This week's most newsworthy study examines methane contamination in wells in Pennsylvania and Texas, linking them with drilling activities. It is authored by the same Duke group who have published on this topic a number of times now.
In the new study, the authors analyse the geochemistry of methane and groundwater around shale gas wells. As well as measuring the geochemistry of the methane, they measure other geochemical variables such as noble gas isotope ratios and salinities, in order to get a better handle of what might be leading to the elevated methane levels.
They find that in some cases the evidence points to a deep source of methane that has migrated relatively rapidly, with little contact with the rock layers that lie in between shallow aquifers and the deep layers in which fracking is conducted. The most obvious conclusion to make is that methane is not getting into shallow layers through cracks and fractures in the rock, but that methane migration through faulty well bores to the surface is a possibility.
The study has, for obvious reasons, garnered a lot of publicity. However, the more I thought about it, the less newsworthy the study becomes. In actual fact, I think it tells us little that we didn't already know.
We already know that faulty cement and/or casing can allow methane migration from depth. We already know that in a handful of cases in Pennsylvania, poor working practice from certain operators has lead to cement/casing problems - these companies have been prosecuted and fined by the Pennsylvania Department of Environmental Protection. So it's hardly surprising that the authors of the study were able to find cases where the geochemical evidence pointed to this issue.
The key questions to ask are:
- Is fault casing/cement commonplace, or a very rare occurrence?
- Is it inevitable that casing/cement will go wrong, or does it only happen as a result of poor operating practice?
- Are horizontal wells that have been hydraulically fractured more prone to casing/cement failure?
- Can faulty wells be repaired, and is any damage permanent or can it be remediated?
What is more interesting to ask is whether these incidents are common-place or rare? The map below shows the locations of samples taken by the Duke team, and the locations of gas wells. The water wells at which evidence for methane migration could be found are highlighted in green.
Out of 20,000 wells drilled in PA, the Duke team find only 4 clusters where geochemical evidence suggests a well integrity issue. The authors did not randomly sample water wells, but deliberately targeted wells known to have high methane levels. The Associated Press reports the following:
"Jackson and colleagues have been studying water contamination around natural gas wells for years and for this study they didn’t chose a random sample, but aimed at areas that seemed to have most complaints of contamination. And even in those areas, it was only in a minority of dozens of sites that they could they connect the contamination to the natural gas wells, he said. In some cases, the contamination was natural and had no connection to gas wells, Jackson said."The first thing to note from this report is that, for wells with high methane levels, the majority of the time the cause is natural, not due to shale gas drilling activities. This is why the issue has become so contentious in the USA, and Pennsylvania in particular.
In fact, much of the time complaints about the industry are found to be without merit, because the problem is a naturally occurring one (or is possibly caused by other factors in the area, such as old coal mines). The Gasland flaming tap is a classic example of this - the homeowner found methane in his water supply and blamed nearby drilling operations, but it later transpired that regulators had analysed the water and found that the methane was naturally occurring.
I'll finish by returning to and answering my original questions:
Is fault casing/cement commonplace, or a very rare occurrence?
I've asked this same question in a number of past posts, and most studies have found that the number of cases where well integrity failure leads to water contamination is very very low - a fraction of a percent. This new Duke study does not alter this assumption, as they have found 4 cases out of 20,000 wells in PA. King and King (2013) point out that
"For US wells, while individual barrier failures (containment maintained and no pollution indicated) in a specific well group may range from very low to several percent (depending on geographical area, operator, era, well type and maintenance quality), actual well integrity failures are very rare. Well integrity failure is where all barriers fail and a leak is possible. True well integrity failure rates are two to three orders of magnitude lower than single barrier failure rates."
Is it inevitable that casing/cement will go wrong, or does it only happen as a result of poor operating practice?
The fact that this problem occurs so rarely suggests that it is one caused by bad practice, as opposed to a systematic problem. Sadly, the Duke authors do not appear to have investigated the status of the gas wells near to where they sampled. As I mention above, some operators have been cited and fined for failing to ensure well-bore integrity by the DEP, so it would be interesting to know whether the wells near to the sites sampled in this study had received any such citation.
Considine et al. also studied this issue, and they concluded that:
"The final conclusion is that the majority of the events were due to operator error, negligence, or a failure to follow proper procedures when drilling. This suggests that the industry has room for improvement, and the frequency of environmental events can be reduced."It is not inevitable that wells will leak. Adequate regulation and good working practices can mitigate this issue. We see this in the UK, where of 2,000 onshore wells drilled, only one has ever had any evidence for well bore integrity failure.
Are horizontal wells that have been hydraulically fractured more prone to casing/cement failure?The is no evidence that this is the case. Failure rates appear to be similar for both types of well. In terms of well integrity, the crucial part is the vertical part of the well near to the surface that runs through the groundwater aquifers. Whether or not the well turns horizontal at depth, and whether or not fluid is pumped down the well for fracking prior to production, would be unlikely to have much of an effect.
Can faulty wells be repaired, and is any damage permanent or can it be remediated?
The answer to this question is very clearly that yes, faulty wells can be repaired, and that once repairs have been done the methane in the groundwater will naturally dissipate. Squeeze jobs can be used to force additional cement into the annulus, filling in any gaps that were allowing methane to migrate.
The Considine et al. report cited above concludes that:
"the Pennsylvania data shows that of the polluting environmental events that resulted in environmental damage, the regulatory agencies and drilling companies acted to completely remediate those damages."Similarly, the Durham well integrity study (Davies et al. 2014) found that the single UK onshore well with an integrity issue had been completely repaired and remediated:
"A thorough investigation commenced in 1997, including the drilling of a number (>11) of additional boreholes, and the carrying out of tracer tests and CCTV examination under the auspices of, and in consultation with, the UK Environment Agency. The leak paths, once identified and verified, were remediated. Monitoring has continued since that time and the observed pollution levels have remained below those set by the Environment Agency as requiring further action."The case of Dimock in Pennsylvania, ground zero for accusations of drilling-related contamination, tells a similar story. Faulty casing and cement led to methane contamination. The DEP investigated and found the operator to be at fault. The operator then repaired their wells, and subsequently methane levels returned to their normal values.
Any incident of well integrity failure leading to methane contamination is one case too many. However, it is clear that such occasions are very rare, and that they are caused by bad practice, and therefore can be mitigated by adequate regulation, such as that found in the UK. Moreover, methane measurements are required around drilling sites in the UK, so if any problems are identified, problems can be rectified immediately.
This new Duke study doesn't tell us anything new about well integrity. What it does provide is further evidence that the hydraulic fracturing process does not lead to groundwater contamination, and that the concept held by some - that hydraulic fracturing can somehow create pathways through 2km of overlying rock along which contaminants can travel - does not hold.