24-Pet-B1 Natural Gas Engineering · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, 98-Pet-B1, Well Logging and Formation Evaluation — May 2016, 3 hours, closed book (approved calculators permitted), 12 questions, all of them marked, values shown per question. neutron and density tools, SP, caliper, Archie, and log crossplots. There is no natural-gas-engineering content in the paper. All twelve questions are answered below.
Reference texts: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (SPE Textbook Series Vol. 4); Asquith & Krygowski, Basic Well Log Analysis, 2nd ed. (AAPG Methods in Exploration 16); Ellis & Singer, Well Logging for Earth Scientists, 2nd ed.; Schlumberger, Log Interpretation Charts / Log Interpretation Principles and Applications.
The exam supplies a formula sheet (page 15) and four chart attachments: an SNP borehole-size correction chart and a nonideal-shale-membrane SP departure chart (page 16), SNP mud-weight and temperature/pressure correction charts (page 17), and a water-oil relative permeability ratio chart plus the Schlumberger Rw-equivalent conversion chart (page 18). Every chart reading below is quoted with the reading tolerance it deserves.
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.
A porosity log responds to a bulk property averaged over its own volume of investigation, and when part of the pore fluid is gas the departure from the liquid-filled response is governed by three quantities.
1. The residual gas saturation within the zone the tool investigates. Density and neutron tools read the flushed zone, so what matters is not the virgin gas saturation $S_g$ but the residual gas saturation $S_{gr} = 1 - S_{xo}$ left after mud-filtrate invasion. Deep invasion sweeps the gas away from the tool and suppresses the gas effect entirely; shallow invasion in a low-permeability or high-mud-overbalance situation leaves plenty of gas in front of the pads and produces a spectacular crossover. This is why an identical gas reservoir can show a strong or a weak signature depending only on the mud programme.
2. The hydrogen index and density of the gas, which are fixed by reservoir pressure and temperature. Gas affects the neutron log through its hydrogen index and the density log through its density, and both are functions of the gas gravity, pressure and temperature. A shallow, low-pressure gas has a hydrogen index near zero and a density near zero, giving a large effect; a deep, high-pressure gas can be compressed to 0.3 g/cm3 or more with a hydrogen index of 0.4 or higher, so the same saturation produces a much smaller separation. Recognising this prevents a deep gas zone being dismissed because its crossover is modest.
3. The porosity of the formation. Both the neutron deficit and the density excess scale with the pore volume that the gas occupies: $\Delta\phi \propto \phi\,S_{gr}$. A 30 p.u. gas sand shows an unmistakable crossover; the same gas at the same saturation in an 8 p.u. tight sand shifts each curve by only two or three porosity units, which is inside the environmental-correction uncertainty. Lithology enters here too, because the matrix parameters chosen for the porosity scale set the baseline the separation is measured against.
