Miscellaneous Subsurface Methods 589 



ability of drilling new wells to drain the oil sand properly. Some major 

 companies make it a practice to survey all wells on a lease before con- 

 sidering the redrilling of any one well. Then an over-all plan for the 

 whole lease is drawn up from the subsurface conditions discovered. The 

 best point at which to set a casing whipstock to commence sidetracking 

 can be determined from the survey. Often it is found that a well that 

 could be deepened to produce from a lower zone has wandered from 

 under the lease. In order to prevent possible legal action by the adjoining 

 landowner, the well must be redrilled from some depth shallower than 

 where it crossed the lease line. Surveys of all wells drilled on a lease 

 give a sound basis for proper determinations of recovery and per-acre 

 yield, since interference between wells can be evaluated. A subsurface 

 map of the wells on a lease is an aid in the selection of input wells in 

 repressuring and flooding operations. 



When it is intended to perforate casing to produce an upper zone 

 it is vital that the measured depth in the well corresponding to the verti- 

 cal depth of the oil sand be known accurately. The location of the well 

 on the structure also should be considered. A well survey supplies all 

 of this information. 



Operators have been very much pleased that they had surveyed a 

 well on those occasions when casing collapsed or liners became stuck 

 permanently. They were able to deflect the sidetracked hole to a new, more 

 productive location under the lease. 



An unusual application of well surveying has been made in an oil 

 field on the Pacific Coast. Oil operators noticed that the cement was begin- 

 ning to break away from the casing in well cellars. A network of surface 

 levels run across the area of the field indicated that the whole area was 

 sinking. This subsidence has continued to where several wells in the field 

 have been shut down during the last two years. It was suspected that the 

 movement was occurring at subsurface slippage planes. The operators 

 decided to run especially accurate, multiple-shot surveys of the wells 

 aff"ected. By making surveys of the same wells at monthly intervals the 

 depth, location, speed, and direction of movement at the slippage planes 

 was detected. 



The Eastman Oil Well Survey Company cooperated with the opera- 

 tors in making special equipment for running these surveys. It was con- 

 sidered advisable to make the survey assembly as small in diameter as 

 possible in order to enable the assembly to pass through the partially 

 collapsed casing at the planes of earth movement. A standard DX multiple- 

 shot machine was shortened by removal of the outer battery case. The 

 shortest protective barrel possible was made to house the instrument. 

 Basket stabilizers, which were free to turn on the outside of the barrel, 

 were made to center the barrel accurately in the casing. These sets of 

 spring stabilizers were made to fit two sizes of casing and also to run 

 through chokes on the wells. The surveying assembly as run into the well 



