January 25, 2012 | 37
With the current negative attention and controversy surrounding shale gas drilling, the words ‘hydraulic fracturing’ or ‘fracking’ have become synonymous with something else: water contamination. But according to recent research out of MIT, the contamination of drinking water near natural gas wells is caused by something totally different. In fact, fracking has nothing to do with it.
Shale gas was long considered inaccessible and unprofitable because of the low permeability of shale formations, but recent developments in horizontal drilling and multi-stage hydraulic fracturing have made shale gas production economically viable. As a result, shale gas drilling has increased dramatically and sparked debates on the safety and environmental impacts of hydraulic fracturing.
You may have seen the commercials supporting fracking that say, ‘clean burning natural gas is the gateway to our low carbon future.’ You may have also seen the Academy Award Nominated documentary Gasland, which highlights some of the environmental risks and discusses contaminated aquifers. Some have even called for a ban on fracking. Unfortunately, in this debate, some of the facts have been clouded by misinformation, anecdotal evidence, and back and forth attempts to discredit those on both side of the argument.
So what is the truth?
Can drilling for natural gas contaminate drinking water? Yes.
Is hydraulic fracturing to blame? No.
Bottom line: water contamination does happen, but not because of hydraulic fracturing. The MIT Future of Natural Gas Study, released in June 2011, examines the causes of 43 reported environmental incidents and finds that, “no incidents of direct invasion of shallow water zones by fracture fluids during the fracturing process have been recorded.”
So what causes the contamination? According to the study, “almost 50% [of the incidents were] the result of drilling operations… most frequently related to inadequate cementing of casing into wellbores.” The table below is from the Future of Natural Gas Study and highlights the frequency and causes of incidents.
While the most common incident is groundwater contamination resulting from drilling operations, the study also states that, “Properly implemented cementing procedures should prevent this from occurring.”
But, to be absolutely clear, hydraulic fracturing is not part of the drilling process. It is part of well completions, and is typically done several weeks after drilling has stopped. This is where much of the confusion sets in. You may have heard something like this before: ‘There has never been a documented case of the fractures in a shale gas production zone migrating upward to the water bearing zones. There are thousands of feet of impermeable rock separating the two, and the rock mechanics make it a physical impossibility.’
That is true. However, that does not mean fracking fluid cannot get into the aquifer. So how does this happen? If fractures never reach the aquifers, then how does the fracking fluid get there?
It goes like this:
Sometimes, drillers have to drill through an aquifer to access a natural gas zone that is farther below. (by the way, an aquifer is an underground layer of water-bearing permeable rock. It is not an underground lake or cavern that is filled with water, like many people imagine.) If the act of drilling through the aquifer and cementing the walls of the hole is not done carefully, there is a higher chance that the cement casing can crack, leaking hydrocarbons and other fluids into the aquifer. If there is a casing leak near the water table, whatever is traveling up or down the well can leak into the aquifer. Since the fracking fluids have to travel through this pipe twice (once on the way down, once on the way back up), it can contaminate the water through the casing leak. In fact, whatever travels through that pipe can leak into the aquifer. Sometimes it is fracking fluid, but more commonly, it is methane, drilling mud or produced formation water.
Aquifer contamination can happen whether you frac a well or not, and bad cementing and casing leaks are possible in any well – vertical, horizontal, fracked or unfracked.
Now, there are significant environmental concerns related to gas drilling, but let’s put them into context. 20,000 shale gas wells have been fracked in the last ten years, and the Future of Natural Gas Study pulled 43 of the most widely reported environmental incidents, and not a single one was caused by fracking. Now, there were likely more than just 43 environmental incidents, indeed, the study mentions, “The data set does not purport to be comprehensive, but is intended to give a sense of the relative frequency of various types of incidents.”
Regardless of the frequency of incidents, any spill or leak is unacceptable, and the pressure should be on the drillers and operators to minimize the environmental impacts of natural gas production. But, the data show the vast majority of natural gas development projects are safe, and the existing environmental concerns are largely preventable.
About the author:
Scott McNally has a B.S. in Chemical Engineering from the University of Texas. He has worked as an Environmental Engineer for Valero Energy Corporation, a Project Engineer for Shell Oil Company, and an energy and climate research intern for the White House Council on Environmental Quality. This is Scott’s second guest blog post at Plugged In – he was invited to be a guest blogger by Plugged In’s Melissa C. Lott. You can reach Scott via e-mail at scottmcnally at gmail dot com.