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Depth Control

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DEPTH CONTROL

WIRELINE STRETCH CHARACTERISTICS

The ability of the wireline to stretch and contract as a function of primarily tension is a mechanical function determined by Hooke's-law and the mechanical properties of the cable design. The counter winding of the armor wires, the number and size of the king wires, and to a lesser extent the behavior of the core to tension, all have an influence on the final characteristics. As such, there is no simple relationship between tension, length of cable and the ensuing stretch. Furthermore/the cable tension while suspended in the borehole is not even: it decreases along the length of the cable towards the cablehead. As such, and stretch being accounted for is an integral of individual stretch increments along the length of the cable from the drum to the cablehead.

Because the cable length is defined in units of 25 m or, 100 ft at.a constant 1000 lb. tension, the accumulated stretch over a suspended length of cable may be more or less than the total stretch imparted to the cable at the constant 1000 lb. tension when defining its length. The combined length and tension which gives a net extension of the cable equal to that at a constant 1000 lb. tension defines the 'ZERO STRETCH' line of the stretch characteristic of the cable. See fig 1.

Increases in length and tension above this line will result in an overall increase in the net stretch of the cable compared to that same length of cable at 1000 lbs. constant tension. This is then referred to as "POSITIVE STRETCH". Likewise, lesser lengths of cable, and lower tensions will result in "NEGATIVE STRETCH". Because of the asymmetric nature of the stretch lines, complex changes in the net stretch will occur as a cable changes length by being moved along the well bore. On fig 1, consider these cases:

case 1

as the cable length increases from point A1 to point B1, the stretch of the cable relative to the 1000 lb. constant tension elongation gradually increases from 0.3 to 6.2 m, ie. over the length of borehole, the total stretch undergone is 6.2 - 0.3 = 5.9 m.

case 2

as the cable length increases from point A2 to point B2, the overall elongation of the cable increases from -0.5 to 0 m, ie 0.0 - (-0.5) = 0.5 m. However, this increase in length is only effective beyond point C2 as between A2 and C2, the elongation of the cable is constant compared to the same length of cable subject to a constant 1000 lb tension.

case 3

as the cable length increases from point A3 to B3, there is an overall elongation of the cable equal to zero as points A3 and B3 lie on the same (negative stretch) line corresponding to the overall elongation of the same lengths of cable subject , to "1000 lbs. tension. However; in this case the cable behavior first displays increasing -VE stretch and then decreasing -VE stretch as the length and tension move through point C3. The net movement from point A3 to point C3 Is approximately -0.7 - (-0.5) = -0.2 m. Likewise from C3 to B3 the net elongation is -0.5 - (-0.7) = 0.2 m.

Obviously the same principles apply equally to a metric stretch table as to a stretch table in feet.

Note that fig 1 is specific to a 7H4 cable calibrated at 1000 lbs., and it gives the true depth. This is important, as each type of cable will have its own characteristics as no two types of cable will behave in the same way.

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