Considering the quality of the Arab D Zone 2-B reservoir, it is possible that ultimate recovery from North Ghawar could approach 70%.  It seems like much of this would have to come from tertiary recovery, but perhaps MRC wells can extend secondary recovery close to that.  A petroleum engineer once told me about horizontal and MRC wells.  He confirmed that once the water reaches a horizontal wellbore or the laterals of MRC wells ("water channeling", versus "coning" for vertical wells), the decline can be quite large and instantaneous.

In any case, we're looking at either a steep decline beginning now, or an extremely steep decline beginning in a few years.  North Uthmaniyah has already declined sharply, while Hawiyah and Haradh will decline much later than Ain Dar, Shedgum, and South Uthmaniyah.

Aramco insiders are actually saying that reservoir models indicate a "catastrophic" decline at Ghawar in 2008 or 2009.