Page 91 - Well Logging and Formation Evaluation

Advanced Log Interpretation Techniques 81

T por = total porosity obtained from summing all the fluid contributions
POR eff = effective porosity obtained by excluding contributions prior to
a predefined T 2 threshold
BVI = the proportion of capillary-bound water to the total volume
CBW = the proportion of the clay-bound water to the total volume
FFI = free fluid index. The proportion of non-clay-bound or non-capillary-
bound fluid to the total volume. POR eff = (BVI + FFI)/100.
PERM = permeability obtained by applying various standard equations

The most commonly used equation for permeability (Coates equation)
is as follows:

4 2
C ) ) ( * FFI BVI)
PERM = ( ( T por
where C is a constant (typical values are around 0.10).
By definition BVI + CBW + FFI = T por . Above the transition zone, i.e.,
at a P c value of >100psi, it should be true that:

S h hydrocarbon saturation) = FFI T por .
(
Below this P c value, FFI will comprise both the free water and the
free hydrocarbon. An example of an NMR log, from a hypothetical
formation lying well above the FWL (free water level), is shown in
Figure 5.4.1.
Fluid differentiation with the tool may be performed by either the dif-
ferential spectrum method (DSM) or the shifted spectrum method (SSM).
DSM works by varying the wait time T w , thereby exploiting the different
T 1 times for different fluid types. SSM works by varying the echo time T e ,
thereby exploiting differences in the diffusivity (D) between different
fluids. This is particularly applicable to gas/oil differentiation, since the
value of D for gas is much higher than that for oil or water. Typical acqui-
sition parameters for the tool are as follows:

Normal T 2 mode:
• Number of echoes: 600
• Wait time: 1.3 seconds (or longer if a significant polarization correc-
tion is required)
• Echo spacing: 0.32ms
• Logging speed: 600ft/hr (i.e., 2 measurements at each depth increment)