One important concept that was covered in the previous section’s video is the tendency of drilling fluids to be forced into permeable formations during the drilling process. Logging while drilling tools measure a formation right after it is first exposed, while wireline methods measure the formation after a delay. This means that if you’re logging while drilling, you’re going to get different results then you would if you used wireline tools to log the same interval at a later time after additional fluids have invaded the formations. Trying to determine the true attributes of an invaded formation is one of the challenges of well logging.
Fortunately, as you saw in the video, formation invasion has some upsides. By using logging tools to figure out how quickly and to what depth a formation has been invaded, geoscientistscan learn about the porosity, permeability, and fluid saturation of that formation.
You’ll hear the term “depth of investigation” frequently in the world of logging. This refers to the distance into a rock formation a particular tool makes measurements. Some tools, such as resistivity and density tools, contain multiple sensors in order to be able to make measurements at different distances from the wellbore. Other tools, such as sonic and NMR (nuclear magnetic resonance), have a wide range of investigation and return information from different depths simultaneously. The depth of investigation of various tools can range from less than an inch to several feet.
We’ll take a look at many of these tools on the next page.
Images: “Formation Evaluation and Invasion Profiles” by Top Energy Training