The neutron porosity is usually abbreviated NPHI or PHIN on log panels and denoted as
\phi_n in equations.
| (1) |
N_n = \phi N_f + V_{sh} N_{sh} + V_m N_m |
which can be re-arranged as:
| (2) |
N_n = N_m + V_{sh} \cdot (N_{sh} -N_m) + \phi \cdot (N_f - V_{sh} N_m) |
The key measurement is compensated neutron log
N_n (log name CNL) from Compensated Neutron Tool.
The key model parameters are:
| rock matrix CNL |
| shale CNL |
| pore-saturating fluid CNL
|
| mud filtrate CNL
|
\{ N_w, \, N_o, \, N_g \} | formation water, oil, gas CNL
|
| a fraction of pore volume invaded by mud filtrate |
\{ s_w, \, s_o, \, s_g \} | original water, oil, gas reservoir saturations
s_w + s_o + s_g = 1 |
The values of
N_m and
N_{sh} are calibrated for each lithofacies individually and can be assessed as vertical axis cut-off on
N_{log} cross-plot against the lab core porosity
\phi_{\rm air} and shaliness
V_{sh}.
The model also accounts for saturating rock fluids with fluid CNL value
N_f.
In overbalance drilling across permeable rocks the saturating fluid is usually mud filtrate.
In underbalance drilling the saturating fluid is identified from resistivity logs.
The total neutron porosity
\phi_n equation is:
| (3) |
\phi_n = \frac{N_n - N_m}{N_f-V_{sh}N_m} |
The effective neutron porosity
\phi_{en} equation is:
| (4) |
\phi_{en} = \phi_n - \frac{N_{sh}-N_m}{N_f - V_{sh}N_m} \cdot V_{sh} |
The fluid density
N_f is calculated in-situ using the following equation:
| (5) |
N_f = s_{xo} \rho_{mf} + (1-s_{xo}) ( s_w N_w + s_o N_o + s_g N_g ) |
The matrix CNL is calculated from the following equation:
| (6) |
N_m = \sum_i V_{m,i} N_{m,i} |
where
V_{m,i} – volume share of the i-th matrix component,
N_{m,i} – density of the i-th matrix component,
\sum_i V_{mi} =1.
Petrophysics / Volumetric Rock Model
