at step 4 6 adjustment of the flow rate of the drains brought into production at step 5 so that the proportion of undesired fluids in drain 7 brought into production at 4 is at a minimum fig6 and 10 show three possible configurations of the geological formation to be worked either respectively the case where the desired fluid a has a density less than that of the undesired fluid b the opposite case that is to say when the density of fluid a is greater than that of fluid b and finally the case where the desired fluid a has a density greater than that of a first undesired fluid b but less than that of a second undesired fluid c in these three figures have been shown the additional drain or drains 10 and 10a intended to produce at least partially the undesired fluid or fluids it will be noted that in these three figures the case has been shown where drain 7 is brought into production and not the drawing off drain 10 this explains that the undesired fluid or fluids b or c reach the production drain 7 of the desired fluid a furthermore it will be noted that in the three fig6 and 10 the drain or drains 10 10a for drawing off the undesired fluids b and or c have been placed in the desired fluid a and preferably proximate the separation surface before working shown with broken lines still within the scope of the invention the drawing off drain or drains 10 may be placed substantially on the separation surface or surfaces of the fluids a b and or a c before working or in the undesired fluid or fluids b and or c over at least a portion of.