Page 56 - Well Logging and Formation Evaluation

46 Well Logging and Formation Evaluation

4000
set packer hydrostatic
open chamber
Pressure (psia) 3000 retract packer

2000

1000
start build-up
0
0 50 100 150 200
Time (s)

Figure 2.8.1 Pressure Measurements During a Pretest

Table 2.8.1
Typical viscosities of borehole fluids
Fluid Viscosity (cp)

Water 0.3
Diesel 2–3
Oil 3–10
Gas 0.015

k )
M = ( m (2.8.1)

where k = permeability of formation, in md, and m= viscosity of fluid
entering chamber, in centipoises (cp).
It may be shown theoretically that the mobility of the formation is
related to the drawdown pressure, drawdown time, and flow rate. From
analysis of the buildup, the contractor will normally give a mobility esti-
mate. For conversion of the mobility to permeability, the viscosity needs
to be known. In most cases the pretest chamber will be filled with mud
filtrate, either water or oil based. Table 2.8.1 gives some values.
While pretests are very useful in that they can prove that some per-
meability is present if a good buildup is obtained, it should be remem-