fig1 shows by way of example one embodiment of a test tube adapted for containing samples of the porous reservoir studied , an aquifer reservoir for example . this test tube includes a tube 1 which may for example be made from stainless steel . at the ends of this tube may be screwed caps 2 and 3 which have orifices for the passage of inlet 4 and outlet 5 ducts , respectively , through which the fluid flowing through the sample may flow . the ends of ducts 4 and 5 have widened sections for receiving perforated plates 6 and 7 . o seals 8 and 9 provide sealing in contact with the internal wall of tube 1 . according to the embodiment of the invention illustrated in fig1 and 2 , two samples of the formation to be studied are taken from a core sample 10 removed from the formation . these samples are taken from the geological formation by means of two corer 11 and 12 ( fig2 ) which are adapted to be housed one after and below the other inside tube 1 , so that the fluid introduced through duct 4 flows therethrough in series . a porous material disk 13 may if required be interposed between the corer tube 12 and the perforated plate 7 . in fig2 arrow 10a indicates the direction of coring in the formation . arrow 10b indicates the flow direction of the fluid in the geological formation . in the output duct 5 is disposed a valve 14 for adjusting the rate of the flow through the samples of the formation studied . it would of course be possible to dispose a single corer tube , or more than two corer tube in series in tube 1 . it would also be possible to dispose in tube 1 several corer tubes taken from different sides of the same porous reservoir . by different sides is here means positions at different distances from the pumping or injection point . fig3 shows schematically a device of the invention in which a plurality of test tubes such as those illustrated in fig1 are placed in thermostatic enclosures 15 and 16 ( the construction of such enclosures is within the scope of specialists and will therefore not be described here in detail ). the enclosures which may be in any number may for example contain a liquid bath in which is placed an agitator driven continuously or periodically by an electric motor , the temperature of this bath being kept substantially constant by a heating resistance whose electric supply is controlled by a detector detecting the temperature of the bath . the different test tubes may be fed in parallel from a source 17 with flow rates controlled by valves 14 . the supply by source 17 coming from the aquifer for example may be cut off by a valve 18 so as to leave room for a supply by another source 19 isolated from the circuit by a valve 20 an a microporous filter 21 . the variations of permeability of the samples as a function of time may be followed , for example , by using the device shown schematically in fig3 which includes the tank 19 containing a reference fluid placed at a sufficient level above the test tubes to exert a hydrostatic load . the test tubes are then supplied with a sterile fluid from source 19 , reservoir or circuit , the fluid being sterilized by the microporous filter 21 , the flow being then controlled in pipe 22 by valve 20 , valve 18 being closed . the variations of permeability of the samples may also be followed by measuring the upstream and downstream pressures by means of manometric tappings 23 , 24 , 25 , 26 , 27 and 28 which may be designed for operating either at absolute pressure , or by relative measurements . fig4 shows an advantageous embodiment of a sample holder reproducing substantially the pressure conditions of the porous medium . this sample holder includes a flexible sealed sheath 29 closed at both ends by two caps 30 , 31 having orifices 32 , 33 for passage of the fluid . the sample holder is immersed in an enclosure 34 containing a fluid 35 kept by a valve 36 at an adjustable pressure p controlled by a pressure gauge 37 . enclosure 34 is equipped with packers 38 for isolating from the pressure and fluid 35 the passage of the ducts connected to orifices 32 , 33 . such sample holders may be used instead of one at least of the tube tubes of fig3 . with the device shown in fig4 if desired , the sample studied may be subjected to an external counterpressure calculated for reconstituting the pressure conditions to which it is subjected to its natural bed : the liquid 35 contained in enclosure 34 bathes the sealed flexible sheath 29 which surrounds the sample and communicates thereto the counterpressure controlled by the pressure gauge 37 and valve 36 . this counterpressure is usually separate from the pressure required for causing the fluid to flow through the orifices 32 , 33 within the sampled studied . the method of the invention is particularly applicable to the diagnosis of the lifespan of working bore holes or wells . it is also applicable to the determination of the limit conditions of fluid flow speed and , consequently , of the optimum working conditions ( flow rate , diameter of the work , dimension of the liners ) to be adopted for avoiding or delaying the clogging of the planned work . the invention is also applicable for providing , by way of correction , a change in a method of working a reservoir which risks being clogged .