venturi 50 creates a pressure reduction which reinforces the induction of the first fraction flowing in the conduit 20 39 partly for this reason and because it reduces the pressure at the bottom of the well the pumping or another method of flow of the second fraction in the conduit 22 39 is used to increase the flow capacity of the first fraction in the conduit 20 39 it is thus possible to control the flow rate in the first conduit for example by varying the flow rate in the casing 22 according to another aspect of the invention the space outside the casings 20 22 in the well casing 18 between the seal 26 and the wellhead 24 forms a chamber 51 which is filled with a chemically inert product which in a preferred embodiment is a gel that is a good thermal insulator intended to reduce thermal convection in the chamber 51 the electrical cable 36 and the hydraulic lines not shown leading to the safety valves 28 29 thus run in a noncorrosive medium which considerably lengthens their lifetime only the final section of the electrical cable running between the seal 26 and the motor 34 is in a corrosive medium bearing in mind its small length this section can be protected by more costly.