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Calculating Cable Stretch

Many field operations with logging cables require a familiarity with how to use cable stretch information. One common need is the necessity to determine the true depth of the logging tool in the well. The true depth can be determined by adding the amount of cable stretch AL to the measured cable length L.  To determine the amount of cable stretch, use the following relation:

DL = (KL/2)[(T + W_t) - 2T_m]

WHERE,

DL = CHANGE IN THE LENGTH OF THE CABLE IN FEET (FT.)

L = LENGTH OF CABLE IN HOLE (IN UNITS OF 1000FT.)

T = SURFACE TENSION (IN UNITS OF 1000 LBS)

W_t= EFFECTIVE TOOL WEIGHT IN MUD (IN UNITS OF 1000 LBS)

T_m TENSION CABLE MARKED AT (USUALLY 1000 LBS)

K = ELASTIC STRETCH COEFFICIENT (FT. /1000FT. /1000LBS.)

As an example, consider a tool whose weight in the mud used in the well is 500lbs., 10,000 ft. of cable in the hole, a 7J46RZ cable with a stretch coefficient of 0.77 ft./lOOO ft/1000 lbs. Suppose the cable has been marked at a 1000 lb. tension and the cable tension measured at the surface is 5,000 lbs.

DL = (0.77x10)/2[(5+0.5)-2xi] = 13.4 ft.

Thus the true depth would be 10,000 ft. + 13.4 ft or 10.013.4pt.

Stretch Calculations

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