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) Residual gas saturation to water. The fraction of pore volume still occupied by trapped, immobile gas after an imbibing water front has swept through (e.g. in an aquifer-drive or waterflooded gas reservoir) — gas that is bypassed and cannot be recovered by further water displacement.
(b) Non-Darcy flow. The departure from a linear pressure-drop-vs-rate relationship at high velocity near the wellbore, where inertial (turbulent) losses add to viscous losses; captured empirically by the rate-dependent term $Dq$ added to the Darcy skin.
(c) Net pay thickness. The portion of the gross formation interval that is both porous/permeable enough and hydrocarbon-saturated enough to contribute economically to flow, after excluding shale streaks, tight zones, and water-bearing intervals identified from logs.
(d) Abandonment pressure. The reservoir (or wellhead) pressure at which production is no longer economic — the rate has fallen to the operator’s economic-limit rate — and the well is shut in for good; it sets the practical endpoint of reserves calculations.
(e) Constant volume depletion (CVD) test. A PVT laboratory test on a gas-condensate sample in which the cell pressure is lowered in steps, condensed liquid is left in place each step, and gas is removed until the cell returns to its original volume — simulating reservoir depletion and yielding produced-gas composition/Z-factor vs. pressure for material-balance use.
(f) Flow-after-flow test. A multi-point deliverability test in which the well is produced at a sequence of increasing (or decreasing) stabilized rates without shutting in between points, each rate/$p_{wf}$ pair being plotted to fit the deliverability equation $q=C(\bar p^2-p_{wf}^2)^n$.
(g) Separator test. A laboratory flash test that mimics the actual surface separator train (stage pressures and temperatures) applied to a reservoir fluid sample, used to determine the optimum separator conditions and the correct field-shrinkage/GOR conversion factors between reservoir and stock-tank volumes.
(h) Looped pipeline. A second pipeline segment laid parallel to (and tied into) an existing line over part or all of its length, splitting the flow between the two to increase total capacity without the pressure penalty of a single larger-diameter replacement line.
(i) Gas well liquid loading. The point at which the gas velocity in the tubing falls below the critical (Turner) velocity needed to continuously lift produced liquids to surface, so liquid begins to accumulate in the wellbore, adding hydrostatic backpressure and eventually killing the well.
(j) Gas lift operation. An artificial-lift method that injects high-pressure gas into the production tubing (via a series of unloading/operating valves) to lighten the fluid column and reduce the flowing bottomhole pressure needed to produce a given liquid rate.
