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Depth Control

Length Calibration

Stretch Characteristics

Stretch Regimes
& Relative Stretch

Depth Determination

Depth Accuracy

Procedures While
Logging

Worked Depth
Control Example

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DEPTH CONTROL

DEPTH DETERMINATION

The determination of the depth of the well is subject to the following prerequisites:

• definition of a depth reference point, or "tie in" point. This may be ,ORT, SRL, JDSG or a depth definition determined by a previous logging job.
  

• knowledge of the number of 25 m (or 100 ft) marked lengths of cable below the tie in point
  

• knowledge of the tension at the tie in point and at the point of interest

The number of marks having passed after the tie in point defines the increase in the length of cable in the hole. The difference in tension between that at the tie in point and that at any point further down the well defines the relative stretch.

The summation of the number of marked lengths of cable and the relative stretch defines the effective length of cable at each cable mark. As the mark lengths are at constant intervals (given 1000 lb. linear tension), each mark represents a defined length of cable subject to a known differential tension, and hence relative stretch. Thus each mark will represent a depth comprised of the tie in depth, the integral number of marks to that point, and the corresponding relative stretch.

eg. 

consider case 1: 

• after correlating the log depth to a previous log. a mark was noted at 2550.0 mat 2500 lbs. Nearl TD, around 5000 m and 5000 lbs., the mark nearest TD, will represent the lowest effective measure point now being at

2550 + n.25 + RS.

n, number of marks to the last mark from 2550,  = 97

RS, relative stretch  = 6.2 - 0.3 = 5.9 m

2550 + 97 x 25.0 + 5.9 = 4980.9 m

In other words, irrespective of what depth is indicated on any sort of depth measurement system, the mark on the cable before TD represents an effective depth of 4980.9 m.

Subsequent marks further up the cable represent depths with corresponding decreases in relative stretch. Near the tie in point the relative stretch-decreases to zero, and the last few marks will represent depths being an exact multiple of 25.0 m beyond the tie in mark depth.

Because each mark represents a defined length of cable, and the behavior of the cable is known for given tensions, the depth definition is based upon the calculation of the stretch and the length of cable each mark represents. As such, it is possible to predetermine assuming a given stretch regime, the effective depths each mark represents. 

e.g.  Consider again case 1 

Tie in mark @ 2550.0 m @ 2500 Ib

TD mark   @ +/- 5000 m @ 5000 Ibs

*NB

1. Linear interpolation between absolute stretch lines usually provides ample accuracy in the determination of the relative stretch.

2. The role of the line measurement device is not in determining the depth of each mark: on the contrary, the mark defines the depth together; with the stretch correction. The line measurement device provides a method of defining the length of cable between the marks.

The line measurement device provides an indication of cable length; which is usually used to help determine the exact depth represented by a mark.e.g. the TD mark representing 4980.9m may have been indicated at 4986.6m. The mark still represents 4980.9m irrespective of the actual value indicated by the line measurement device. Hence the indicated value is adjusted to read that depth which the mark represents, in this case 5.7m less.

3. While logging, the marks and the tension define the depth logged, and the line measurement derived depth is adjusted as is necessary such that the line measured depth tracks the mark and tension defined depth. Each mark on the cable will represent a calibrated length of cable, and the line measured depth must continually conform to this.

As the logging progresses, it may become apparent that the initially presumed stretch regime is incorrect, and instead of a straight line relationship existing between points A1 and B1, there is actually some sort of progression which implies a slower initial loss of tension. (e.g. case 5, A5 to B5 vs A5 to B5 via D5). If this is the case, usually only minor adjustments have to be made to the figures in the relative stretch section of Table 1 to come up with the real case.

e.g. consider case 1

The incremental differences are small but cumulatively they can show a significant departure of the true depth from the projected depth. Because of this, particularly over long log intervals or in crooked holes, it is worthwhile checking the tension at a number of intervals (e.g every 500 m) while running in hole to try to build up a prognosis of the stretch regime before the logging commences.

After the depth of the lowest mark has been calculated, the incremental distance between the mark depth and pick-up can be ignored in times of further adjustments for stretch, and the pick up depth (and hence TD) can be directly read off the adjusted line-measured depth.

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