742 Subsurface Geologic Methods 



major importance. Only by proper sealing can a successful test be com- 

 pleted. Practical experience has indicated that the most effective seats 

 are in the top of the formation to be tested. In recent years, electrical and 

 caliper logs have borne out this accepted practice, many times locating 

 where least expected packer seats that otherwise would have been over- 

 looked. This is especially true where the well depth has been carried 

 several hundred feet below a desired testing zone. Sand, lime, chalk, and 

 dolomite usually accurately hold to bit diameter, a fact which makes them 

 desirable for packer seats. 



Increased depth and mud weight often impose greater pressure on 

 the outside of the empty drill pipe than its strength will withstand. To 

 overcome this condition a fluid cushion is placed in the otherwise empty 

 drill pipe in sufl&cient quantity to secure the pipe against collapse. The 

 quantity of fluid required for the cushion varies with well depth and mud 

 weight, and should be placed in the pipe while going into the well to 

 eliminate air pockets. Fluid cushion also reduces the shock load at the 

 opening of the tool valves. This is an important factor in formation test- 

 ing and should always be given consideration. 



Subsurface-pressure-recording devices have removed any doubt con- 

 cerning pressure changes during the testing procedure. Pressures that 

 were previously theoretical have been proved a reality by a continuous 

 recording of all pressure changes that develop while making a formation 

 test. Information obtained from pressure-versus-time recorded charts has 

 induced continued development in this phase of testing such that at the 

 present time the instruments are highly accurate. These recorded pres- 

 sures are now invaluable in compiling information by geological, engi- 

 neering, or production departments, in checking the operation of the sub- 

 surface equipment, in calculating productivity, in reservoir analysis, and 

 in selecting production equipment. Efforts are being extended to accumu- 

 late sufficient material whereby formulas can be set up on a method for 

 evaluation of productivity of wells from drill-stem tests. The basic ma- 

 terial for this project necessarily involves pertinent information obtained 

 from pressure-recording-device charts. 



Operational procedures that produce predominant pressure changes 

 are (1) lowering the tools into the well, (2) seating the packer, (3) open- 

 ing the tool valves, (4) flow from formation, (5) closed-in pressure valve 

 closed, (6) opening of fluid by-pass, and (7) removing the tester as- 

 sembly from the well. 



Adverse conditions that are readily recorded by these units include 

 plugged choke, plugged tool, plugged perforations, sloughing of uncon- 

 solidated formations below the packer, and irregular diameters of the 

 walls of the drill hole. Formation-testing procedures usually involve the 

 use of at least one pressure recorder. More recently it has been found 

 that the use of two gauges is more practical. If one only is used, it should 

 be blanked off from the flow stream. When two are used, the lower 



