Cuttings refer to the small rock pieces chipped by the bit during the well drilling process. The mud stream transports these rock fragments to the surface from the bit where they’re ‘caught’ to be analyzed.
These cuttings are essential since they’re often the only physical lithological data recovered from wells. The data is used as supportive information for formation identification, reservoir description, correlation of wireline logs to lithology, show evaluation, and other special geophysical, geological, and engineering analyses.
What You Need to Know About Catching Representative Samples
At the time of well planning, the types and number of sample sets and intervals are typically determined. The drill cuttings are collected as composite samples to reflect different lithologies drilled over the interval. The length varies on the required detail and the drilling rate needed for lithological and reservoir descriptions.
With slow drilling rates, the sample collection must represent the entire drilled interval instead of just the final few feet. Moreover, the collection of representative samples must be when the drilling rate is high, making it challenging for mud loggers to collect or analyze the samples accurately.
The situation allows the sample interval to be increased, or you can assign an additional mud logger for the task. The drilling rate can also be slowed to obtain better samples and evaluate prospective reservoir zones, kick-off points, and casing. Supplemental samples must also be caught to ease show evaluation when a change in background gas or drilling break occurs.
The cuttings are usually caught from the possum belly, rig shale shakers, or the de-silter and de-sander outlets. Most rigs have multiple or single shaker screens to separate the mud from the cuttings.
When the returning mud travels across the screens, the solids travel to one end of the screen while the mud fluids fall through the mesh. A catching board placed at the screen’s foot catches the composite sample. The box also prevents samples from falling onto the ground or into the mud pit. This is the shale shaker technique.
For the possum belly method, a metal box or bucket is placed in the possum belly near the flow line’s entrance. The technique helps prevent conglomerate and sandstone cutting disaggregation.
Unconsolidated samples are also taken from the de-sander or de-silter outlets when they use the underflow of the shakers rather than being supplied from a pit. These samples comprise both formation solids and particles related to the drilling fluid. Mud loggers need to discriminate between the two.
The object is to identify lithology changes and new formations’ appearance. Ultraviolet light and microscopes are tools used to reconstruct the characteristics of the originating strata.
The estimate of the percentages of staining, porosity, and lithology needs to be made carefully because factors such as grain size, shape, distribution, and color can impact the relative percentages. When undertaking lithology estimates, there are some potential contamination sources to consider.
Drilling Cutting Contamination: The Top Five Primary Sources
#1- Recycled cuttings
When drilling cuttings at the de-silters, de-sanders, or shale shakers aren’t removed from the fluid efficiently, they can be recycled through the mud system. These can be recognized as abraded, small, rounded fragments of rock in the sample. The recycled microfossils can lead to serious problems in correlation.
While cuttings are always washed through a coarse sieve to remove the cuttings from prior drilled intervals, some may remain in the sample.
The cavings can be recognized as splintery large rock formations that are convex or concave in the cross-section. They’re identical, with formations lithologically from the open hole’s higher sections.
When found in excessive qualities, they can indicate a severely unbalanced drilling mud condition or scenarios where stabilizers catch the hole’s side, or the pipe rotation is too rapid.
Occasionally, metal is found in samples that frequently originate from the inside wearing of the casing by drill strings. It can be remedied by using rubber protectors for the drill pipe. Bits that are lost sidewall bullets or other products of milling operations are also potential sources of metal contamination. If large amounts of metal are found, the tool pusher should be informed promptly.
Contamination of cement is typically encountered when drilling while well sidetracking or after casing. Cement is often mistaken for siltstone, although it can be readily identified via a phenolphthalein solution test. Due to higher pH levels of manufactured cement, it stains purple.
#5- Mud Chemicals
Certain mud chemicals are often confused with rock types. Bentonite, for instance, can be identified as montmorillonite clay or lignite as lignosulfonate in poorly mixed mud systems. Additionally, LCM (lost circulation material) such as fibers, mica flakes, and nut shells can be common contamination culprits during lost circulation zone drilling.
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