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Part of rock volume containing the hydrogen atoms.


The neutron porosity is usually abbreviated NPHI or PHIN  on log panels and denoted as  \phi_n  in equations.


The key measurement is compensated neutron log   N_{log} (log name CNL) from Compensated Neutron Tool.

The key model parameters are:

N_m

rock matrix CNL

N_{sh}

shale CNL

N_f

pore-saturating fluid CNL

N_{mf}

mud filtrate CNL

\{ N_w, \, N_o, \, N_g \}

formation water, oil, gas CNL

s_{xo}

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:

(1) \phi_n = \frac{N_{log} - N_m}{N_f-N_m}


The effective neutron porosity  \phi_{en} equation is:

(2) \phi_{en} = \phi_n - \frac{N_{sh}-N_m}{N_f - N_m} \cdot V_{sh}

The fluid density  N_f is calculated in-situ using the following equation:

(3) N_f = s_{xo} N_{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:

(4) 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.

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