jol 1, page-4

Closed loop testing involves pumping cool water down Habanero 1, circulating it through the hot rocks 4.2 km below, and returning it super-heated (240 degrees) via Habanero 3. The superheated water's temperature will then be measured, cooled, and returned back down Hab 1 in a closed loop process. This will be done for a period of 6 weeks, all the while monitoring temperature changes in the system.
The important variable is how much the temperature drops over the 6 week period, and whether it matches simulations. There are many other unknowns too regarding permeability of the rocks, scaling, etc.