"the pressure kept rising past where it should have stopped (perhaps the pre-production pressure). The engineers determined that water injection on the east flank of Ghawar was responsible for the pressure buildup."

we are seriously in the dark here. imo, it is possible for injection into ghawar to create a pressure response 20km east, however to raise the pressure above original would require enough water injection to raise the pressure in the entire aquifer. there seems to be a missing piece or two.

All it would probably take would be a localized pressure difference between the aquifer on the west vs. the east side of Harmaliyah, and maybe that is what they were referring to. What the story said was:

Although Harmaliyah oil field had been dormant since 1998, the reservoir's pressure had been steadily rising, alarming engineers. This pressure (averaging 360 psi higher than other area wells) threatened to cause unintended oil migration into inaccessible nooks and crannies of nearby reservoirs -- an event that could damage nearby reservoirs (Arab C/Hanifa) and render large volumes of petroleum forever irretrievable.

This isn't much to go on. Looking at the existing wells (circa 2003), most of those connected are near the GOSP in the middle of the field (but I can't tell about those in the low resolution area).

I don't know what to make of the C/Hanifa thing, either. The thesis cited at top says:

Harmaliyah Field contains two oil-bearing reservoirs: Arab-D and Lower Fadhili (Saudi Aramco, 1982, unpublished report). The Arab-D, a member of the Late Jurassic Arab formation, is the major reservoir producer in this field.