Geothermal Well Design Calculator

Target Depth feeds into BOTH the drilling cost and the casing cost components of Total Well Cost -- drilling cost is Target Depth × Drilling Cost per foot, and casing cost is Target Depth × a per-foot cost that itself scales with Casing Diameter. Because depth multiplies through two of the three cost components at once, it moves Total Well Cost more than any single other input, including Drilling Cost per foot alone.

This calculator's cost model is built entirely from physical drilling parameters -- how deep you drill, what diameter casing you install, and your target flow rate -- not from what temperature the reservoir happens to be. In reality, drilling into a hotter reservoir can require more specialized (and more expensive) high-temperature equipment and materials, which this simplified planning model doesn't capture.

Thermal Output is calculated purely from how much hot fluid you're extracting (Target Flow Rate) and how much hotter it is than the assumed 80°F return temperature (Reservoir Temperature) -- it uses the standard water heat-transfer approximation of about 500 BTU per hour per gpm per degree Fahrenheit. Well depth determines whether you'll actually reach that reservoir temperature and flow rate in the real world, but once you specify those two figures directly, this calculator's thermal output estimate doesn't need depth as a separate input.

Fluid Velocity converts your Target Flow Rate and Casing Diameter into a flow speed in feet per second inside the casing. Excessively high velocity in a production well can increase pressure drop, erosion, and scaling risk, while a casing that's too wide for the flow rate wastes drilling and casing cost on unnecessary diameter -- Fluid Velocity is a quick sanity check on whether your chosen Casing Diameter is a reasonable match for your Target Flow Rate.

It's the standard approximate heat-transfer constant used throughout HVAC and thermal engineering for water: it combines water's specific heat capacity with its density (about 8.34 lb per gallon) and a 60-minutes-per-hour conversion, giving roughly 500 BTU of heat transferred per hour for every gallon-per-minute of flow and every degree Fahrenheit of temperature difference. It's a well-established approximation for water, not a figure specific to geothermal systems.