Energy

The Sustainable Geothermal Rate: How Fast Can You Mine a Reservoir?

By Lucian — builder & engineer, LK Forge

A geothermal reservoir isn't a fixed hot rock you can drain at any speed for free — it's fed by a slow, natural recharge, and mining heat faster than that recharge cools it down. We benchmarked the Geothermal Energy lab's reservoir model for a 150°C resource over 30 years, sweeping the brine flow rate, to find exactly where "sustainable" ends and decades-long decline begins.

 ·  5 min read  ·  measured from the reservoir-drawdown model

44.6 kg/s
the largest flow that stays sustainable at 20% reinjection
0%
reservoir decline over 30 years at 25 kg/s — essentially flat
24.46%
decline at 150 kg/s, over three times the sustainable rate
39.95%
decline at 300 kg/s, the fastest rate benchmarked

Why extraction rate matters

A binary-cycle geothermal plant pulls hot brine from the reservoir, runs it through a heat exchanger, and reinjects the cooled brine back underground. Reinjecting some fraction of that flow (here, 20%) returns heat and pressure to the rock, but the rest is heat permanently mined out. The reservoir has its own natural recharge — slow conductive heat flow from the surrounding rock — and as long as mining stays at or below that recharge rate, the reservoir's temperature holds steady indefinitely. Mine faster, and the reservoir has no way to keep up: its temperature falls, year over year, for as long as the over-extraction continues.

sustainable ⇔ heat mined at start ≤ natural recharge into the reservoir

That's a temperature-independent test evaluated at the resource's starting condition, which is why it draws a clean line: everything at or below 44.6 kg/s in this benchmark held essentially flat, and everything above it declined for all 30 simulated years, with no sign of leveling off.

Flat below the line, falling above it

Reservoir temperature decline over 30 years, as a percentage, plotted against the brine flow rate that produced it. The dashed line marks 44.6 kg/s, the largest flow that stays sustainable at 20% reinjection.

0% 10% 20% 30% 40% 44.6 kg/s 050100150200250300 flow rate (kg/s) decline

The exact numbers

Every row below is a direct sample from the lab's reservoir model: a 150°C resource, 70°C reinjection, 20% reinjection fraction, simulated for 30 years.

Flow (kg/s) Sustainable 30-yr decline Final Tres Net MWe start → end
25 yes 0% 150°C 0.837 → 0.837
50 no 1.7% 147.45°C 1.675 → 1.598
100 no 14.94% 127.59°C 3.35 → 2.09
150 no 24.46% 113.3°C 5.025 → 2.104
200 no 31.34% 102.99°C 6.7 → 1.939
300 no 39.95% 90.07°C 10.05 → 1.529

Sustainable flow at 20% reinjection: 44.6 kg/s. Resource: 150°C, 25°C ambient, 70°C reinjection, 30-year horizon. Generated 2026-09-10.

Try it yourself

Open the lab, set your own flow rate and reinjection fraction, and watch the reservoir hold steady or cool away over decades in real time.

Open the Geothermal Energy lab →
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Common questions

What makes a geothermal extraction rate "sustainable"?

In this model, extraction is sustainable when the heat mined from the reservoir at the resource's natural starting temperature is no more than the reservoir's own natural recharge — heat conducting back in from the surrounding rock. Mine at or below that rate and the small amount of extra reinjected brine keeps the reservoir topped up indefinitely. Mine faster, and you are drawing down stored heat faster than nature replaces it. For this benchmark (150°C resource, 70°C reinjection, 20% reinjection fraction), that boundary sits at 44.6 kg/s — below it the reservoir holds flat (25 kg/s here declined just 0% over 30 years), above it every row in the sweep declined, from 1.7% at 50 kg/s up to 39.95% at 300 kg/s.

Why does net electricity output fall over decades if you over-extract?

Net electric output depends on the temperature gap between the reservoir and the reinjected brine — that's what sets both the heat available and the plant's Carnot ceiling. Pull heat out faster than the rock can replace it and the reservoir cools, which shrinks that gap and the power with it, even though the flow rate never changes. In this benchmark, 150 kg/s starts at 5.025 MWe but falls to 2.104 MWe after 30 years as the reservoir cools from 150°C toward 113.3°C. The same drop shows up at every unsustainable rate we tested: 200 kg/s goes from 6.7 MWe to 1.939 MWe, and 300 kg/s — the highest flow benchmarked — starts strongest at 10.05 MWe but ends lowest of all at 1.529 MWe.

Does a higher flow rate always mean more electricity?

Only at first. Right after startup, more flow always means more heat moved and more electricity — the sweep starts at 0.837 MWe for 25 kg/s and climbs to 10.05 MWe for 300 kg/s, roughly in step with flow. But that ordering does not hold 30 years later: 150 kg/s ends at 2.104 MWe and 200 kg/s ends at 1.939 MWe, barely different despite 200 kg/s starting well ahead, because the faster rate has cooled its reservoir further in the meantime. Chasing flow past the sustainable rate borrows output from the future.