Neutron Porosity

Part of rock volume containing the hydrogen atoms.

The neutron porosity is usually abbreviated NPHI or PHIN  on log panels and denoted as  $//<![CDATA[ \begin{array}{l}\phi_n\end{array} //]]>$  in equations.

The key measurement is compensated neutron log   $//<![CDATA[ \begin{array}{l}N_{log}\end{array} //]]>$ (log name CNL) from Compensated Neutron Tool.

The key model parameters are:

The values of  $//<![CDATA[ \begin{array}{l}N_m\end{array} //]]>$ and  $//<![CDATA[ \begin{array}{l}N_{sh}\end{array} //]]>$ are calibrated for each lithofacies individually and can be assessed as vertical axis cut-off on  $//<![CDATA[ \begin{array}{l}N_{log}\end{array} //]]>$ cross-plot against the lab core porosity  $//<![CDATA[ \begin{array}{l}\phi_{\rm air}\end{array} //]]>$ and shaliness  $//<![CDATA[ \begin{array}{l}V_{sh}\end{array} //]]>$. 

The model also accounts for saturating rock fluids with fluid CNL value  $//<![CDATA[ \begin{array}{l}N_f\end{array} //]]>$.

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  $//<![CDATA[ \begin{array}{l}\phi_n\end{array} //]]>$ equation is:

The effective neutron porosity  $//<![CDATA[ \begin{array}{l}\phi_{en}\end{array} //]]>$ equation is:

The fluid density  $//<![CDATA[ \begin{array}{l}N_f\end{array} //]]>$ is calculated in-situ using the following equation:

The matrix CNL is calculated from the following equation:

where 

$//<![CDATA[ \begin{array}{l}V_{m,i}\end{array} //]]>$ – volume share of the i-th matrix component,

$//<![CDATA[ \begin{array}{l}N_{m,i}\end{array} //]]>$ – density of the i-th matrix component,

$//<![CDATA[ \begin{array}{l}\sum_i V_{mi} =1\end{array} //]]>$.