50 DEEP BOREHOLE SURVEYS AND PROBLEMS 



of the survey but also the time loss which could otherwise be 

 taken up in drilling. 



According to recent findings^ the direct and indirect 

 costs of making separate directional surveys with every 

 500 ft. of additional hole amount approximately to 2 to 3 

 per cent of the total cost of a producing well. The increased 

 cost due to the changes in drilling practice in order to 

 keep a hole straight and the cost of straightening a crooked 

 hole ordinarily range from 5 to 10 per cent of the total 

 cost of the hole, depending upon the work required to 

 correct possible crooks in the hole. Thus apart from 

 any considerations (in oil-well drilling) of improved 

 spacing, better drainage and higher recovery per well 

 and per acre which arise from correct surveying of boreholes, 

 it will be seen that good surveying will tend to lighten the 

 burden of straightening costs. This because it also yields 

 enlightening data on dry wells and causes of dryness. 



Accuracy of Borehole Surveys. — Respecting the accuracy 

 to be expected in a contract for borehole survey work it 

 may be mentioned that demands here vary in stringency 

 with the importance of the survey. Freezing shaft con- 

 tracts frequently require a minimum limit of reliability 

 in readings of 1 in 150, i.e., 1 off the vertical for every 150 

 deep. Or again they may desire a deflection record not 

 exceeding 2}^ deg. off the vertical, since beyond this no 

 frost wall is safe at depths of over 100 yd. Hence the 

 desired accuracy decides the type of apparatus being 

 employed, whether crude methods with unreliable direction 

 records, like pricking bobs without orientated rod couplings, 

 or the more precise pendulum and gyroscopic methods which 

 often yield accurate results up to 1 in 3,000. The purpose 

 of the boring will therefore, in the end, decide the nature 

 of the survey apparatus. The purposes for which boreholes 

 are put down are as follows: 



1. To locate a seam, stratum, oil zone, salt or any other 

 mineral. 



1 Murphy P. C, and Judson, S. A., Bull. Amer. Assoc. Petroleum Geol. 

 Vol. 14, p. 603, May, 1930. 



