I read Simmons differently. I believe that his key points are: (i) The oil column in Ghawar has thinned making MRC wells almost a requirement [the oil columns he alludes to would still make someone operating in TX or OK deliriously happy]; (ii) injection of water is one of the techniques used in secondary production [tertiary techniques are next]; (iii) when an MRC well waters out it is because the oil water contact had more or less uniformly risen to the level of the laterals which are by design at the very top of the pay; (iv) at the time a well designed layout of MRC wells in a highly porous and highly permeable field such as Ghawar starts to experience rising water cuts, it may be a sign that the oil water contact has almost uniformly reached the top of the reservoir -- no more thousand plus barrels of oil per well per day.
Simmons provides the Yimbal [spelling] field as an example of how rapidly a giant oilfield can decline using MRC well and massive water injection during primary production. The concern is not the injection of water, but the fact that the technique masks normal production declines and bringing the point of depletion forward in time.
Beyond that, the main message that I get from Simmons is that we don't know enough about OPEC reserves to know what is in store.
This brings me back to your belief that there is 3 trillion barrels in still to be produced oil.
"This brings me back to your belief that there is 3 trillion barrels in still to be produced oil."
It's not a controversial statement to say that we have 3 trillion barrels of oil available. This is from the IEA. Actually, one of the resource charts I've seen shows *7 trillion* barrels,
including various unconventional. Warning this is a PDF:
http://www.iea.org/textbase/nptable/Oil%20cost%20curve,%20including%20technological%20progress,%20availability%20of%20oil%20resources%20as%20a%20function%20of%20economic%20price.pdf
What that chart shows is effectively 5.5 trillion recoverable barrels of oil at a price point of $70/barrel or less... oil shale, tar sands/bitumen, deepwater, artic, and then the conventional OPEC and nonOPEC resources. If you notice, the total conventional resources is about 3 trillion barrels.
See also:
http://www.iea.org/Textbase/npsum/oil_gasSUM.pdf
Key statement:
"Proven reserves amount to 2.2 trillion BOE."
Proven reserves are very cautious, sure to be exceeded estimates.