24-Pet-B2 Oil and Gas Evaluation and Economics · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams May 2016, 98-Pet-B2, Natural Gas Engineering — 3 hours, closed book (non-communicating calculator permitted), 7 questions of 20 marks each. 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.; 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) Gas formation volume factor. $B_g=0.02827\,ZT/p$ (ft$^3$/SCF) — the ratio of gas volume at reservoir conditions to its volume at standard conditions, used to convert reservoir gas volumes into surface (sales) volumes and vice versa.
(b) Volumetric gas reservoir. A gas reservoir with no significant aquifer support, so reservoir pressure declines essentially in proportion to cumulative gas withdrawn (constant hydrocarbon pore volume) — the condition that makes the linear $p/Z$ vs. $G_p$ material-balance straight line valid, and the basis for Question 4(a) below.
(c) Residual gas saturation to water. The gas saturation left trapped, immobile, behind an advancing water front once imbibition has occurred (here $S_{gr}=0.25$) — capillary forces snap off and isolate gas blobs in the pore throats, and this trapped fraction cannot be produced by further pressure depletion once water has invaded that pore volume (the basis for Question 4(b)).
(d) Water influx. Aquifer water encroaching into the hydrocarbon pore volume as reservoir pressure declines, partially or wholly offsetting the pressure drop that would otherwise accompany production — it both helps maintain pressure (higher current deliverability) and, for a gas reservoir, traps residual gas behind the advancing front, lowering ultimate recovery relative to volumetric depletion.
(e) Constant composition expansion (CCE) test. A PVT laboratory test in which a fixed mass of reservoir fluid is expanded stepwise at reservoir temperature with no fluid removed at any stage, tracking total volume vs. pressure to locate the bubble/dew point and generate the single-phase $Z$-factor (or liquid relative-volume) curve above it.
(f) Stock-tank liquid. The liquid remaining in the stock tank at atmospheric (standard) conditions after a produced wellstream has passed through the separator train — for a dry-gas reservoir like this one it is negligible/absent, distinguishing it from a wet-gas or gas-condensate stream.
(g) Gas recycling. Reinjecting produced (lean) gas back into a gas-condensate reservoir after stripping the liquids, to maintain reservoir pressure above the dew point and suppress retrograde condensate drop-out in the pore space, at the cost of deferring sales-gas revenue until a later blow-down phase.
(h) Natural gas storage. Injecting gas into a depleted reservoir, aquifer, or salt cavern during low-demand periods and withdrawing it during peak demand, using the same volumetric/material-balance relations that describe a producing gas reservoir to size working gas and cushion (base) gas volumes.
(i) Gas reserve. The portion of the original gas in place (OGIP) that is technically and economically recoverable under defined operating conditions to a stated abandonment point (e.g. the $q_{min}$ economic-limit rate used in Question 5(b)/(c) below) — a subset of OGIP, not the full volumetric figure computed in Question 4(a).
(j) Orifice meter. A differential-pressure flow meter that infers gas rate from the pressure drop across a thin plate with a precisely sized circular bore installed in the pipeline (per AGA-3/ISO 5167), widely used for natural-gas custody transfer because it has no moving parts and is well characterized empirically.
