Question 4 of 4: True/False — EOR Concepts

Problem 4: True/False — EOR Concepts (20 marks — 5 marks each)

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. FALSE. Dispersion is strongly scale-dependent. Laboratory core-flood dispersion is governed by pore-scale mixing and is small; field-scale "dispersion" observed in an actual reservoir is dominated by macroscopic heterogeneity — permeability layering, channelling, fracture networks — that a core sample cannot capture, and is typically orders of magnitude larger than the lab-measured coefficient. As a result, a miscible slug that stays intact and effective over a lab core-length displacement can be diluted below the miscibility threshold far sooner in the field, causing earlier loss of miscibility and lower recovery than lab data alone would predict; the two are not directly comparable without an explicit scale-up correction.

b. FALSE. Higher initial water saturation does not improve steam-flood efficiency; it worsens it. A larger fraction of the injected heat is consumed simply raising the temperature of (and, near the steam front, vaporizing) the connate/mobile water already in place, which contributes nothing to oil recovery — this is pure thermal "overhead," not a useful convective mechanism. The convective heat transport that does matter (a hot-water bank forming ahead of the advancing steam zone as steam condenses and gives up its latent heat) is a property of the steam-condensation process itself and occurs regardless of the reservoir's initial water saturation; it is not enhanced by starting with more connate water. Thermal efficiency (oil recovered per unit heat injected) is generally better at lower initial water saturation, where more of the pore volume — and more of the injected heat — is associated with oil.

c. FALSE. Thermal methods are generally more, not less, efficient at lower reservoir pressures (this is exactly the screening criterion of Problem 3: $p\lt1300$ psia is favourable). At higher pressure, the saturation temperature of steam is higher, so more energy per unit mass of steam must be generated and delivered downhole (and more is lost to wellbore heat losses along the way) to reach the same operating condition; higher pressure also compresses the latent-heat advantage of steam (the latent heat of vaporization decreases as pressure/saturation-temperature rises, shrinking the "useful" heat carried per unit mass of steam relative to its sensible heat). Both effects make thermal recovery less efficient, not more, as reservoir pressure increases.

d. TRUE. Water is denser than oil, so a flood in a steeply dipping (30°) bed is gravity-stable when the water enters at the bottom of the structure and pushes the oil upward along the dip: injectors are placed downdip (low on structure) and producers updip. Gravity then holds the water beneath the oil, keeps the water-oil contact flat, and damps the viscous fingering an unfavourable mobility ratio would otherwise cause, improving sweep and delaying breakthrough, provided the rate stays below the critical (gravity-stable) rate, above which water tongues ahead along the base of the bed. Injecting at the top of the structure and flooding down-dip does the opposite: the denser water slumps and underruns the oil along the bottom of the bed, giving early breakthrough and poor vertical sweep.