Petroleum engineering needs knowledge of the mud log because much of the design of the next wells to be constructed that belong to the same reservoir as the well being drilled is based on the lithology of the well being drilled.
What is mud logging?
It is an activity done by engineers and geologists, which consists of keeping a detailed daily log of an oil well while it is being drilled, examining the rock cuttings that come from the bottom of the well and that are cut with the drill bit and transported by the drilling fluid or drilling mud.
In my experience as a drilling operations engineer, I can tell you that this is an activity that was performed by a private company, since the state company PDVSA did not perform it, however the supervision of the drilling work and the activities carried out by the mud logging company are the responsibility of the drilling operations engineer.
This provides well owners and producers with information on the lithology and fluid content of the well during drilling. Historically, it is the oldest type of well logging. In some circumstances, compressed air is used as the circulating fluid instead of mud.
What it could provide to us as the company that owns the oil well, is that while drilling the well, you can analyze the drill cuttings samples and thus know what rock you are going through while drilling, an example of this is that for example you can be going through a sandstone in an interval of 40 feet, and then move to a shale in an interval of 30 feet and so on you can go recording the various intervals of rock that you are going through as you drill.
As additional information you can also know with the mud log the type of fluid contained in the well, i.e. whether oil or gas formation is entering the well.
Below is an image of how the geologist can view a drill cuttings sample under the microscope:
If a diversity of lithological fragments of subsoil rocks is seen in a single sample, then the percentage present has to be estimated, e.g. such a percentage of limestone, such a percentage of siltstone, such a percentage of sandstone and so on.
Mud logging can also help us to be able to determine the exact depth to which the hydrocarbon is in the subsurface, evaluating the generalized work, it can be said that it also identifies the lithology of the subsurface, monitors the entry of natural gas formation to the well, shows the analysis of the rock cuttings that circulate to the surface as they are transported by the drilling mud.
Mud logging works generally begin to be executed when the drilling depth in the oil well is at a depth when it is really needed, that is generally when the intermediate hole begins to be drilled, since from this hole it is about to start drilling the production hole, this is important for the engineer in charge to know so that he is prepared for the logistics involved in calling the company that will provide the mud logging services.
What is the scope of the mud logging activity?
The mud loggimg company also installs sensors and tools at the bottom of the drill pipe in order to log various parameters as the well is being drilled, one of the clear examples is the following log that I am going to show you in the following image:
In this log you can see that from left to right there are several graphs, the blue one is the porosity, which measures the porosity of the rock that is being penetrated, which is very important to diagnose and know if we are going through the producing formation, the black graph is the rate of penetration (ROP), It is important to measure the ROP because if there is an abrupt change in the rate of penetration is because there may be an indication of the presence in the well of formation fluids such as natural gas and oil.
The green and yellow color is a lithological description of the type of rock and at what depth it is being found in the subsurface, in this case the gray is limestone and yellow is sand. You can also plot the density of the drilling fluid (drilling mud) and the corrected d exponent, which is a value used in a mathematical correlation of great importance to evaluate certain geological aspects of the reservoir.
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