Question 4 of 12: Porosity-Log Response in Gas-Bearing Formations

Question 4: Porosity-Log Response in Gas-Bearing Formations (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Gas has a much lower hydrogen concentration and bulk density than oil or water, so both the neutron and density porosity tools mis-read a gas zone, and the direction/size of the error is set by three factors:

• Reduced hydrogen index (HI) of gas. The neutron tool infers porosity from the hydrogen concentration of the pore fluid, calibrated to a water-filled, 100% HI reference; gas has HI « 1 (a function of gas density/pressure), so the neutron tool under-reads porosity in a gas zone — it "sees" less hydrogen than the true φ would give if water-filled.
• Reduced bulk density of gas. Because ρ_gas « ρ_water, the density tool (φ_D=(ρ_ma−ρ_b)/(ρ_ma−ρ_f), formula sheet) over-reads porosity when ρ_f is assumed to be water/mud filtrate but is actually gas.
• Excavation effect. At high porosity and low S_w, the neutron under-reading is larger than a simple HI-weighted average of gas and rock would predict, because the neutron slowing-down length in a gas-filled, high-porosity rock is disproportionately long — the net result is that the neutron-density "crossover" (φ_N reading well below φ_D) is the classic gas indicator, but its exact magnitude also depends on this excavation correction, not on HI/density contrast alone.