Question 7 of 12: SNP Environmental Correction, Order of Magnitude

Question 7: SNP Environmental Correction, Order of Magnitude (7 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.

Given. SNP (sidewall neutron porosity) reading φ_SNP=20% (assuming the standard 7⅞" borehole, fresh water 8.3 lb/gal mud, 75°F calibration conditions); actual hole = 6"; actual mud = 12 lb/gal barite, fresh-water base (≈83% water by volume per the correction chart); actual formation temperature = 175°F; actual formation pressure = 5200 psia.

Find. The order of magnitude of the correction the SNP surface panel applies to the 20% reading.

Approach. The SNP panel automatically applies two of the three environmental corrections from its own caliper and mud-weight input — borehole-diameter and mud-weight — reading them off the manufacturer's correction charts at φ=20%; the temperature/pressure correction is a separate, manually-applied chart reading, not something the panel itself corrects for.

1. Borehole-diameter correction (panel-applied). From the "Correction for 6" and 10" Diameter Boreholes" chart (computed from the caliper signal and automatically applied by the panel), at φ=20% derived assuming a 7⅞" hole, the 6" HOLE band reads $$\Delta\phi_{bh}\approx +2.0\ \text{p.u.}$$
2. Mud-weight correction (panel-applied). From the "Correction for Weighted Muds" chart (also automatically applied by the panel), the 12 lb/gal barite mud (≈83% water by volume) band at φ=20% reads $$\Delta\phi_{mud}\approx +1.6\ \text{p.u.}$$ Together, the correction the panel itself has already applied is $$\Delta\phi_{panel}=\Delta\phi_{bh}+\Delta\phi_{mud}\approx 2.0+1.6=\boxed{3.6\ \text{p.u.}}$$
3. Temperature/pressure correction (NOT panel-applied). The "Correction for Temperature of 175°F" chart carries a separate "Combined Temperature and Pressure Corrections" curve (distinct from the wider "Temperature Correction Range" band, and with no "applied automatically by panel" label) — at φ=20% it reads $$\Delta\phi_{T,P}\approx +0.5\ \text{p.u.}$$ This is a chart the log analyst applies by hand at the wellsite; it is the correction the panel does not make.

Substituting, the order of magnitude of the correction the SNP panel applies is a few porosity units — $\boxed{\approx 3\text{--}4\ \text{p.u.}}$ (2.0 p.u. from hole size + 1.6 p.u. from mud weight); a further ≈0.5 p.u. combined temperature/pressure correction must still be applied manually, giving a total environmental correction on the order of $\boxed{\approx 4\ \text{p.u.}}$ if the reader wants the fully-corrected porosity.