Exploring The Mechanism Of Coiled Tubing Lock-Up
Lock-up in CT is like death and taxes, it’s an inevitability of life. If you are going to try and set weight at the end of CT, the amount of weight you will be able to set will ultimately be governed by lock-up.
The key to running a successful operation is understanding how and why lock-up occurs, comprehending factors that influence it and planning the operation to minimise the effect of those factors. These are the topics I Intend to cover in this series of blog articles.
Holding a length of coiled tubing in your hand is an instructive experience. In terms of size, weight and stiffness, it’s comparable with scaffold tubes used in the construction industry. It’s readily apparent that a six-foot length, held vertically and balanced on your toe, will exert a downward force equivalent to the weight of six feet of steel pipe, and equally so that a twelve-foot length will exert twice that force. What is not so intuitive is that once the length of pipe you are holding gets longer, it behaves more like a length of garden hose.
Most of us can visualise that if we try to compress a short, 8-inch length of garden hose between our palms, that hose will buckle. We can also see that if we hold a slightly longer length vertical, buckling also occurs. In this latter case, it is not compressive force from our palms which causes buckling, it is gravity acting on the hose’s own weight.
The relevance of the above is twofold:
The force exerted at the bottom end of a length of coiled tubing results from the coiled tubing’s own weight.
It is the CT’s flexibility and the resultant interactions with the wall of the hole that cause lock-up.
Any long slender member will buckle if sufficient axial load is applied. Consider a wellbore 7500ft deep, completed with 5-1/2 inch tubing, through which a CTD operation will be conducted. The ratio of diameter to length is 1:17000. On the same length scale, our well is in excess of 5 times more slender than a human hair, so ‘long’ and ‘slender’ are clearly appropriate adjectives in this case.
The weight of a typical piece of 2-3/8 inch coiled tubing in air is 4.742 pounds per foot.
According to a resource* widely respected in our industry, critical buckling load for a vertical tube is given by:
Where E is the material Youngs Modulus,
I is the second moment of area of the pipe section
W is the weight per unit length.
Evaluating for 2-3/8 inch CT shows that buckling is likely for loads in excess of about 200 lbf.
Accordingly, we can expect 2-3/8 inch CT in a vertical hole to be prone to buckling at depths over 43 ft, as a result of its own weight.
Now is the time to make the important distinction between buckling and lock up. Buckling is the state where the CT deforms under compressive axial load so that the CT repeatedly, regularly touches the walls of the borehole, as illustrated in Figure 1.
In Figure 1, at every position where the buckled CT contacts the wall of the hole, a component of the axial load carried by the CT acts towards the wall of the borehole. The size of this wall component depends on the contact angle, and hence on the difference between the OD of the CT and the ID of the hole it is passing through. The wall component, multiplied by a friction coefficient, results in an axial force which opposes the motion of the CT.
The condition where the sum of these forces exceeds the own weight force propelling the CT down the hole is lock-up.
In planning any CT intervention, it is important to evaluate whether the CT and BHA can be conveyed to the end of the hole and set down with sufficient force to operate without encountering lock-up.
In this article we have only considered vertical holes. Next time we will look at what happens when we convey CT into a deviated or horizontal well.
*CTES Coiled Tubing Manual, 2005.
Author: Richard Stevens
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