24-Pet-B2 Oil and Gas Evaluation and Economics · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams May 2019, 17-Pet-B2, Natural Gas Engineering — 3 hours, open book (non-communicating calculator permitted), 7 questions of equal (10-mark) value. NOTES item 5 states only the first five questions in the answer book are marked; all 7 are solved.
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.; McCain, The Properties of Petroleum Fluids, 3rd ed.; Mohitpour et al., Pipeline Design and Construction, 3rd ed. (ASME Press); GPSA Engineering Data Book (component critical-property tables); Wichert & Aziz (1972), “Calculate Z's for Sour Gases,” Hydrocarbon Processing; Mandhane, Gregory & Aziz (1974), “A Flow Pattern Map for Gas-Liquid Flow in Horizontal Pipes,” Int. J. Multiphase Flow.
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.
Approach. Redraw the single dome-shaped envelope printed on the exam sheet (peaking at the critical point, C.P.) and label each requested feature directly on it, working from the critical point outward along each branch.
[Figure not reproduced: Pressure–temperature phase envelope of a typical natural gas reservoir mixture, with the six requested features labelled on the redrawn dome. See the official exam paper.]
(a) Liquid phase region. The region enclosed to the left of and above the bubble-point line — at any (T, p) point there, the mixture exists as a single liquid phase.
(b) Gas phase region. The region outside the envelope to the right of and below the dew-point line (including the entire area at temperatures above the cricondentherm) — the mixture exists as a single vapour phase there.
(c) Two-phase region (liquid and vapour). The interior of the dome, bounded by the bubble-point line on the upper-left and the dew-point line on the lower-right, meeting at the critical point — any (T, p) inside this envelope splits into coexisting liquid and vapour in proportions given by the lever rule.
(d) Bubble-point line. The upper-left branch of the envelope, running from low pressure up to the critical point — along this curve the first bubble of gas comes out of solution from an otherwise liquid mixture as pressure is reduced (or temperature raised) at constant composition.
(e) Dew-point line. The lower-right branch, running from the critical point down and around to low pressure — along this curve the first drop of liquid condenses out of an otherwise gaseous mixture; the retrograde (backward-bending) upper part of this branch is why isothermal pressure depletion of a gas-condensate reservoir between the cricondentherm and the critical temperature causes liquid to drop out on the way down in pressure.
(f) Cricondenbar point. The point of maximum pressure anywhere on the envelope (on the dew-point branch, just past the critical point) — above this pressure the mixture cannot exist as two phases at any temperature.
(g) Cricondentherm point. The point of maximum temperature anywhere on the envelope (on the dew-point branch, at low pressure) — above this temperature the mixture cannot exist as two phases at any pressure, however high.
