Page 241 - Drilling Technology in Nontechnical Language

232 Drilling Technology in Nontechnical Language Second Edition

Specific gravity (weight). Specific gravity is the relative weight of

a mineral compared to the weight of an equal volume of pure water. The

average specific gravity (SG) of a rock or mineral would be about 2.5.

Metallic ore minerals generally have specific gravities above 3.5. Minerals
can be readily recognized from their SG.
Fractures. Some rocks are naturally fractured. Often these fractures

will fill up with minerals and can be seen as veins of a different color
within the rock.

Fluorescence. Crude oils will fluoresce under ultraviolet light (that is,
they will glow, giving off light of a different wavelength than the source
light). If a cutting from an oil-bearing formation is exposed to a UV lamp,

the fluorescent properties can identify the type of oil present. Different

colors of fluorescence indicate different grades of oil. Moving from low to
high API gravity, colors seen will be brown (below 15°API), orange, yellow,
white, blue-white, and violet (45°API). (The density of an oil is expressed

in degrees API. API gravity of 10° is equal to freshwater density [specific

gravity of 1]. An API gravity of 45° is equal to a specific gravity of 0.801.
The higher the API gravity, the lower the SG, and the more valuable the
oil. This is because a lighter oil contains more light hydrocarbon elements,

which require less refining to produce petroleum.)
On modern drilling operations, specialist contractors are used to
monitor and analyze surface data and samples. This activity is called
mud logging.

Mud logging

Mud logging involves taking measurements and samples during
drilling. This usually includes the following:
1. Taking samples of cuttings, mud, and formation fluid shows and

analyzing them. (A show is an indication of hydrocarbons down

hole, usually seen by examining drilled cuttings for fluorescence.)
2. Logging and recording all important drilling parameters.
3. Detecting and warning of the presence of problems such as kicks,
H S, and washouts.
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4. Producing analyses and reports.
5. Recording quantities and descriptions of cavings.

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