Question

What specific change in the Density-Neutron log crossover signature occurs when gas replaces oil in the pore space of a clean sandstone reservoir?

Accepted Answer

When gas replaces oil in a clean sandstone reservoir, the Density-Neutron log crossover signature shifts from a small separation or no separation to a significant positive separation, meaning the neutron porosity value decreases while the density porosity value increases. This occurs because the Density log measures electron density, which is very low in gas compared to oil, resulting in an artificially high calculated density porosity. Simultaneously, the Neutron log measures hydrogen index, which is drastically lower in gas than in liquid oil, causing the neutron tool to record an artificially low porosity. In a clean sandstone, oil typically causes the density and neutron logs to track closely or show a slight crossover, but gas creates a wide separation where the density porosity curve plots to the right of the neutron porosity curve. This distinctive pattern is known as the gas effect, providing a reliable petrophysical indicator of gas presence because the low density of gas makes the formation appear less dense and the low hydrogen content of gas makes the formation appear to have fewer fluid-filled pores.

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What specific change in the Density-Neutron log crossover signature occurs when gas replaces oil in the pore space of a clean sandstone reservoir?