The neutron porosity is usually abbreviated NPHI or PHIN on log panels and denoted as in equations.
| LaTeX Math Block |
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| N_n = \phi N_f + V_{sh} N_{sh} + V_m N_m |
which can be re-arranged as: | LaTeX Math Block |
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| 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 (log name CNL) from Compensated Neutron Tool.The key model parameters are: | rock matrix CNL | | shale CNL | | pore-saturating fluid CNL
| | mud filtrate CNL
| | LaTeX Math Inline |
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| body | \{ N_w, \, N_o, \, N_g \} |
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| formation water, oil, gas CNL
| | a fraction of pore volume invaded by mud filtrate | | LaTeX Math Inline |
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| body | \{ s_w, \, s_o, \, s_g \} |
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| 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: | LaTeX Math Block |
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| anchor | phie_rhob |
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| alignment | left |
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| \phi_n = \frac{N_n - N_m}{N_f-V_{sh}N_m} |
The effective neutron porosity equation is:
| LaTeX Math Block |
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| anchor | phie_rhob |
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| alignment | left |
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| \phi_{en} = \phi_n - \frac{N_{sh}-N_m}{N_f - V_{sh}N_m} \cdot V_{sh} |
The fluid density is calculated in-situ using the following equation:| LaTeX Math Block |
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| 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: | LaTeX Math Block |
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| 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,. | 