Part of rock volume containing the hydrogen atoms.
The neutron porosity is usually abbreviated NPHI or PHIN on log panels and denoted as in equations.
The key measurement is compensated neutron log (log name CNL) from Compensated Neutron Tool. The key model parameters are: | rock matrix CNL | | shale CNL | | pore-saturating fluid CNL
| | mud filtrate CNL
| | formation water, oil, gas CNL
| | a fraction of pore volume invaded by mud filtrate | | original water, oil, gas reservoir saturations |
The values of and are calibrated for each lithofacies individually and can be assessed as vertical axis cut-off on cross-plot against the lab core porosity and shaliness . The model also accounts for saturating rock fluids with fluid CNL value . 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 equation is: \phi_n = \frac{N_{log} - N_m}{N_f-N_m} |
The effective neutron porosity equation is:
\phi_{en} = \phi_n - \frac{N_{sh}-N_m}{N_f - N_m} \cdot V_{sh} |
The fluid density is calculated in-situ using the following equation: 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: N_m = \sum_i V_{m,i} N_{m,i} |
where – volume share of the i-th matrix component, – density of the i-th matrix component, . |