydraulic fracturing. The technique involves injecting mixtures of fluid and sand deep underground at high pressures to release fossil fuels from otherwise impermeable rock.

“Understanding how and why water injection is triggering seismicity in the Delaware Basin helps us place an upper limit on the potential size, and thus the potential hazard, associated with these types of induced earthquakes,” said Mark Zoback, the Benjamin M. Page Professor of Geophysics (emeritus) at Stanford.

Historically, earthquakes were rare in the Permian Basin. But seismicity in Texas has been increasing since around 2009, when horizontal drilling and fracking in the region helped to fuel the shale gas boom that made the U.S. the world’s largest producer of oil and gas. In the Delaware Basin, horizontal drilling was quickly followed by a surge in seismicity, mostly in the southern portion of the basin. The north, meanwhile, has been much less active.

Shallow quakes

One of the most seismically active areas of the Delaware Basin is a roughly 10-by-10-mile patch straddling the Reeves-Pecos county line, where 21 earthquakes of magnitude 3 or greater have struck in just the last few years. Between 2014 and 2020, 77 horizontal wells were drilled, fracked, and put into operation in the area. During that period, “these wells produced over 1.5 million barrels of oil and five times that amount of water,” Ellsworth writes with coauthors Karissa Pepin and Yixiao Sheng, PhD ’20. Almost all the wastewater was injected locally into a rock formation roughly a mile underground.

Using an artificial intelligence model created at Stanford to detect very small earthquakes often overlooked by other methods, the researchers analyzed seismic data collected by a state-run network of seismic monitoring stations known as TexNet.

The AI model picked up more than 450 earthquakes centered within the study area, and over 3,000 more that originated elsewhere. With few exceptions, the Reeves-Pecos quakes started within the rock layer where operators have injected millions of gallons of wastewater from oil production into a dozen wells. And they happened between a mile and a mile and a half below the surface, along the same faults where the team’s companion analysis of satellite data reveals significant downward slip close to disposal wells. No earthquakes were associated with fracking of production wells.

In the seismically quieter northern portion of the Delaware Basin, satellite data analysis led by Pepin shows broad swaths of land have been rising in areas where large volumes of wastewater were injected and sinking where there was more oil extraction. In the shakier southern Delaware Basin, the ground has been rising and falling along long, narrow strips.

The research suggests this motion is caused by slip on shallow faults where two blocks of Earth’s crust meet along an inclined plane, like a loaf of bread sliced diagonally. When the fault is “activated” by fluid injection, the upper block slides downward along the lower block under the force of gravity.

Vulnerable faults

It remains unclear whether wastewater injections in the Delaware Basin are triggering earthquakes directly or indirectly, by causing gradual or “aseismic” slip that paves the way for more abrupt fault movements later on. But disposal wells located close to shallow, pre-existing faults in the region appear particularly prone to causing the Earth to slip.

The results support a hypothesis, published in 2021 by Zoback and former postdoctoral scholar Noam Dvory, that oil production in the northern Delaware Basin throughout the 20^th century reduced fluid pressure in the area’s underground rock formations. Farther south, operators over the past 20 years have drilled disposal wells in previously unexploited sediments where pressure remains high and faults are close to their stability limit. As a result, Zoback and Dvory theorized, even relatively small pressure changes from wastewater disposal could be enough to disturb the delicate balance and set faults in motion.

So far, earthquakes in the Delaware Basin have been too weak to cause serious damage at the surface. “Deep injection appears to be responsible for the larger, potentially damaging earthquakes that have occurred in the area to date,” said Zoback.

But displacement along the shallow faults in the southeastern portion of the basin is growing. According to the researchers, faults roughly a mile below the surface have shifted by as much as nine inches in just three to five years. According to Ellsworth, “Even small amounts of fault slip might pose a hazard to the oil field operations. It could lead to shearing of wells that could be problematic and expensive.”

Ellsworth is also a Professor (Research) of Geophysics in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). Pepin is a PhD student in Geophysics. Sheng is now affiliated with Université Grenoble Alpes Grenoble France.

The research was supported by the Stanford Center for Induced and Triggered Seismicity (SCITS) and the Department of Energy.

Media Contacts

Bill Ellsworth
School of Earth, Energy & Environmental Sciences
(650)723-9390, wellsworth@stanford.edu

Karissa Pepin
School of Earth, Energy & Environmental Sciences
kspepin@stanford.edu

Howard Zebker
Geophysics and Electrical Engineering
zebker@ee.stanford.edu

Josie Garthwaite
School of Earth, Energy & Environmental Sciences
(650) 497-0947, josieg@stanford.edu