Downhole logs are typically the data set that nobody wants to acquire during an expedition, but that everyone is happy to have once the expedition is completed!
What is the reason for this? When you log, you don’t core, and cores are more fun to study than logs. They can be touched, photographed, sampled for various purposes and dated. Downhole logs are basically vertical wiggles on a screen. Far less sexy, I have to admit! I am nevertheless a fervent defender of downhole logging. And this is not only because acquiring logs is part of my job as a “Petrophysics staff scientist” (a “PSS” from EPC) on Exp 364. Logs are powerful. And there are MANY good reasons for acquiring logging data, including when plenty of good quality cores are recovered, as is the case on Exp 364.
How does it work? Simple. A tool (or a set of tools that we can combine together to form a tool chain) is connected to a winch and lowered in the hole after a coring phase has been completed. Once the tool has reached the bottom of the hole, it is pulled up slowly while different parameters characterizing the geological formation are recorded and transmitted to a computer on the deck. The resulting signal is called a “log”. One log is produced per parameter.
I know… this may not look very exciting at first view, but our tools are really cool!
On Exp 364, we use what we call “slimline tools”. They are very small in size (4 cm in diameter, 0.6 to 2.2 m in length) and light (<20kg). We are very far from the oil industry tool strings which can exceed 1 ton and must be placed in the hole using cranes. Our tools can be manipulated very easily, even by myself (note that I have neither the dimensions nor the muscles of a driller). More importantly, non-slimline tools would be too large to fit into the hole we are currently drilling!
I will spare you the technical descriptions of all the tools we have, and all the parameters we can acquire. Among them however, there is one which is clearly my “chouchou” (i.e. my favorite) and I would like to introduce: the “acoustic televiewer”. It allows 360° imaging of the internal wall of the borehole. It gives you information on the size of the hole, its shape, if it is tilted (or not) and also, it allows mapping the geological features (such as sedimentary beddings or faults) that are visible on the borehole wall…. And that’s how, magically, the downhole logging becomes VERY important on Exp 364… Especially for my colleagues, “structuralists”, who are aiming to study the faults observed in the cores in order to understand how the impact crater formed and how the rocks got weakened by the shock. We also have the full support of the co-chiefs!
Why are these guys so enthusiastic? Because cores are not oriented in space. I mean: if, in a core, you observe a fault dipping (tilted) for example at 30°, you cannot say toward which direction the fault plane was originally inclined at depth, before the core arrived on deck. Was it dipping toward the North? The South East? The acoustic televiewer can provide an answer to this unknown as the images of the borehole wall produced by the tool are oriented with regard to the magnetic north. Consequently, if you see a fault in a core and if you know at which depth this core has been taken in the hole, you can search for its expression on the acoustic image log and restitute its true orientation… That’s how you make a structuralist “happy”. Pretty cool no?
So, what does the job consist of exactly, and who are the people involved?
Downhole logging is clearly a team effort. The story began 1 year ago in close collaboration with the ESO Operation Manager (Dave Smith) in order to design the best logging program possible to help achieve the scientific objectives of Exp 364, keeping in mind the technical constraints of the drilling/coring strategy (in our case, the small diameter of the hole, and the subsequent need for slimline tools). One year later, we are currently offshore, in the middle of the action phase! The borehole should be logged in 3 logging sessions. Each one starts with the rig up of the equipment (including the transfer and positioning of our 300kg winch on the drill rig). The DOSECC drillers are always ready to give a hand for this and are full of creativeness, helping to weld or design a piece of steel that will make the difference between a good set up, and an optimal one (special thanks to Chris for our return pulley).
Then comes the acquisition phase sensu stricto… I have to say, it can be pretty boring… especially when everything goes well. So the more boring it is, the better it is! You sit on a chair, start the acquisition and wait until the hole gets logged downhole and uphole, keeping an eye constantly on the data quality and the cable tension. The slower the acquisition speed (between 0.5 and 30 m/min depending on the tool used), the longer you sit on your chair! We’ve already performed 2 logging sessions of ~24 hours each. The last one should last up to 48 hours depending on the final depth of the hole and the number of tools we will run. For this reason, several people are involved in the acquisition phase: the 2 PSS (Erwan, from the University of Leicester, and myself) and two logging engineers working in alternate shifts, 12 hours long. Jehanne, Gilles and Laurent are the 3 logging engineers from the University of Montpellier involved in Exp 364. Jehanne and Gilles are based onshore and join the L/B Myrtle for the acquisition phase only. Laurent is on standby on the boat for the entire duration of the expedition. He is the only one on the boat spending 75% of the expedition “wandering”, and the remaining 25% checking the tools, calibrating them, and of course acquiring logs. So if you see a French guy walking around 12 hours a day, with a large smile, he is not a clandestine passenger, no, no. He’s simply a logging engineer, looking forward to the next logging session!
Logging sessions are synonymous for me to a lot of excitement, a certain amount of stress (hum…), and very few hours of sleep. For the others, it’s kind of “weekend” time… The scientists have the opportunity to catch up with core description and measurements. While the drillers get to have some rest, as the hole is occupied by our tools.
What’s going to happen now?
We have already logged two intervals (0-500m; 500-700m) and got tremendous data (!!) as the hole is very stable and of good quality. The remaining part of the hole will be logged during the very last days of Exp 364, once the last core has arrived on deck and coring operations declared over. This will be a busy time for logging team but will allow the acquisition of some missing key data, such as the fault orientations. This last logging phase will be for me the end of a fantastic scientific and human adventure!
Johanna Lofi – Petrophysics Staff Scientist