It is 8 wells per section. A section is 640 acres.

A BP presentation I found on the internet had the 1 mile by 1 mile graphic. I presume 640 acres is equivalent to 1 mile by 1 mile, but haven't checked.

I found the place I said "per acre" instead of "per square mile" and fixed it.

I presume 640 acres is equivalent to 1 mile by 1 mile, but haven't checked.

Correct, in terms of area, 640 Acres = 1 square mile = 1 Section
http://www.metric-conversions.org/area/acres-to-square-miles.htm

Land survey Jargon (In US and Canada):
A square mile, or an acre, can be any shape, but a Section (as in a Section of a Township) must be a square with a side length of 1 mile.

A Township is a square shape with a side length of 6 miles i.e. contains 36 Sections.