ABSTRACT
The central part of the Paris Basin of France is one of the largest hot water resources developed to date for district heating purposes. Exploitation addresses reservoirs hosted chiefly in consolidated carbonate rocks and, occasionally, in deeper, clastic sandstone formations. Depths and temperatures range from 1,600 to 2,000 m and 50 to 85°C respectively.
For obvious environmental and pressure maintenance reasons, the production technology is based on the so-called doublet concept. It associates a production well, a heat exchanger transferring the geothermal heat to a distribution network and an injection well conveying the heat depleted brine into the source reservoir.
Among the fifty four such schemes presently operating in the Paris area, one third are undergoing severe exploitation problems. Those consist of well damage which, in most instances, has affected first the injection well, then the producing well. This damage originates from a thermochemically sensitive formation fluid, a hot, slightly acid brine with a fairly low gas liquid ratio. It results in casing corrosion and scaling by heavy metal, mainly iron, sulphide deposits. Yet the origin of this scale, either corrosion generated, bacterial or native, is not clear.
Most workover programmes, based on conventional cable and drill stem technologies, have succeeded in restoring initial well injectivities and productivities by combining casing cleaning/scraping and open hole jetting. However their efficiencies, in terms of durability, are limited to hardly one year.
Chemical inhibition has been attempted on a few sites with varying success. It was based on injection at shallow depths of corrosion and scale inhibitors and bactericides.
Alternative methods are contemplated. They consist of (a) a versatile, cost effective, workover technique using a high pressure large diameter flexible coiled tubing, and (b) of continuous injection down hole of crystal growth inhibiting agents.
Existing restoration and inhibition treatments are reviewed and selected examples presented. Candidate procedures are discussed in the light of damage monitoring and identification of source mechanisms.
