Question 4 of 4: True/False — EOR Concepts

Problem 4: True/False — EOR Concepts (25 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. TRUE. Adding polymer raises the injected water's viscosity $\mu_w$, which lowers the mobility ratio $M=\lambda_w/\lambda_o=(k_{rw}/\mu_w)/(k_{ro}/\mu_o)$ toward (or below) unity. A lower, more favourable $M$ suppresses viscous fingering and channelling through high-permeability streaks, so the flood front advances more uniformly (better areal and vertical sweep) instead of a thin, fast finger of water racing ahead to the producer — delaying the arrival (breakthrough) of water at the producing well relative to a plain-water flood at the same total injection rate.

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. TRUE. Miscibility in a "miscible" gas flood is not achieved on first contact for most real solvent/oil pairs — it develops through multiple-contact mixing at the displacement front. Mechanical dispersion and molecular diffusion are exactly the mechanisms that mix injected solvent with reservoir oil at the pore/microscopic scale, generating the transition zone in which interfacial tension vanishes and local (microscopic) displacement efficiency approaches its ideal miscible value (100% of contacted oil mobilized, no residual saturation left behind by capillary trapping). Without dispersion, an idealized "sharp" immiscible-like front would never develop the true local miscibility the process depends on.

d. 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.

e. TRUE. Within the fully-heated steam zone (behind the steam front, at or near steam temperature), oil viscosity is reduced to near that of water and, critically, the lighter components of the crude can co-vaporize with the steam (steam distillation) and be carried forward with the vapor — a mechanism with no counterpart in a cold waterflood, which is limited to viscous/capillary displacement down to a residual oil saturation set by $k_{ro}$-$k_{rw}$ curves. Steam distillation can, in principle, strip essentially all of the distillable fraction of the oil from the pore space it contacts, so the steam zone's own local (microscopic) displacement efficiency can approach 100% — even though overall field recovery is still limited by macroscopic (areal/vertical) sweep efficiency, which is a separate factor.