Energy independence for the United States – the mere fact that it’s being talked about tends to spark a warm, fuzzy feeling among the folks who recognize the positive financial and geopolitical impact from the somewhat recent surge in domestic production.
Hydraulic fracturing (HF) technology inarguably underpins the United States’ ascent to prominence as a leading hydrocarbon producer. After all, the abundant shale formations primarily responsible for this production need all the help they can get to give up their contained treasure, and induced fracturing of these dense rocks is key.
Even so, it’s a process that certain politicians, citizens in general and a number of grass roots organizations consider to be akin to the work of the devil.
Love it or hate it, hydraulic fracturing technology used to tap this abundant domestic hydrocarbon resource is crucial to the nation’s energy needs – and, in turn, the economy.
Certain unsubstantiated accusations related to HF, including poison-contaminated drinking water, have kind of faded into the background. Currently, earthquakes supposedly caused by water injection are the headline-makers.
Flowback and produced water resulting from HF and hydrocarbon production have resulted in an increase in wastewater disposal via underground injection wells. As a result, studies suggest that recently felt seismic events may be associated with injection wells in several states, including Arkansas, Ohio, Texas and Oklahoma.
One of most recent headline makers concerns a string of small earthquakes northwest of Fort Worth. Researchers are investigating the possible tie to injection wells in the area.
“The scientific community has known that earthquakes can potentially be caused by fluid injection or extraction activities since the 1920s, commonly referred to as ‘induced seismicity,’” said Matthew Tymchak, hydrologist at Gradient Corp. in Cambridge, Mass.
“But in many cases, such as Oklahoma, there is debate about whether the earthquakes resulted from fluid injection, natural tectonic processes or remote events,” Tymchak noted.
“Also, it’s not clear why earthquakes have appeared near some injection wells but not others having apparently similar geology, target reservoirs and injection rates,” added Samuel Flewelling, Tymchak’s hydrologist colleague at Gradient.
Natural or Induced?
Distinguishing between tectonic (natural) and induced seismicity can cause sleepless nights for members of the scientific community.
Tymchak said the National Research Council in 2012 noted that previous attempts to evaluate potentially induced seismicity have relied on inferences based on spatial and temporal proximity of earthquakes to human activity.
Spatial correlation has been a qualitative approach that essentially plots locations of earthquake epicenters – or in some cases, hypocenters – and injection wells, according to Tymchak. Even though the induced seismic event likely would be near an injection well, it’s essential to have knowledge of a region’s seismic history and other ancillary data to have sufficient info to identify induced versus tectonic events.
As to temporal correlation, Flewelling noted that it entails a comparison of the timing of injection volumes and subsequent downhole pressure changes with increased seismic activity in the area.
The ability to distinguish between induced and tectonic seismicity is considerably enhanced where there is a site-specific seismic monitoring network near a fluid injection site, particularly if a known fault is present near the well.
Unfortunately, there are no site-specific monitoring networks for most injection wells. Additionally, traditional seismic monitoring networks usually are inadequate to detect most low magnitude events that would typify a region’s potential seismic response to fluid injection.
“What you generally have is seismic catalogs from larger regions like states, and monitoring networks detecting events are not so dense,” Flewelling said. “You can’t locate those events quite as accurately, and there are many injection wells operating in the area, so it becomes very difficult to do a site specific analysis.
“We’re seeing some approaches where people are trying to look at spatial correlation of events with injection wells, or time correlation of events with the timing of injection,” he noted. “But if there is not an intensive monitoring program, it’s difficult to say definitively whether or not these events are induced.”
On the Job
But these savvy researchers are on it.
They have nailed down some approaches to explore to distinguish between natural and induced seismic events. These include recently developed quantitative approaches, such as the analysis of frequency magnitude distributions or the number of earthquakes that occur for a given magnitude.
“We’ve been exploring whether frequency magnitude distributions could be a more useful tool for evaluating potential induced seismicity with the existing monitoring network in addition to looking at spatial and temporal correlation,” Tymchak said.
“Frequency magnitude distributions have been used to characterize seismicity of particular regions and have been shown to vary with different tectonic processes, as well as for induced seismicity around individual wells,” he noted. “We are evaluating whether the frequency magnitude distribution of seismicity detected by the current monitoring network in a particular region should change if there are induced events, and if so, how.”
This whole topic has potential impacts beyond the oil and gas industry.
The USGS intends to update the country’s earthquake hazard maps, which will take induced seismic events into account.
“Part of this process will inherently involve picking which seismic events in the past have been induced,” Flewelling stated. “It’s not entirely clear how such a differentiation should be made.
“One of the points of our work,” he added, “is to move the discussions and qualitative approaches to be more quantitative, or mathematical, if possible.”