A new, free software tool created by Stanford researchers could help operators identify the likelihood of wastewater injection wells causing fault slips
Although routine, the disposal of wastewater from hydrocarbons production carries risk – most notably that the injection of water into the disposal formation could trigger earthquakes. Although many pose no problems, seismicity has trended upwards dramatically over the past decade as production from onshore wells has increased, according to the US Geological Survey (USGS). About 2 billion gallons (7.5 billion litres) of wastewater are injected daily into an estimated 180,000 wells in the US, mostly in Texas, Oklahoma, Kansas and California. Currently, there are no methods available to predict whether wells could trigger sizable quakes, or fault slips. However, a new free tool from scientists at the University of Stanford may change that.
“Faults are everywhere in the Earth’s crust, so you can’t avoid them. Fortunately, the majority of them are not active and pose no hazard to the public. The trick is to identify which faults are likely to be problematic, and that’s what our tool does,” said Stanford School of Earth, Energy & Environmental Sciences’ professor of geophysics, Mark Zoback. The tool was developed by Zoback and his graduate student Rall Walsh at the Stanford Center for Induced and Triggered Seismicity (SCITS), an industrial affiliates’ programme involving 10 Stanford professors, while the software was developed in collaboration with ExxonMobil.
The USGS notes that three conditions must be met for injection to induce an earthquake: presence of a fault; stresses acting on the fault favourable to slip; and a pathway for the pressure increase from injection to interact with the fault. Zorback’s Fault Slip Potential (FSP) tool uses similar information.
The first component is how much wastewater injection will increase pore pressure at a site, and the second is knowledge of the stresses acting in the earth. This information is obtained from monitoring earthquakes or wells drilled previously in the area. The tool then draws on details of pre-existing faults in the area – information typically available in data already collected by oil and gas companies as they explore for new resources.
No more slip-ups
Zoback and Walsh have started testing their FSP tool in Oklahoma, which has experienced a sharp rise in the number of earthquakes since 2009, owing largely to wastewater injection operations. Their analysis suggested that some wastewater injection wells in Oklahoma were unwittingly placed near stressed faults already primed to slip. “Our tool provides a quantitative probabilistic approach for identifying at-risk faults so that they can be avoided,” Walsh said. “Our aim is to make using this tool the first thing that’s done before an injection well is drilled.” The two also noted that regulators could use the tool to identify areas where proposed injection activities could prove problematic so that enhanced monitoring efforts could be implemented.
The FSP program is freely available for download at SCITS.stanford.edu