How does a deep well extract all along its length? My belief was the production well will experience a gradient (so the bottom of the well will have the temperature you need), and the temperature will drop as you get closer to the surface (which also contributes to the calcite scaling problem geothermal can experience).

Also, I think a lot of wells add concrete casing (or metal, as indicated in the article) around portions of the well, which would prevent extraction around those zones of the well.