Mud log

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(Figure 1)
A 1 inch = 100 feet (5 foot average) mud log detailing a Show while drilling an oilwell

(Figure 2)
A 1 inch = 100 feet (every foot) mud log showing corrected d-Exponent trending into pressure above the sand

A Mud log is a graphical representation of the ROP (Rate Of Penetration), lithology, hydrocarbons and other drilling parameters generated while drilling an oil well. The act of gathering this information is considered mud logging and is done by the mud logger. Other types of logs that are simular in scope are well logs.

[edit] History

Mud logs were one of the first types of logs created in the early days of the petroleum industry. They were created to give a visual reference of the lithology of the formations that were being drilled. With this information, someone wanting to drill another well in the vicinity of the logged well, could get some idea of what to expect.

ROP (rate of penetration), also known as the penetration rate or drill rate was added to show the speed that the formations were being drilled. With this information one could get a better idea as to the amount of time it might take to drill the well.

Later it was discovered that gas hydrocarbons could be extracted from the mud system, as it came out of the hole, and plotted on the mud log to give a visual reference of the formations that contained the gas.

As the petroleum industry matured, more sophisticated equipment was developed to monitor nearly every aspect of the drilling process and many of these were incorporated onto the mud log. In the height of the industry, the mud log and mud logger was considered the central source of information about what was going on with the drilling process of the well.

[edit] Details of the Mud Log

The ROP (rate of penetration) in (Figure 1 & 2) is represented by the black line on the left side of the log. The farther to the left that the line goes, the faster the rate of penetration. On this mud log, ROP is measured in feet per hour but on some older, hand drawn mud logs, it is measured in minutes per foot.

The porosity in (Figure 1) is represented by the blue line farthest to the left of the log. It indicates the pore space within the rock structure. An analogy would be the holes in a sponge. The oil and gas resides within this pore space. Notice how far to the left the porosity goes where all the sand (in yellow) is. This indicates that the sand has good porosity. Porosity is not a direct or physical measurement of the pore space but rather an extrapolation from other drilling parameters and therfore not always reliable.

(Figure 3)
Sample of drill cuttings of shale while drilling an oil well in Louisiana. For reference, the sand grain and red shale are approximately 2 mm. in dia.

The lithology in (Figure 1 & 2) is represented by the cyan, gray/black and yellow blocks of color. Cyan = lime, gray/black = shale and yellow = sand. More yellow represents more sand identified at that depth. The lithology is measured as percentage of the total sample, as visually inspected under a microscope, normally at 10x magnification (Figure 3). These are but a fraction of the different types of formations that might be encountered. Color coding is not nessecarily standardized among different mud logging companies, though the symbol representation for each are at least, very similar. In (Figure 3) you can see a sample of cuttings under a microscope at 10x magnification after they have been washed off. Some of the larger shale and lime fragments are seperated from this sample by running it through sieves and must be considered when estimating percentages. Also, this image view is only a fragment of the total sample and some of the sand at the bottom of the tray can not be seen and must also be considered in the total estimation. With that in mind this sample would be considered to be about 90% shale, 5% sand and 5% lime (In 5% increments).

The gas in (Figure 1 & 2) is represented by the green line and is measured in units of ppm (parts per million) as the quantity of total gas, but does not represent the actual quantity of oil or gas the reservoir contains. In (Figure 1) the squared-off dash-dot lines just to the right of the sand (in yellow) and left of the gas (in green) represents the heavy hydrocarbons detected. Cyan = C₂ (ethane), purple = C₃ (propane) and blue = C₄ (butane). Detecting and analyzing these heavy gases help to determine the type of oil or gas the formation contains.