Welge's Graphical Method: 2018 #9

Problem 9. The following statement most TRUE concerning modifying the saturation distribution with Welge’s graphical method is:

(A) For gas displacement, the tangent to the fractional flow curve is drawn from the irreducible water saturation.
(B) In actual fact, the saturation front tends to be smeared by gravity at the trailing edge and smeared by capillary action at the leading edge.
(C) For waterdrive reservoirs, a line from the irreducible-water saturation tangent to the fractional-flow curve is drawn; the saturation at the tangent is the flood trailing edge.
(D) For waterdrive reservoirs, a line drawn from the irreducible-water saturation tangent to the fractional-flow curve may be extended to an oil saturation of 1 to find the average water saturation at breakthrough.

This problem is a confusing word garbage dump; it looks like it's written by a lawyer. Be ready for this sort of thing.

So I don't read the details yet. Once I know the theme (Welge) I just run to the Guidebook TOC and scan for Welge. It's on 14 WFL 4. At 10 seconds, I've eliminated (A), since it says that for gas reservoirs you start the line at fw = 0. In the next 30 seconds, (C) and (D) are as easy to eliminate just by looking at the graph and sorting things out.

So by process of elimination I'm selecting (B). And if you understand this whole capillary action, leading edge, and saturation front business, you can see how it makes sense. Need more confirmation? I prefer it. There must be an SPE direct quote here somewhere...

The Handbook Series? I check the index, and I get 1106-1108. Sorting through this mess I find nothing helpful, but do finally find Welge mentioned on page 1108 without any clue for this problem. So I cut my losses and move on.

The Textbook Series? It's a waterflood question so I check TS3 index and find Welge on page 122. But again, this page merely refers me to another section, and neither section gets answers. Ouch.

But it's also a reservoir question. So I check TS8, which has an index entry for Welge with two reference pages (161-162). After several minutes of scanning, I find (B) is a direct quote from page 161! Bingo. As I mentioned last post: be warned that while SPE TS8 is not popular, it is the "official" SPE reservoir reference, and it's got some gems that few other texts have, like hydrodynamic traps and Welge. Of course, by now I'm 5+ minutes down (at least).

The Guidebook would have got you through this one in a minute or less. My general approach? Scan the Guidebook TOC (the cover) and review the relevant page. If I strike out here and have time later on I move to the HS index. Finally, I'm in the TS series, checking books I think relevant.

Honestly, you wouldn't even need to be a petroleum engineer, or even really understand much, to solve this one with the Guidebook. But many, many engineers get flustered by this sort of problem. Just reading the dang thing is a frustrating experience. 

Note: now we have the CBT and no extra resources allowed, one would have to just work through the solution as they best knew how. However, I doubt the CBT will have this level of difficulty for this reason. Only time will tell, but it doesn't hurt to study and understand difficult problems!