Sonic drilling is an excellent way to collect high quality (and quantity) continuous core samples. It offers an efficient way to drill in difficult overburden conditions while getting a great borehole log of the formation. But drilling in some formations can generate heat which may elevate the core sample temperature during collection. We’re often asked if the heat generated affects any vapors of concern (VOCs) that may be in the sample, and in this blog post we’ll tackle that question.
Heat isn’t always generated in noticeable amounts. When penetration rates are good, there may be very little. Formations like sand, gravel and softer clays are easier drilling formations, and the cores collected are fine for laboratory analysis of VOC samples.
But formations that are more difficult to drill, such as cobbles, boulders, and caliche, will produce heat as the energy transferred down the drill string is converted. The slower the penetration, the more heat is generated. In very difficult formations the change in sample temperature can be significant. In a few instances, we have seen melting of the plastic sleeve that collects the core sample as is extruded.
This is actually a difficult question. It is challenging to really calculate how much change occurred during drilling, and even harder to determine how much the heat effected the in-situ amount of VOC’s in the sample. That’s due in part to the fact that most drilling methods can affect the actual value of VOC’s in a sample, so we cannot easily compare samples side-by-side.
For instance, any type of air rotary or percussion method of drilling is injecting highly compressed air during drilling which is likely to have some effect on the quantity of contaminant in the sample. An auger can generate heat in difficult formations. And mud drilling contains…mud! In difficult conditions, every type of drill either has problems penetrating a formation and/or collecting undisturbed chemical samples of the material.
So, how do we deal with the heat issue? The most foolproof way is to drive a split-spoon sample up to 24” ahead of the drilling. This is possible with any sonic drill setup. While the core barrel is out of the borehole, a lined split-spoon sample can be driven ahead of the coring activity. This can be driven with a 140 lb drop hammer or advanced sonically. Obviously, this method only works if the formation allows penetration of a split-spoon (and we’ve broken plenty over the years!).
One drawback to this option is the significant added cost with this “extra” procedure. It also reduces some of the speed and efficiency benefits of sonic drilling and coring. Another drawback is that this method eliminates the advantage of collecting the sample out of core and picking out an ideal location to take the sample (change in formation, staining, aquitard, etc.).
If we instead choose to collect VOC samples from the sonic core, there are ways to reduce the effect of the heat. Some of these are:
Ultimately, sonic drilling can create a change of temperature in the core sample. This can be a very small insignificant amount or a very substantial change in temperature. A good sonic drill operator has ways to minimize the heat that gets transferred to the sample during drilling.
If you have a sampling project that may involve VOCs, reach out to me with your questions. If you need a contractor experienced with minimizing heat while sampling, request a quote.