Formation Evaluation and Well Logging
Formation Evaluation and Well Logging tests how logs quantify lithology, porosity, saturation, and producibility. Expect GR and SP, resistivity, density-neutron, sonic, NMR, Archie saturation, and invasion corrections.
Core rules
- GR distinguishes shale from clean sand. SP helps estimate Rw and indicates permeability.
- Density porosity is phi_D = (rho_ma - rho_b)/(rho_ma - rho_f). Neutron responds to hydrogen index; density-neutron crossover can indicate gas.
- Archie: Sw^n = Rw/(Rt phi^m). Defaults of m ≈ 2 and n ≈ 2 are common but must be calibrated.
- Resistivity is affected by invasion, bed thickness, and adjacent beds. Use deep resistivity for Sw.
- NMR gives bound and free fluid plus permeability proxies; sonic porosity works best in compacted, known-lithology formations.
Worked example
Given Rw = 0.05 ohm-m, Rt = 5 ohm-m, phi = 0.20, m = 2, and n = 2: phi^m = 0.04; Rt phi^m = 0.20; Sw^2 = 0.05/0.20 = 0.25; Sw = 0.50. The zone is 50% water saturated. If porosity is entered as 20 instead of 0.20, the result is wrong by orders of magnitude.
Traps
Entering porosity as a percentage, using the wrong m or n, or reading Rt in a wet zone will fail saturation checks. Do not ignore shale corrections, bad hole, or neutron limestone-scale offset in a sandstone. Invasion and thin beds can make deep resistivity unrepresentative.
Key points
- GR and SP separate shale and indicate permeability.
- Density porosity uses grain and fluid densities.
- Archie: Sw^n = Rw/(Rt phi^m).
- Use deep resistivity and calibrated m and n.
- Bad hole and shale reduce log accuracy.