Have you ever had a site where several gas migration tests were completed but received no clear answer as to whether or not the well was leaking. Then, when you exposed the wellhead, the gas migration (GM) was blatantly obvious. Maybe it looked something like this…
There is a huge problem when an abandoned well is convulsing gas but the GM testing done prior to exposing the wellhead were inconclusive as to whether or not it is leaking. It should never happen.
There are reasons why it does happen and precautions that should be taken. Inconclusive soil gas migration testing is generally a result of poor sampling, poor field measurements or both. On this well (the one in the video), we took surface readings 1m, 2m... away from well centre in all directions. All but one result was above 250 ppm methane. Some of the readings were even non-detect! The background sample locations at the site also showed trace methane readings so from the surface measurements, nothing conclusive could be determined. This, from a well that is actually leaking quite profusely. What does that mean for wells that have small leaks?
Surface readings have a place in verifying if a well is leaking but just because your surface readings are good, doesn’t guarantee that there isn’t a potential problem (in the future) and may give you a false sense of security. For wells in contentious areas, for wells in areas known to have issues or vintages of wells with known issues, the best long term strategy should include the use of gas probes at a minimum of 50 cm depth to ensure that good soil gas samples are acquired. These probe samples will tell a very different story. For the same samples discussed above, readings from sub-surface probes at those same locations were 50,000 to 200,000 ppm methane in ALL samples providing definitive identification of gas migration.
Taking the extra precaution of measuring using probes provides another key ingredient to a successful investigation. The higher concentration gas samples can then be used for isotopic analysis providing the molecular signature (fingerprint) of the source zone. The higher concentration gas samples will allow better isotopic analysis.
The goal of a better designed investigation (with best practices) is to provide a solid answer the first time rather than having to keep going back to a site to investigate the same issue over and over again. Think about that the next time you have a potential GM issue and decide if it might be worth a little more investment now to save money later.