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24-Pet-B1 Natural Gas Engineering · Undated paper

Question 8 of 10: Neutron–Density versus Gamma-Ray Crossplot and Fluid Typing

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Examinations, 17-Pet-B1, Well Logging and Formation Evaluation — May 2019, 3 hours, closed book (Sharp or Casio approved calculators permitted), 10 questions, all marked. Every question on this paper is Well Logging & Formation Evaluation content, solved to the paper as printed. Every datum here was read from the paper: the Question 9 SP track prints "SSP −80 mV" and "PSP 47 mV", its gamma-ray track prints 28, 92 and 44 API against Zones B, C and A, and the attachments supply the gas-sand chart and SP departure chart used in Questions 7 and 10.

Reference texts: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (SPE Textbook Series Vol. 4); Asquith & Krygowski, Basic Well Log Analysis, 2nd ed. (AAPG); Ellis & Singer, Well Logging for Earth Scientists, 2nd ed.; Schlumberger, Log Interpretation Charts / Log Interpretation Principles and Applications.

Question 8: Neutron–Density versus Gamma-Ray Crossplot and Fluid Typing (20 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.

QuantityValue
Log suite (page 8)Caliper + gamma ray in track 1 (GR 0–100 API); compensated formation density porosity (solid) and compensated neutron porosity (dashed), both 60–0 p.u. on a sandstone matrix, 3 p.u. per division
Adjacent shale, φD,sh / φN,sh38% / 16%
Shale / clean gamma ray, γsh / γclean85 API / 30 API
Zones to plotA, A1, A2, B1, B2, C, D, E1, E2, E3
Zones to classifyF (9402 ft), G (9599 ft)

Find. The crossplot of (φN − φD) against gamma ray for the labelled zones, and from it the fluid in Zones F and G.

Approach. Shale and gas both separate the neutron and density curves, so the separation alone cannot tell them apart. The crossplot gives shaliness its own axis. The clean point and the shale point supplied in the question define a straight shale line, along which a liquid-bearing rock of any shaliness must plot. A zone displaced from that line by more than the reading uncertainty is displaced by something other than shale: gas.

  1. Anchor points of the shale line. A clean, liquid-filled sand on the correct matrix scale reads the same porosity on both tools, so its ordinate is zero at the clean gamma-ray level. The shale reads the given averages. $$(\gamma,\ \phi_N-\phi_D)_{clean}=(30\ \text{API},\ 0)\qquad (\phi_N-\phi_D)_{sh}=16-38=-22\ \text{p.u. at } \gamma_{sh}=85\ \text{API}$$
  2. The shale (liquid) line. Shale volume enters both tool responses linearly, so liquid-bearing rock of any shaliness lies on the straight line joining the anchors: $$(\phi_N-\phi_D)_{line}=(\phi_N-\phi_D)_{sh}\,\frac{\gamma-\gamma_{clean}}{\gamma_{sh}-\gamma_{clean}}=-22\,\frac{\gamma-30}{55}\qquad \boxed{(\phi_N-\phi_D)_{line}=-0.40\,(\gamma-30)\ \text{p.u.}}$$
  3. Read the log at each zone. Gamma ray is read on its 0–100 API track, and both porosity curves on the 60–0 p.u. scale, at each labelled depth tick. The table gives the readings, the ordinate φN − φD, the shale-line value at that gamma ray, and the displacement Δ = (φN − φD) − (φN − φD)line.
  4. Classify by displacement. A zone on the line has a separation fully explained by shale. A zone well below it has extra separation that shale cannot explain. With a reading tolerance of about ±3 p.u., a threshold of Δ < −6 p.u. (twice the tolerance) marks gas.
Zoneγ, APIφD, p.u.φN, p.u.φN−φDShale lineΔVerdict
A343815−23.0−1.6−21.4gas
A13038.514−24.50.0−24.5gas
A2354116−25.0−2.0−23.0gas
B1463324−9.0−6.4−2.6liquid
B2463026.5−3.5−6.4+2.9liquid
C473126−5.0−6.8+1.8liquid
D363718−19.0−2.4−16.6gas
E1482725.5−1.5−7.2+5.7liquid
E2462826−2.0−6.4+4.4liquid
E34529.526.5−3.0−6.0+3.0liquid
F (9402 ft)623420−14.0−12.8−1.2liquid (shaly sand)
G (9599 ft)722915−14.0−16.8+2.8liquid (shale / very shaly sand)
20 30 40 50 60 70 80 90 100 10 5 0 −5 −10 −15 −20 −25 −30 Gamma ray, API units φN − φD, porosity units clean point (30 API, 0) shale point (85 API, −22) A A1 A2 D B1 B2 C E1 E2 E3 F G shale (liquid) line 6 p.u. below: gas cut gas-bearing zone liquid-bearing zone
The required crossplot of φN − φD against gamma ray. The solid red line is the shale (liquid) line from the clean point (30 API, 0) to the shale point (85 API, −22 p.u.); the dashed line lies 6 p.u. below it. Zones A, A1, A2 and D fall far below the line (gas). Zones B, C and E cluster on it, and Zones F and G, the two asked about, fall on it (liquid).

Zone F, 9402 ft — liquid-bearing, not gas. Zone F shows a raw separation of 14 p.u., which on a plain neutron–density overlay looks like a gas crossover. The crossplot corrects that impression. At 62 API the zone is about 58% shale on a linear index, and shale alone accounts for 12.8 of the 14 units. The remaining 1.2 p.u. is inside the reading tolerance, so there is no evidence of gas. Zone F is a shaly, liquid-bearing (most likely water-bearing) sand.

Zone G, 9599 ft — liquid-bearing, not gas. Zone G plots 2.8 p.u. ABOVE the shale line: its separation is smaller than shale alone would produce at 72 API. Its gamma ray is the highest on the section and the caliper steps out over it. The point sits near the shale end of the trend, consistent with a water-bearing shale or very shaly sand of no reservoir interest.

What the crossplot does find. Zones A, A1, A2 and D lie 17 to 25 p.u. below the shale line, far outside any reading error, at gamma rays within 6 API of the clean line. Those are the gas-bearing sands of this well. Zones B, C and E plot within 6 p.u. of the line, none of them below the gas cut: shaly, liquid-bearing sands. This is why the crossplot is built rather than reading the overlay. F (−14 p.u.) and G (−14 p.u.) would both be called gas from raw separation alone, and F would be ranked beside the genuine gas in Zone D (−19 p.u.).

Check: the gamma-ray and porosity values are read from the printed log. The log prints depth numbers only at 9400, 9500 and 9600 ft, with no intermediate numeric grid. Each reading carries about ±3 p.u. and ±5 API. The gas verdicts for A, A1, A2 and D clear the threshold by 10 p.u. or more. At Zone G the two track-1 curves separate below the caliper step, so the gamma ray there could be about 57 API rather than 72 API. At 57 API the shale line sits at −10.8 p.u. and G's −14 p.u. is only 3.2 p.u. below it, so the liquid verdict holds. Zone F's verdict likewise holds across 57–67 API. The printed shale values (φD,sh = 38% above φN,sh = 16%) put the shale point on the negative side of the ordinate. That is unusual for shale on a sandstone scale, but it matches this log, where the density curve reads above the neutron almost throughout. The values are used exactly as printed.