Wastewater from oilfields is sinking deeper and is leading to an increase in the number of high-magnitude earthquakes in some parts of the US. The reason: sinking wastewater - which has a higher density than naturally occurring fluids - increases fluid pressure at greater depths, causing larger earthquakes.
“Oilfield wastewater tends to sink when it has a higher dissolved solids concentration (and thus, higher density) than fluids deep (4+ km) within the Earth’s crust. As this high-density wastewater sinks, it displaces lower density fluids and increases fluid pressure. Pressure changes so deep - at depths up to five miles or greater - can cause more high-magnitude earthquakes,” says the study published in Nature Communications.
The researchers used computational modeling and earthquake data analysis from across a broad region of northern Oklahoma and southern Kansas, roughly 30,000 square miles.
According to the study, there is a larger proportion of high-magnitude earthquakes at depths greater than 8 km in north-central Oklahoma and southern Kansas, but there are fewer total earthquakes at these depths.
They found that the number of earthquakes greater than a magnitude 4 increased more than 150% from 2017 to 2018, while the number of earthquakes with magnitude 2.5 or greater decreased 35% during the same period.
“This implies, while the overall number of earthquakes is starting to decrease, the percentage of higher-magnitude earthquakes is increasing. The year-over-year increase in magnitude-4 earthquakes from 2017 to 2018 is highly unusual because the overall earthquake rate is falling. We think this is probably a result of high-density wastewater sinking and increasing fluid pressure at greater depths, where higher magnitude earthquakes tend to occur, even though there are fewer overall earthquakes at greater depths,” Dr. Ryan M. Pollyea, who led the study from Virginia Tech College of Science’s Department of Geosciences, told MEA Worldwide (MEAWW).
The study is the first to show that oilfield wastewater in Oklahoma and Kansas has a much higher density than natural fluids in the earthquake (seismogenic) zone.
First study to show wastewater density plays a role in earthquakes
Scientists have known since the late 1960s that injecting wastewater deep underground can trigger earthquakes, and research has shown that oilfield wastewater disposal causes earthquakes across the middle US. However, this study is the first to show that the composition (that is, the concentration of dissolved solids) of oilfield wastewater can also increase fluid pressure enough to cause earthquakes.
This is a critical finding, say geoscientists, as earthquakes have become common throughout much of the central US, where the average rate of magnitude-3 or greater earthquakes have gone up from approximately 19 annually before 2008 to over 400 each year since then.
According to the research team, in many cases, these earthquakes occur when oilfield wastewater is disposed of by pumping it into deep geologic formations. As wastewater is injected deep underground, fluid pressure builds up and migrates away from injection wells. This destabilizes faults and causes’ injection-induced’ earthquakes, such as the damaging 5.8-magnitude earthquake that struck Pawnee, Oklahoma, in 2016.
“To understand why fluid density matters, we developed a wastewater injection model that shows how high-density wastewater will sink and increase the fluid pressure within the portion of Earth’s crust where earthquakes happen. This increasing fluid pressure is what destabilizes faults and causes earthquakes. We also found evidence of this process in the earthquake data for northern Oklahoma, which shows that average earthquake depth gets systematically deeper soon after injections begin. We then show that the relative proportion of higher magnitude earthquakes increases with depth in the study area. Thus, our study implies that when high-density wastewater sinks, it may trigger earthquakes with a larger relative proportion of high magnitude events, although it’s important to recognize that there are far fewer earthquakes at such depths (deeper than approximately 8 km),” Dr. Pollyea told MEAWW.
By analyzing earthquake data for several Oklahoma counties, the team found that earthquakes are getting deeper at the same rate high-density wastewater sinks.
“We show evidence that average annual earthquake depth systematically increased (deepened) in three Oklahoma counties over the last 10 years," says the team.
Earthquakes to continue long after wastewater injection stops
The researchers also found that earthquakes near oil fields could continue long after drilling stops. The reason: high-density wastewater will continue sinking and increasing fluid pressure for over a decade, enough to cause earthquakes for 10 to 15 years after wastewater injection rates decline or stop.
This could prolong the earthquake hazard in regions like Oklahoma and Kansas, and implies that earthquake frequency is likely to remain above background levels in areas where oilfield wastewater comprises much higher density.
“This study shows that we really need to consider fluid properties - density, in particular - when developing models of injection-induced earthquake hazard. Our study shows that fluid pressure can remain in the geologic system long after injection operations cease, which means it will take much longer than previously thought for earthquake rates to return to background levels,” Dr. Pollyea says.
According to him, for regions that may be considering whether or not to permit wastewater injection wells, it is critical to know as much geology as possible: to identify the structural and hydrogeological properties of the disposal formations, as well as the fluid properties of both wastewater and fluids occurring deeper in the basement rocks.
“This information can be used to model how the geologic system will respond to the wastewater injections, for example, to estimate how long fluid pressure may remain elevated and how far pressure fronts might travel for various injection scenarios."