Page 237 - Drilling Technology in Nontechnical Language

228 Drilling Technology in Nontechnical Language Second Edition

Physical Sampling at the Surface

In the early days of drilling, the main source of information on the
formations being drilled through came from examining the drilled cuttings

returned to the surface (fig. 10–1). The depth that the sample came from
can be estimated (but not known precisely) by recording the time that the
sample appeared at the surface and by knowing the time taken to circulate
a cutting from the bottom. Subtracting the transit time from the time on
the surface then allows the wellsite geologist to work out the bit depth at
the time that the cutting was generated and so gain an idea of the depth
from which the sample came.

Fig. 10–1. Drilled cuttings at the shale shakers

A solid particle will fall through the drilling fluid in the annulus at a

speed that depends on its size, shape, and density relative to the fluid and

the viscosity of the fluid. This downward speed is called the slip velocity.
During circulation, the upward speed of the fluid in the annulus is called

_Devereux_Book.indb 228 1/16/12 2:12 PM