How Safe Is Our Drinking Water?

Abstract

Beyond the Headlines Editor’s note: Professionals in the oil and gas industry often receive questions about how industry operations affect public health, the environment, and the communities in which they operate. Of particular concern today is the impact of hydraulic fracturing on the environment. In this new column, JPT is inviting energy experts to put those questions and concerns about industry operations into perspective. Additional information about the oil and gas industry, how it affects society, and how to explain industry operations and practices to the general public is available on SPE’s Energy4me website at www. energy4me.org. Do shale oil and gas drilling present a real threat to drinking water supplies? This is likely the single greatest concern in the minds of those opposed to the exploitation of this resource. Can oil and gas wells leak fluids into the Earth? Yes. Can it be prevented? Yes, again. In this essay, we will discuss the mechanisms involved, the measures to prevent these occurrences, and the most recent scientific studies on the topic. On the last point, I am happy to report that, to date, the news is uniformly very good. Happy because this resource must be developed in a sustainable fashion. It has transformed the United States economy and improved the lot of every US citizen. We have a duty to get it done right. Other countries with similar resources need the US to succeed. There are two potential sources for contaminating fluids. One is the hydraulically fractured zone in the reservoir and the other is the vertical portion of the wellbore. Microseismic monitoring involves “listening” to the minor tremors generated by the hydraulic fracturing operation. Thousands of such operations have been monitored and fractures do not extend more than 1,000 ft in a vertical direction. Leaving a margin of error, 2,000 ft of vertical separation ought to be sufficient. Most producing zones are at vertical depths greater than 4,000 ft and fresh water rarely extends beyond a few hundred feet. Another measure of comfort comes from the propensity of rock fractures to “heal” due to Earth stresses. This tendency is the reason that proppants (sand or other ceramic material) are used to keep the cracks in producing zones open to allow fluid to flow into the well. This propping mechanism is not dissimilar to the piers and beams used in coal mines. The most definitive study (Hammack et al. 2014) in this regard is one by the US Department of Energy in the Marcellus Shale. Researchers injected tracers during the fracturing operation and found no evidence in freshwater bodies above. Studies of Potential Contamination of Fresh Water Two types of fluids are candidates for leaking into surrounding rock: gases and liquids. One is produced gas and the other is produced liquids, including portions of the fracturing fluid returning to the surface. In general, gas is able to move through the formations more easily than liquids. Consequently, if gas infiltrated freshwater bodies, this would have little bearing upon the possibility of liquid contamination. This point has been evident in all of the studies reported to date. In a 2011 paper, Osborn et al. reported methane contamination of water wells in proximity to shale gas wells in Pennsylvania, but found no liquid contaminants. This study did not have baseline measurements. A later 2012 study (Kresse et al.), with baseline measurements, together with US Geological Survey investigators, reported that neither type of fluid contaminant was found in proximal freshwater bodies. In 2014, Darrah et al. studied several existing wells in Pennsylvania and Texas using isotopes of noble gases as tracers. They found methane contamination of water wells, but concluded that in all instances, it was because of faulty well construction. They ruled out direct contamination from the fractured intervals (see story on page 42). All the four studies support a longheld industry belief that properly constructed wells are not likely to leak fluids. An upset condition is far more likely to result in methane, rather than liquid, contamination. This is important because methane in drinking water has no adverse health effects. Public concerns center on potential contamination from the ingredients in fracturing fluid. The studies to date offer considerable comfort in this area.