Page 29 - Well Logging and Formation Evaluation

Basics 19

tative of the formation salinity. This measurement is usually performed
at ambient conditions but may also be performed at in-situ pressure.
• Saturation exponent (n). In this measurement, the resistivity of the
plugs is measured as a function of water saturation, with the resistive
fluid being either air or kerosene. This measurement is usually per-
formed at ambient conditions.
• Capillary pressure (P c ). The saturation of a nonwetting fluid (either
air, mercury, or kerosene) is measured as a function of P c applied. In a
drainage cycle, 100% brine is gradually replaced by the nonwetting
fluid. For an imbibition cycle (following a drainage cycle), brine is rein-
troduced to replace the nonwetting phase.

Different techniques are available to make these measurements. In the
traditional approach, m, n, and P c would be measured using the porous
plate method, with air as the nonwetting phase. Since the measurement is
limited to 100psi, additional P c measurements would be performed using
mercury injection up to 60,000psi, thereby also determining the pore-size
distribution.
Many oil companies no longer favor these measurement techniques for
the following reasons:

1. Measurements using mercury involve destruction of the plugs and
present a potential environmental/health hazard.
2. P c measurements involving air/mercury are not representative of true
reservoir conditions and may give misleading results.
3. Porous plate measurements are slow and involve the repetitive han-
dling of the samples to measure the saturations using a balance. If grain
loss occurs, then the results are inaccurate and the electrical measure-
ments tend to be operator dependent.

Preferred techniques for undertaking these measurements are as
follows:

• Measurement of m and n should be performed using a continuous injec-
tion apparatus. While not steady state, this technique has been shown
to give reliable results. In the procedure, the sample is mounted verti-
cally, flushed with brine, then kerosene-injected at a continuous rate
while the resistivity and saturation are continually monitored.
• P c should be measured using a centrifuge capable of up to 200 psi pres-
sure. The sample is flushed with brine, then the amount of fluid expelled