Given that geothermal sources cannot be regarded as renewable, but exhaustible instead, heat mining prevails in any exploitation undertaking whatsoever.

Optimum heat recovery, pressure maintenance and waste disposal concerns make (re)injection of the heat depleted brine into, preferably, the source reservoir a major engineering and environmental issue.

This poses the problematic of reservoir life, assessed from production well thermal breakthrough times, and of sustainable resource extraction and reservoir management, which ambition at mining heat over significantly long, say at least fifty year, times.

In so doing it should be borne in mind that geothermal operators need to reconcile this farsighted objective with severe constraining factors, such as well lifetime and drill site limitations, the latter particularly acute in city geothermal district heating environments.

The foregoing are discussed and illustrated in a case study addressing the following issues:
(i) elaboration of the early system conceptual model and related breakthrough predictions;
(ii) matching forecasted vs. actual breakthrough times, and model redesign and calibration by:
- comparing 2D (single layer equivalent) and 3D (multilayered) reservoir structures,
- assessing proper bed and cap rock thermal boundary conditions,
- quantifying (analytically and numerically) interbedded impervious layers, stationary and transient, heat transfers and their impact on reservoir cooling kinetics;
(iii) enhancement of system and reservoir life via appropriate production/injection multiwell (doublet/triplet) arrays and novel, long lasting, completions;
(iv) last but not least, elaboration of relevant future heat demand and offer scenarios securing the sustainable management outcome.

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