A new stimulation technique, known as soft acidizing, has been applied to damaged geothermal, district heating, injection wells in the Paris Basin. It
consisted of injecting continuously from surface highly diluted HCI solutions mixed with an iron sequestrant additive. The injected acid volume (I0 m3 HCI 15X) is equivalent to that normally squeezed into the reservoir via a drill string during conventional-petroleum/geothermal/ground water well- acid jobs. Only do injection times differ -60 hrs against 1 hr and the etching process which, in the conventional procedure concerns the reservoir alone whereas soft acidizing addresses both well casing and/or near well formation damage.

The technique has been successfully implemented in the framework of a field test programme (three geothermal injection wells) encompassing the whole damage spectrum : casing andlor near well formation damage.

The acidizing process has significantly increased well injectivities often above nominal figures and also optimum injection rates. The latter feature is manifest on wells displaying prevailing casing friction losses.

Economic performance may reach pay back times ranging from 8 to 24 months depending on whether additional electric heat supply and electricity consumption or only electric power saving issues are contemplated. Actually the perenniality of the soft acidizing impact may be questioned as suggested by post treatment well monitoring. However none of the tested wells do exhibit the sharp and fast increases in injection pressure noticed in the past.

Extension of the protocol to producer wells and intermittent acid injection through down hole chemical injection lines to secure durable performance of both wells are also discussed.

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