Mud Logging

Mud logging is the most common type of sample retrieval because it doesn’t cost the company anything extra to retrieve the samples. During drilling, rock cuttings are carried to the surface by the drilling mud and removed from the mud in the shale shakers. In most cases, a wellsite geologist, or “mud logger,” is present to collect and analyze the rock cuttings that come to the surface. The mud logger works for a mud logging company that is contracted by the well operator.

Mud logging is a challenging job. Rock cuttings are typically less than an inch in length, and are usually coated with drilling mud. To make matters worse, depending on the density and shape of the cuttings, they move up the annulus at different rates. This means that the order the cuttings come out of the well isn’t always the same order in which they were drilled. In addition, there is a time delay between when a layer is penetrated by the bit and when the drill cuttings associated with that layer reach the surface. Careful calibration of drilling rate, rock layers, and other aspects of the drilling process is critical to proper analysis of cuttings.

At sampling intervals determined by the operators, the mud logger analyzes drill cuttings in the mud lab, a prefabricated laboratory on the drill site. This analysis measures attributes such as porosity, lithology (rock type and rock characteristics), and oil staining.

In addition to detailed sampling at defined intervals, the mud logger is responsible for maintaining a continuous log of drilling operations. A log usually includes information about the following:

  • Rate of bit penetration (ROP) [In other words, how fast is the rig drilling]
  • Porosity
  • Lithology
  • Gas content
  • Mud weight
  • Temperature
  • Pressure

Mud Logging Equipment

In order to collect all this information, a mud logging company needs to install a variety of equipment on the rig. Let’s look at some of the most important equipment:

  • Gas trap: This device is installed on the mud return line. It traps gas released from the mud returning from the annulus. The gas is then sent to a combustible gas detector and a chromatograph, which analyze the gas for hydrocarbons.
  • Rate of penetration sensor: A rate of penetration sensor is able to measure how quickly the drill bit is progressing through the rock at the bottom of the hole. This information can assist with the assessment of rock types and correlation of cuttings with specific units.
  • Depth sensor: The depth sensor keeps track of the current depth of the drill bit below the surface. This information is important because it is impossible to plot mud logging data without a good understanding of the depth of the bit at the time the mud reached the surface.
  • Mud pit level sensors: Although the level of a mud pit isn’t always useful to the formation evaluation process, it’s critical to keep track of it during drilling. A sudden rise in mud pit level is a classic indicator of a kick, the precursor to a blowout.
  • Automatic recording equipment: All the information collected by the sensors is sent to a central computer, where the values are recorded continuously.

Looking at a Mud Log

The wellsite geologist records all the drilling attributes in a single chart known as a mud log. The mud log shows changes in processes and characteristics that occur as drilling progresses. Traditionally, mud logs were recorded by hand, but now they are typically recorded digitally.

The main header of a mud log contains reference information such as the well number, well location, dates of drilling, and the name of the mud logging geologist. Header formats vary considerably, especially with the advent of digital mud logs, but they are usually self-explanatory. The body of a mud log contains the results of the logging. Each column contains data of one or more types, gleaned from the cuttings and mud that return from the bottom of the hole.

The mud log provides a continuous record of drilling activity that is useful for formation evaluation, meeting record-keeping requirements, and determining what went wrong when problems occur.

Images: “Mud log with corrected d-exponent trending into pressure” by Mudgineer via Wikimedia Commons licensed under CC BY SA 3.0; “597px-Mud_Log_cDExp1” by Mudgineer, CC BY-SA 3.0 via Wikimedia Commons