Stepping on the accelerator - Oklahoma earthquakes and injection wells

Introduction

In my last blog I showed some quick data exploration of available earthquake and injection well data, showing both a strong temporal and spatial correlation of earthquakes. 

Of course, strictly speaking correlation by itself does not prove causation, but at the same time it doesn't dismiss the fact, either. If I step on the gas pedal in my car, it accelerates. 

That last analogy got me thinking. If I were going to "gather evidence" that pushing on the accelerator makes my car go, what experiment would I run?

The Simple Experiment 

The simplest experiment might be to push on the gas pedal, measure speed, and then push down again, and see if the speed increases. Without really understanding how my engine works, I would gather evidence (one way or the other) about the relationship between the two. And let's face it, many people out there have little to no idea how an internal combustion engine works, let alone look under the hood of their car, yet through repeated "evidence gathering" are pretty confident their gas pedal makes their car go.

The Data

Data sources were discussed in the previous blog. Oklahoma well data are sourced from the Oklahoma Corporation Commission. Earthquake data are reported by the USGS.

Stepping on the gas

In this situation the gas pedal is assumed to be "well mechanical integrity testing" of the well. Of course, the mechanical link is more complex. Actual well use, or perhaps ultimate waste disposal, may be correlated. But the idea is exactly like the car example above. Push on the gas, see how fast the car goes. Push down more on the gas, and see if the car goes faster...

This graph plots the number of earthquakes and well MIT dates in each calendar year since 1970. There are two steps up in well activity, and in each case it i appears to be accompanies by a rise in earthquake activity. Note that the correlation is not perfect. In the years from 1994 and 2006 a constant level of activity was accompanied by a decrease in earthquake activity. In 2001 no earthquakes were measured, for instance. 

The correlation

When we step on the gas in a car, there's a bit of a lag while "stuff happens" and then the car moves forward. Looking at the above graph it appears there's about a two year lag in the steps of drilling activity and the apparent steps in earthquake activity. The analysis below looks at the frequency of earthquakes in year n as a function of the drilling activity in year n-2. (I also looked at other variations, and this one seemed to offer the cleanest relationship).

The fit works out to about 0.15 quakes per well (The 90% confidence interval is 0.13 to 0.17 quakes per well and the rSquared value of the fit is 0.90). The earthquake and drilling activity since 2007 obviously have the biggest influence on that behavior.

Conclusion

If we step on the gas pedal in our car, the car moves forward, although there are many steps in between stepping on the accelerator (mechanical linkages, computer analysis, fuel flow modulation, spark plug ignition, etc.) the correlation of one action to the other leave little doubt about causality.

Similarly, this data strongly suggests that putting wells into production ultimately is correlated to earthquakes. While the mechanics between the two are not explained by this data, our confidence in the linkage between one and the other is certainly increased by this data.

It looks very much like the gap pedal might be making the car go....

Next steps

There are lots of opportunities to dig in deeper.

As a next step I want to look at the "swarms" that are apparent in the geographical data as shown in the previous blog. Perhaps by narrowing the geographical regions of analysis below the state level to even a few km from the sites of earthquakes (as suggested might be the case here) one might find much stronger correlations.

It's be great to get the actual flow data, but so far I haven't been able to find it online. That data might shed light on the gap in late 90's on earthquake data, one way or the other. It may also explain why the two year gap exists. Is that a meaningful timeframe relevant to real drilling operations? It may even suggest ways to mitigate consequences.

If someone knows where or how to get that data, please make a comment or send a link.