The depth of feed water influences maximum discharge-pressure of hot water geothermal wells

The maximum wellhead pressure at which hot water wells discharge is an important parameter for geothermal power and as it slowly declines with years of exploitation presents a moving target for project designers. It can also decrease rapidly for newly closed-in wells (within days or even hours) to a point at which auto-discharge is impossible and tedious techniques have to be employed to restart flow. The common cause of this phenomenon is reduction in the temperature of the hot water feeding the well; in the former case is the result of a general decline in the reservoir water enthalpy, and in the latter is due to cooler denser water from higher in the uncased part of the well percolating down and flooding the lower more permeable levels from which a discharging well mainly draws its fluids. The inter-relationship of feed water temperature, depth and maximum discharging-pressure is determined in this study with illustrated examples demonstrating application