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As the drill bit cuts through the different formations, the cuttings are brought to the surface by the drilling mud. Traces of oil or gas may also be brought up in the mud
. The practice of mud logging tries to identify, record, and/or evaluate lithology, drilling parameters, and hydrocarbon shows.
The information obtained by the mud logger is presented in the form of various logs such as the driller’s log, the cuttings log, or the show evaluation log. The mud logger takes this information, correlates it with data from other wells, and determines whether the well may be able to produce hydrocarbons in commercial quantities. In addition, the mud logger monitors the wellbore for stability to prevent blowouts or kicks.
To comprehend all of the information available, we need to understand four important areas of mud logging; rate of penetration and lag, gas detection, formation evaluation arid sample collection, and show evaluation.
1. Rate of Penetration and Lag
Rate of penetration (ROP) is the oldest and most common way of measuring and evaluating formation characteristics and drilling efficiency.
The formation’s lithology (rock type and hardness), porosity, and pressure affect the ROP. The drilling parameters that affect ROP include the weight on the bit, the bit’s speed (rpm), the drillstring configuration, the type of bit selected and its condition, and hydraulics.
2. Gas Detection
Gases extracted from the mud system are usually the first indication that hydrocarbons are present downhole.
Gas enters the drilling fluid from one of three sources:
(1) a gas-bearing formation, (2) a formation feeding gas into the mud, or (3) contamination.
As the bit drills through a formation, it opens or exposes some of the pores. Fluid from these opened pores mixes with the drilling mud. This gas, along with cuttings, or piece, from the drilled formation, is pumped back up toward the surface. As the gas and cuttings rise, the pressure drops and more gas comes out of the pores in the cuttings. This «liberated gas» is an important piece of data por log interpretations.
If the hydrostatic pressure is less than the formation pressure, even more gas can flow into the wellbore. The amount of flow from the formation into the borehole depends upon the pressure differential (the difference between the hydrostatic pressure and the formation pressure), the porosity and permeability, the properties of the formation’s fluids, and the length of time this condition lasts. When the formation fluids enter continuously, the well is said to kick. Swabbing (lifting the drillstring rapidly) also encourages formation fluids to flow in the well because the wellhore pressure drops.
Engineers can identify this gas that enters the borehole during swabbing, called connection gas, and can use the data to enhance formation evaluation and improve well safety-occasionally gas is introduced into the drilling fluid from a source other than the formation, particularly when oil-based drilling fluids are used. This is called contamination gar.
3. Collecting Samples
One of the most important jobs of the mud logger is to collect a representative sample of drill cuttings from the shale shakers
and prepare it for lithological identification and hydrocarbon show evaluation.
When the cuttings arrive at the shale shaker, they are covered in mud, unsorted by size, and generally unidentifiable. The shale shaker sifts or separates the larger cuttings from the drilling fluids and finer—microscopic or dust-sized pieces of formation. The fluid is filtered for reuse, and the cuttings arc routed to the reserves pit. The mud logger collects some of the cuttings before they arc routed to the reserve pit and lost.
Once the samples are collected, the logger can examine them unwashed and wet, washed and wet, or washed and dried. The unwashed samples arc collected directly from the shale shakers and are untreated. They are placed in labeled sample bags and are shipped to a laboratory. Washed samples are also collected from the shale shaker; however, the excess drilling mud is flushed away and then the samples are sieved to remove the coarser cavings (formation fragments that originate from the sides, nor the bottom, of the borehole) before they are put in sample bags. Washed and dried samples go through the same steps as washed samples. Before they are bagged, though, they are air dried or dried in ovens. It is from this set of samples that cuttings are taken for microscopic analysis for lithology identification and to observe oil shows.
4. Show Evaluation
A show is the presence of hydrocarbons in a sample over and above background levels. Show evaluation is the complete analysis of the hydrocarbon-bearing formation with respect to lithology and type of hydrocarbon present.
A complete show evaluation identifies the presence and type of hydrocarbon, determines the depth and thickness of the show, assesses the porosity and permeability, and assigns a show value that indicates the potential productivity of the formation.
Two types of shows are recognized: gas and oil. A gas show is hard to identify, but the mud logger may see a significant increase in gas levels. An oil show is an increase in heavier-than-methane gas levels as well as a physical indication of oil.