Minchinyon, I think said that already, its a preferential flow...

Minchinyon, I think said that already, its a preferential flow path, it perhaps might be described as a route of least resistance to the water molecules, which upon entering the up pipe are able to speed up because of less resistance, thus causing a drop in pressure as the space between the water molecles is stretched. Van Der Waals and Hydrogen bonding would then drag the water still in the rock towards the outlet wells.
But the water between the pipes is still not flowing in a sealed pipe, its an open system with water that exceeds or just matches the (saturated?)voids / fracture pressures at where ever you care to define as the field boundary.

I think I might just have answered my question. The pressure in the system will drop off towards the periphery of the field because of friction, adhesion and hydrostatics until it matches the surrounding rock water pressures, this could be defined as the field boundary, since there would be so little or no nett exchange of water across this boundary (which would be 3d and not planar, so better descried as a boudary envelope perhaps). This obviously necessitates the surrounding rock being saturated. If its not saturated, and is permeable (can that happen at these depths?)then water must be lost from the field at some rate.

Ok enough amateur speculation, hydrogeologists or any body who can re-arrange Darcy's in a number of different ways step forward. Or Minchinyon try a little harder with your explanation, which was short..... on information, I want to understand how the field size / boundary is established. I want to understand how the water molecules at this boundary behave / move. The water molecules can't all take the shortest route once they leave the injection well. There must be a spreading of water molecules re-injected throughout the field, at some point this spreading must stop, and a reconvergience start to happen.

Anybody into bio mimicry, check out Janine Benyus here,

http://www.ted.com/index.php/talks/view/id/18

There is a neat story she tells about sea shells inspiring a new coating for the inside of pipes to stop scaling. It mimics the protein layer laid down by sea shells to prevent non stop deposition of carbonates onto the shell from sea water. Any one know of it, or think it may be of use to GDY's plumbing set?