Page 15 - A Primer of Oilwell Drilling, 7th Edition

ormation evaluation is the process used by operators to determine
Fif rock layers contain hydrocarbons. Formation evaluation can 11
Petroleum Extension-The University of Texas at Austin
determine if sufficient quantities of hydrocarbons are present and
if the rock has enough permeability to allow a commercial comple-
tion. The techniques addressed in this chapter are the examination
of cuttings and drilling mud, well logging, drill stem testing, and
formation
coring.

Evaluation
EXaMININg CUTTINgS aNd dRILLINg MUd

One of the oldest formation evaluation techniques is to simply look
at the cuttings and the drilling mud returning from the bottom of
the hole (fig. 200). A geologist or trained technician who examines
the returning drilling mud and cuttings is called a mud logger.
The rock type can be identified from the cuttings. This is
important because reservoirs typically fall into broad categories
by rock type. For example, reservoir rocks are often sandstone and
limestone, which develop the correct combination of porosity and
permeability needed to contain hydrocarbons and allow them to fow.
A rough idea of the porosity of a rock can be determined by viewing
cuttings under a microscope. if a rock contains oil, trace amounts
of oil will coat the cuttings even after they have been circulated in
drilling fuid and brought to the surface.
Oil is a polarizing compound. it will have a fuorescent shine
when viewed in a black light box. The oil stain on cuttings can be
confirmed by fushing the oil off the cuttings with a solvent. The
streaming solvent will also fuoresce under the black light. in this
way, an oil stain can be differentiated from other rock mineral that
might also fuoresce. Using this method to determine the presence
of oil does not work if an oil-based mud is used as a drilling fuid.

Figure 200. A handful of cuttings made by the bit

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