It seems that conduction matters more for a reservoir with high surface area to volume ratio. However, in our example, the conduction is similar for the base-case and thin reservoir scenarios because the surface area is the same. Perhaps the initial amount of heat in the reservoir (which is related to the reservoir volume) is more critical than conduction for avoiding early temperature depletion. It is prudent to consider if the reservoir's initial amount of energy will last for the geothermal project's proposed lifetime.