Where no "extra" marks have been made and no "skips" noticed, there 

 may accumulate fractional-foot errors, which may be designated as "creep." 

 Over long-continued drilling, involving several connections or even round trips, 

 a correction may be required, and it is difficult to know where it should be made. 

 Having written the kelly-down depth at each connection and the correct depth 

 at a round trip, accurate in each case to the nearest hundredth of a foot, it will 

 be obvious what depth should be assigned just before or just after making a 

 connection. For example, if a connection has been made at 6079.04 feet in 

 Figure 18-5 and a foot mark is shown immediately after the connection, it would 

 be reasonable to indicate the depth of the mark prior to the connection as 

 6078 feet, as the total number of feet drilled corresponds to the number of foot 

 marks on the chart. Had the connection been made at 6078.97 feet, however, and 

 a foot mark shown just prior to making the connection, with a normal drilling 

 interval following the connection before the next foot mark was recorded, it 

 would be reasonable to assign a depth of 6079 feet to the foot mark prior to 

 the connection. Apparent errors of this type would be the result of "creep." 



The foregoing is only suggestive of some generalizations that may be made 

 in preparing the chart for time determinations. Frequent use of drilling-time 

 charts will increase the speed and accuracy in obtaining proper depth desig- 

 nations. The chart reproduced in Plate 18-1 provides 24 inches for recording 

 twelve hours of time, or two inches per hour. The hour is divided into twelve 

 divisions of five minutes each, and these are further divided into one-minute 

 divisions. For high speed, when the requirements demand it at the sacrifice of 

 accuracy, the eye can measure the distance between foot marks within an accuracy 

 of approximately twenty to thirty percent. Many uses of drilling-time logs re- 

 quire greater accuracy than this, however, particularly when minor breaks in 

 the over-all pattern are used to correlate with minor breaks on an electric-poten- 

 tial log. To obtain greater accuracy, with an error of less than one percent, 

 measurements may be made with an engineer's scale, using the thirty-divisions- 

 per-inch scale for the purpose. This scale, placed on the chart having two inches 

 per hour, will provide sixty divisions per hour or one per minute. 



The use of a printed form to tabulate the time readings may be considered 

 as an extra step, and some may suggest plotting directly from observation of 

 the chart. It has been found that this extra step not only avoids many errors 

 that might otherwise be made but actually saves time. In addition one often 

 wishes to plot the same data on more than one scale, in which case the time 

 saving is considerable. The same person may make the readings and write the 

 tabulation, but if one person keeps his eye on the scale and chart and another 

 writes down the time on the form, it will save two-thirds of the time required 

 for this operation. Additional time can be saved in the following step, plotting 

 the log from the tabluation, by having one person read the units off as another 



383 



