24-Pet-B2 Oil and Gas Evaluation and Economics · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams December 2015, 98-Pet-B2, Natural Gas Engineering — 3 hours, closed book (non-communicating calculator permitted), 7 questions of 20 marks each (only the first five as they appear in the answer book are officially marked). All 7 questions are solved, not just the first five.
Reference texts: Katz et al., Handbook of Natural Gas Engineering; Lee & Wattenbarger, Gas Reservoir Engineering (SPE Textbook Series Vol. 5); Ahmed, Reservoir Engineering Handbook, 5th ed.; Mohitpour et al., Pipeline Design and Construction, 3rd ed. (ASME Press); McCain, The Properties of Petroleum Fluids, 3rd ed.
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) Four common flow-measurement techniques. Orifice-plate (differential-pressure) meters; turbine meters; positive-displacement meters; and Coriolis (mass-flow) meters — ultrasonic meters are an increasingly common fifth option worth naming as well.
(b) Four factors governing technique selection. Expected accuracy/repeatability requirement for the application (custody transfer vs. allocation vs. well testing); fluid type and phase (single-phase gas/liquid vs. multiphase); operating range and turndown ratio needed (rangeability); and practical considerations — pressure drop tolerance, installation/straight-run piping requirements, and capital/maintenance cost.
(c) Accuracy and repeatability. Accuracy is how close the meter’s indicated value is to the true value of the flow rate — a measure of systematic bias, normally stated as a percentage of reading or of full scale. Repeatability (precision) is the closeness of agreement among successive readings of the same steady flow under the same conditions — a measure of random scatter, independent of whether those readings are centred on the true value.
(d) Rangeability and linearity. Rangeability (turndown ratio) is the ratio of the maximum to the minimum flow rate the meter can measure within its stated accuracy — a meter with 10:1 rangeability can accurately measure anywhere from 10% to 100% of its top rate. Linearity is how closely the meter’s output tracks flow rate as a straight-line (constant-gain) relationship across that range; a highly linear meter needs only a single calibration factor, while a nonlinear one needs a calibration curve.
These four sub-questions build on each other in a way worth spelling out: the choice of technique in (a) is driven directly by the selection factors in (b), and once a technique is chosen its actual field performance is only meaningful once described using the vocabulary in (c) and (d). An orifice meter, for instance, is cheap and robust (a strong (b) factor) but has comparatively poor rangeability — typically only about 3:1 to 4:1 — because its differential pressure varies with the square of flow rate, so at low flow the signal becomes too small to read accurately. A turbine or ultrasonic meter, by contrast, can offer 10:1 to 20:1 rangeability with good linearity across most of that range, which is exactly why allocation and custody-transfer metering on producing wells (whose rates decline over field life) increasingly favours these technologies over a fixed-bore orifice plate sized for early-life rates only.
