Perhaps the best procedure for measuring the length of a drillstring is to strap the string while pulling it out of the wellbore with the string hanging on the elevators and after rotating each stand free of wellbore drag. In vertical holes this procedure should very accurately give the length of the drillstring and then the measured depth of the wellbore and it should be sufficiently accurate for most directional wellbores if not precisely correct in principle.
EXAMPLE CHANGE IN LENGTH OF BHA & PIPE
Changes in drilling fluid density and in bottom hole assembly will have only a minor effect on drillstring length as demonstrated in the following problem. Suppose a drillstring is composed of 10,000 feet of 5 inches, 19.5 lb. per foot drillpipe and 600 feet of drill collars with 8 inches external and 3 inches internal diameters and is suspended in 8.33 lb. per gallon drilling fluid. The length of the stretch of the string including the drill collars would be
72 Ls2 r
Ls = drillstring length, feet
E = Youngs Modulus, 30 * 106 psi pg = density of steel, lb/in.3
rf = density of drilling fluid, lb/in.3
Wd = weight of drill collars in air, lb.
Lp = drillpipe length, feet Wp = drillpipe weight per foot, lb/foot tr e =
If the mud density is changed to 16.7 lb/gallon (.0722 lb/in3) and the drill collars to 180 feet of length with 7 inches by 3 inches diameters; then the stretch of the string becomes e = ( 2833 _ 2 . 0722 (1 _ 3)] + 40.8; 18224-10000 Q _ ^
The extreme changes in bottom hole assemblies and drilling fluid densities selected in the above problem caused a difference of 8.9 feet in the stretch of the two strings and this difference would have been smaller in a directional wellbore.
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