Page 238 - Drilling Technology in Nontechnical Language

Chapter 10 – EVALUATION 229

the annular velocity. The net speed at which a particle will move up the
annulus is calculated by subtracting slip velocity from annular velocity. As
drilled cuttings from one formation are generated in a variety of sizes and
shapes, it follows that their slip velocities will differ too. Samples from one

specific depth will arrive at the surface over a period of time and not all at
the same time. For this reason, the actual depth of a particular sample is
unlikely to be precisely known.
Another source of inaccuracy in sample depth determination is that
some cuttings may settle at the side of the hole (in a deviated well) or in
an enlarged section of the hole. Later on, pipe movement or an increase
in pump speed might disturb these beds of cuttings and allow them to
continue upwards. In this case, samples from much higher in the well
might suddenly appear at the surface, mixed in with the newer (deeper)
cuttings. There is plenty of scope for confusion!

When the bit drills from one formation into a distinctly different
formation, there will almost always be a detectable change of rate of
penetration or drilling torque if the WOB and RPM are kept the same. The
depth at which this ROP or torque change takes place is used to adjust the
sample depths for greater accuracy. For instance, if drilling from shale into
salt, there will usually be a sudden increase in the rate of penetration. The

first samples of salt arriving at the surface should have come from the very
top of the salt formation, though mixed in with this sample will be particles

of shale still returning up the annulus. The depth of this first salt sample
can be assumed to be the depth where the ROP increase was noted.
Drilling mud circulated around the well can also provide valuable
information. Pore fluids or gas will enter the mud, even if a kick has not

occurred, simply because pore fluids will be released from the drilled

cuttings coming up the annulus. This might then cause a detectable change
in mud chemistry, for instance by increasing the salinity of the mud. Oil
could be easily visible, as shown in figure 10–2.

Just a few types of rocks and minerals make up most of the Earth’s

crust. All of these are readily identified by simple tests. It is important

for the geologist to identify these rocks and minerals in the field without
elaborate equipment. Minerals generally occur as small grains making up
the rocks. The main attributes that the geologist uses to identify a rock are
described next.

_Devereux_Book.indb 229 1/16/12 2:12 PM