In the first study of its kind, satellites show how wastewater disposal from oil and gas operations in eastern Texas may have deformed the ground to trigger the region's largest-ever quake — a magnitude 4.8 event in 2012.
The study published in the 23 September issue of Science also provides insights into why wastewater injection causes earthquakes near some wells, but not others.
Such observations that unequivocally link wastewater injection and seismicity are scarce, and these results suggest that satellites could be an important new tool in tracking the impact of wastewater injection in regions prone to induced temblors.
"We need to better understand why induced seismicity happens, and how to minimize it, and our approach opens up new possibilities for understanding the risk in ways that can reduce earthquake hazard," said Manoochehr Shirzaei, assistant professor in the department of Earth and space exploration at Arizona State University.
An intensity map for the magnitude 4.8 induced earthquake that took place near Timpson, Texas in 2012. | USGS
Scientists have known for decades that earthquakes can be induced by industrial processes. Since 2008, however, when seismicity in the central U.S. began to surge, earthquakes induced by the underground fluid injection processes have become a major focus. Fluid injection can occur with conventional oil and gas extraction methods, which extract fuel from underground pools, and with unconventional methods like fracking, which recover oil and gas from small voids in rocks.
At the end of oil and gas drilling, water that was used to create small fractures in deep rock to retrieve the fuel is injected back into the ground. This type of injection happens in 90% of cases, spanning both conventional and unconventional extraction processes. To avoid polluting fresh water supplies, the water is introduced between impermeable layers of rocks. Pumping large volumes of it deep into the earth increases the fluid pressure on earthquake faults, which decreases their strength, bringing them closer to slippage.
Previous studies attempting to evaluate the threshold between wastewater disposal and earthquake activity have focused on pore pressure in the ground, which can be hard to measure directly.
Shirzaei and colleagues took a different approach, using a constellation of satellites — the Interferometric Synthetic Aperture Radar (InSAR) satellites — to detect alterations in Earth's crust driven by underlying pressure fluctuations. The InSAR satellites use radar to illuminate large areas of the Earth's surface, measuring how the distance between the satellite structure and the ground surface changes over time. "This technique produces surface deformation maps with unprecedented resolution and accuracy," Shirzaei explained.
He and his team used InSAR satellites to study surface changes following wastewater injection at two sets of east Texas wells separated by less than 15 kilometers. Using their satellite-based observations — recorded in 2007, 2010, and 2014 — they were able to estimate pore pressure changes at these wells over time, including during the 2012 magnitude 4.8 Timpson earthquake, the region's largest temblor.
The estimated pore pressure changes are of a magnitude sufficient to cause earthquakes elsewhere, they say, suggesting wastewater injection in the area triggered the Timpson event.
"Where the wastewater is injected can make a huge difference, and our approach can be used to locate regions most susceptible to induced seismicity following injection activity."
Satellite observations revealed that ground uplift occurred at the eastern wells but not at their western counterparts, where the Timpson quake actually erupted.
"That uplift occurs near the eastern wells but not at the western wells is an extremely important observation," said Shirzaei. "It helps us to answer the question of why wastewater injection causes earthquakes at some wells but not others."
At the eastern wells, Shirzaei explained, rock is compressible, meaning that it can deform gradually, without breaking, in response to wastewater injection. But where deformation does not occur — which the satellites revealed to be the case at the western wells — rocks are stiff and can break, resulting in earthquake activity in the aftermath of large pressure changes, including those caused by wastewater injection.
"Where the wastewater is injected can make a huge difference," said Shirzaei, "and our approach can be used to locate regions most susceptible to induced seismicity following injection activity."
Studying surface uplift with satellites can also reveal the extent of injection activity's influence. This study measured uplift more than eight kilometers from the wells in question, the researchers say, suggesting that the entire region may require monitoring.
The results will guide hazard mitigation planning by drilling regulatory commissions, drill operators, and town planners in determining where to build homes, among other considerations.
What's more, as the approach of Shirzaei and colleagues uses globally available satellites, many regions can take advantage of their data.
"Disposal of oilfield waste by injection is a common practice around the world and may pose a hazard anywhere it is done," Shirzaei said. "Our approach to pinpointing the injection sites that are most problematic can be widely applied."
[Credit for associated image: Eric Kounce/ Wikimedia Commons]