Petrophysics / Core Analysis Division - Tests And Services...

Special Core Analysis

Electrical Resistivity Studies
The evaluation of saturations and thus oil reserves from resistivity logs requires laboratory determination of both the cementation factor (m) and the saturation exponent (n) under conditions as representative as possible of reservoir ones.
A special laboratory equipment using actual fluids (crude oil and formation brine) at reservoir temperature and under effective overburden pressure has been developed.
Formation resistivity factor (for "a" and "m" values), two-electrode method. This service is used for determining the formation resistivity factor for plug samples at ambient and overburden conditions. The electrical resistivity of a fully saturated plug sample is
measured by placing it between two electrodes. The formation factor (FF) is calculated by dividing the resistivity of the fully saturated sample (Ro) by the resistivity of the
formation brine (Rw).

Cementation exponent
From the log plot of formation resitivity factors against the log of fractional porosities for all analyzed plug samples taken from the core, y intercept (a) and cementation exponent (m) can be calculated.

Resistivity index (for "n" value) two-eletrode method
This service is used for determining resistivity index values (“n” values) for plug samples at ambient and overburden conditions. The electrical resistivity of a partially saturated plug sample (Rt) is measured by placing it between two electrodes. The resistivity index (RI) is then calculated by dividing Rt by the measured resistivity of the fully saturated sample (Ro).


Air-Brine Capillary Pressure (Porous Plate Method)
A brine-saturated plug sample is placed on a porous ceramic plate and sealed inside a steel cell. Humidified air is introduced into the cell at various pressures and brine is displaced from the sample through the porous plate. The percentage brine saturation is then calculated for each injection pressure and capillary pressure curves can be plotted against the saturation levels.

Geotech has two different porous plate cells set up to suit different injection pressures. A 216mm diameter x 178mm stainless steel porous plate cell is used for low pressure capillary pressure measurements and a 15bar (220psi) ceramic plate extractor cell is used for higher pressure tests.

COMPARISON OF CAPILLARY PRESSURE TECHNIQUES:

Centrifuge Capillary Pressure
  Full curve drainage or imbibition tests
  Typically use native fluids, gas/ oil/ water at reservoir temperature.
  Fast and inexpensive

Mercury Injection
  Can be used for small samples, including sidewalls
  Typically drainage test, mercury displacing air
  Excellent for quantifying pore throat size distribution
  Fast, inexpensive, destructive to sample

Porous Plate Capillary Pressure
  Typically a drainage test
  Can use native fluids, oil/ water/ gas
  Equilibration may be slow