Miscellaneous Subsurface Methods 743 



recorder is blanked off while the other is placed in the flow stream. This 

 arrangement permits detection of the part of the equipment that becomes 

 plugged, should such occur, and also indicates sloughing around the 

 recorders and perforations. 



If the well conditions permit, pressure-recording devices are placed 

 below the packer. If not, above-packer recording devices are placed above 

 the packer and below the choke. This is true whether one or two recorders 

 are desired. Pressure values remain the same regardless of the location 

 of the recorders, as long as they remain below the bottom hole choke. 

 If placed above the choke, recorded pressures will indicate only the weight 

 of the fluid recovery and gas pressure trapped within the running-in string. 



Requirements for testing between packers (whether in open hole or 

 in casing) include the use of one pressure-recording device below the 

 bottom packer and one or two recorders in the testing section between the 

 packers. The recorder below the bottom packer is used to indicate the 

 sealing results of the lower packer, and the two in the test zone record the 

 pressure changes during the operational procedure of the test. In many 

 cases it has been found where highly permeable formations have been 

 sealed below the bottom packer, dehydration of the drilling fluids below 

 the packer into the permeable formation have indicated the static pres- 

 sure of the formation sealed off. This is not true in all formations. Less 

 permeable formations below the packer that lend no avenue of escape 

 by dehydration maintain the original hydrostatic pressure during the 

 course of the test, and this pressure is released only when the bottom one 

 of the two packers has been removed from its seat. The addition of more 

 than one packer above a test zone has no material effect on the pressure 

 recording. 



Various types of subsurface pressure-recording devices have been 

 developed for the purpose of securing recorded pressures during the 

 progress of drill-stem tests. Of these the spring and piston type and the 

 Bourdon tube type are the ones most frequently used. The design of these 

 devices was originally patterned after wire-line, bottom-hole-pressure 

 instruments used for taking bottom-hole pressures of wells on production. 

 For formation testing the spring and piston type of devices were first 

 used. This type of gauge permitted the development of a rugged and 

 sturdy unit that would withstand the rough treatment so common in 

 formation testing. Although rugged in construction its calibrated springs 

 and highly polished pistons operate to a degree of accuracy sufficient for 

 standard use on formation tests. Periodical calibrations of these units by 

 dead-weight testers and corrected to temperatures have continued their 

 value. Units of this type are usually referred to as standard pressure- 

 recording devices, (fig. 398) . 



Developed in more recent years is the Bourdon tube type of recording 

 device whose duties in formation testing remain comparable to that of the 

 standard pressure-recording devices. The mechanical reaction of this unit 



