Order of Magnitude of the SNP Panel Correction: 24-Pet-B1 Natural Gas Engineering, May 2016, Q7

Question 7: Order of Magnitude of the SNP Panel Correction (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.

Quantity	Value
Raw SNP porosity from the calibration curves	25 p.u.
Borehole diameter	6 in (calibration hole is 7⅞ in)
Drilling fluid	12 lb/gal barite, fresh-water base (83 % water by volume)
Formation temperature	160 °F (chart is drawn for 175 °F)
Formation pressure	3800 psia

Find. The size of the correction the SNP surface panel applies automatically — not the total environmental correction.

Approach. The question turns on a distinction printed on the charts themselves. Read each attachment chart at 25 p.u., separate the corrections labelled "computed from caliper signal and automatically applied by panel" and "correction applied automatically by panel" from the one that is not so labelled, and sum only the automatic ones.

Waterfall of the SNP environmental corrections at 25 p.u. The panel applies the borehole-size and mud-weight terms automatically; the temperature and pressure term is left to the analyst.

Borehole-size correction — panel applied. The upper chart on attachment page 16 plots the correction for 6-in and 10-in boreholes against porosity derived assuming a 7⅞-in hole, and is captioned "CORRECTION COMPUTED FROM CALIPER SIGNAL AND AUTOMATICALLY APPLIED BY PANEL". Entering the 6-in band at 25 p.u. and reading the mean of the shaded band, $$\Delta\phi_{bh} \approx +2.2\ \text{p.u.}$$ The sign is positive because the calibration assumes a larger (7⅞-in) hole; a narrower hole puts less moderating mud in front of the skid, the count rate rises, and the uncorrected reading is too low.
Mud-weight correction — panel applied. The upper chart on attachment page 17 gives the correction for weighted muds against porosity derived assuming fresh water (8.3 lb/gal) in the hole, and carries the caption "CORRECTION APPLIED AUTOMATICALLY BY PANEL". Entering the 12 lb/gal barite (83 % water) band at 25 p.u., $$\Delta\phi_{mud} \approx +1.7\ \text{p.u.}$$
Sum the automatic corrections. These two are the corrections the panel makes on its own, from the caliper signal and the entered mud weight: $$\Delta\phi_{panel} = \Delta\phi_{bh} + \Delta\phi_{mud} = 2.2 + 1.7$$ $$\boxed{\Delta\phi_{panel} \approx +3.9 \approx +4\ \text{porosity units}}$$ which is about 16 % of the raw 25 p.u. reading. The panel therefore outputs approximately $$\phi_{SNP} = 25.0 + 3.9 = 28.9\ \text{p.u.}$$
Identify what the panel does NOT correct. The lower chart on page 17, "correction for temperature of 175 °F" with its "Combined Temperature and Pressure Corrections" curve, carries no automatic-application caption. At 25 p.u. the combined temperature-and-pressure curve gives roughly $+0.6$ p.u., and because the true formation temperature here is 160 °F rather than the chart's 175 °F the term is at the low end of its range. This correction must be applied by hand, bringing the fully corrected porosity to about 29.5 p.u.

The answer to the question as asked is therefore the panel-applied total: an order of magnitude of a few porosity units, about +4 p.u. — not tenths of a unit, and not tens.

Correction	Magnitude at 25 p.u.	Applied by
Borehole diameter, 6 in	+2.2 p.u.	panel (from caliper signal)
Mud weight, 12 lb/gal barite	+1.7 p.u.	panel
Total panel correction	≈ +3.9 p.u. (≈ +4 p.u., 16 % of the reading)	panel
Panel output porosity	28.9 p.u.	—
Temperature and pressure (160 °F, 3800 psia)	≈ +0.6 p.u.	analyst, by hand
Fully corrected porosity	≈ 29.5 p.u.	—

Check: the three departure values are read from the shaded bands of the supplied charts and carry the width of those bands, roughly ±0.4 p.u. each. The conclusion — a panel correction of order +4 p.u., with the temperature/pressure term excluded from it — is unaffected by reading anywhere within the bands.

Topic: SNP environmental corrections and the division of labour between the surface panel and the analyst.

Key relations: $\phi_{corrected} = \phi_{raw} + \Delta\phi_{bh} + \Delta\phi_{mud} + \Delta\phi_{T,P}$, with the first two terms applied automatically by the SNP panel and the third left to the interpreter.

Why this works: The SNP skid carries a caliper, so the panel always knows the hole diameter and can look up the borehole correction in real time; the mud weight is entered by the engineer at the start of the run and is likewise available. Temperature and pressure vary continuously with depth and were not fed to the analogue panels of that generation, so that correction was deliberately left off the recorded curve — which is exactly why the question asks for the correction "applied by the SNP panel" rather than the total environmental correction. Answering with the total would earn no marks; the marks are for noticing the captions.

Common pitfall: Adding all three chart values and reporting +4.5 p.u. as "the panel correction", or the mirror-image error of applying the borehole and mud-weight corrections a second time to a curve that already carries them — which double-counts about 4 porosity units and would turn a 29 p.u. sand into a 33 p.u. one.

Source: Schlumberger, Log Interpretation Charts, SNP borehole-size, mud-weight and temperature/pressure departure curves (reproduced as attachment pages 16–17 of this paper); Bassiouni, Ch. 12.

Question 7 of 12: Order of Magnitude of the SNP Panel Correction

Question 7: Order of Magnitude of the SNP Panel Correction (7 marks)