Geothermal reservoir management. The Paris Basin district heating case
|Location||International Geological Congress,oslo 2008|
|Holding Date||04 October 2008|
The case study addresses the development and management of a large geothermal district heating (GDH) scheme exploiting, since the early 1970s, a dependable carbonate reservoir located in the central part of the Paris Basin. The geothermal reservoir consists of a hot water aquifer, of regional extent, hosted by Dogger pervious oolithic limestones and dolomites, of mid-Jurassic age, at depths and temperatures ranging from 1450 to 2000 m and 56 to 80°C respectively.
Development of the resource consisted in the completion of 54 GDH grids, based on the, mass conservative, well doublet concept of heat extraction, of which 34 remain online to date, achieving a ca 200 MWt installed capacity, a 1000 GWht yearly supply securing the heating of some 120 000 equivalent dwellings and, last but nor least, the savings of 350 000 tons of CO2 emissions.
The rewarding experience, built up from a thirty year exploitation record, reflects the learning curve phases, from infancy to teenage and maturity, inherent to any new technology and energy route. As a result, GDH got soon faced to three major shortcomings, namely:
(i) technical problems : a hostile fluid environment characterised by a thermochemically sensitive (aqueous CO2/H2S system) geothermal brine which caused severe, corrosion/ scaling induced, damage to well tubulars and production/injection facilities;
(ii) financial problems : deemed the most critical, they resulted from a massive debt charge (no equity) aggravated by high inflation and a depleted energy market further to the second oil shock;
(iii) managerial problems: they related to the lack of experience and expertise of geothermal operators, most of them belonging to the public/municipal sector, in handling industrial installations including a significant mining segment.
This bleak outlook could be progressively overcome thanks to innovative, State supported, specifically designed chemical inhibition (continuous, coiled tubing type, downhole injection lines, free annulus fiberglass lined wells) and well restoration (jetting tools, workover waste processing units) technologies, customised monitoring protocols and sound management of GDH plants and grids.
Abandonment of the twenty poorly reliable doublets was the tribute to be paid. Sustainability was another key issue. It clearly addressed the longevity of existing, approaching the 25 year deadline assigned by former feasibility assessments, GDH systems and whether or not there was a life after. Reservoir simulation of production well cooling kinetics led to the selection of an optimum doublet-triplet-doublet well scenario, completed from the existing well head platform, securing an overall 75 year reservoir thermal life.
So, everything considered, GDH in the Paris Basin has a good chance. In spite of a technically hostile and economically competitive environment, GDH scored well. It demonstrated so far its technological and entrepreneurial maturity and gained wider social acceptance and credibility.