Miscellaneous Subsurface Methods 741 



an outlet on the side. One valve serves as a master cut-off below the out- 

 let, whereas the other, at the top, when closed may be used to divert the 

 flow through the side port or upward through the kelly joint when it is 

 open. The usual procedure, while testing, is to divert the flow through 

 the side port and attached choke units, then through a flowing hose to a 

 separator and tanks prepared for the purpose. The strength of this type 

 of head is sufficient to withstand heavy pull when the pipe is removed 

 from the well (fig. 397) . A multiple choke unit may be attached to the 

 side outlet to provide for the use of various sizes of surface chokes that 

 may be changed at will to produce pressure and flow changes that are 

 invaluable in preparing completion programs. 



Improved methods of oil-well-drilling procedures have had a tre- 

 mendous effect on the condition of the bore hole as well as on the drilling 

 fluid. Practices used for maintaining the wall of the bore hole for drilling 

 or coring are of great aid to formation testing. For this reason, to a large 

 degree, permissible testing depth has increased to any drilling depth de- 

 sired. Theory, followed by experience, has shown that formation testing 

 is more effective in small drill holes than large ones. Drilling programs, 

 that have adopted smaller holes and smaller casing sizes, have enhanced 

 formation testing success to a large degree. Although many tests are made 

 in wells with a bore-hole diameter of eleven inches or greater, the most 

 desirable diameters are less than 9'^ inches. 



For any given well depth and mud weight, the smaller the hole bore 

 the less total differential load the packer must withstand and the less 

 circumferential distance that must be sealed off. Coupled with the above 

 facts and of great importance is that the less the annular clearance between 

 packer bottom shoe and hole bore, the better are the chances for effective 

 packer sealing. These things explain why smaller, rather than larger, 

 holes are desired for formation testing. 



Condition of the drilling fluid and its effect on testing should not be 

 overlooked. The quality of the mud must be such that it will remove 

 cuttings from the well bottom to provide a solid bottom on which to 

 anchor the testing equipment. Shales, debris, and foreign material of any 

 kind that would impair fluid passage through the screen section should be 

 removed before attempting a formation test. The weight of the drilling 

 mud as used to control the well during the progress of drilling is ordin- 

 arily sufficient for protection while making a drill-stem test. However, it 

 is frequently found that the pressure as recorded on a subsurface gage 

 indicates a mere two or three hundred pounds over the formation pressure 

 of the tested zone. This indication should demand and receive immediate 

 attention. Low water-loss muds of all types greatly assist success in ob- 

 taining correct data from a test since in such cases the formation has less 

 foreign fluid content to produce before giving up its contained natural 

 fluids. 



The selection of the packer seat, when testing an open hole, is of 



