A significant limitation in the application of coiled tubing technology is the ability of the coiled tubing to push. Push may be required to move a heavy bottom hole assembly (BHA) along a highly deviated or horizontal well. Push may simply be required to push against the drag forces induced by the coiled tubing's own weight, as it is run into a well. Either way, it is often important to know how far coiled tubing can be pushed into a well, or what weight/length of BHA can be pushed to a certain depth. Accurate modeling is required to avoid a costly mis-run with coiled tubing that fails to get to the necessary depth.
Recent projects carried out offshore Brunei illustrate how critical accurate force modeling is and how small well factors can greatly affect the depth that coiled tubing can reach. This paper will also demonstrate how difficult it is to infer down hole drag conditions from surface readings unless the coiled tubing is nearing its point of maximum reach.
An in-depth, theoretical engineering treatment of the buckling of tubulars supported within larger tubulars is not the subject of this paper. Other works have shown the theory behind this behavior1,2,3,4,5,6. This paper focuses more on the observed response of coiled tubing seen during specific campaigns of work and makes comparisons with an established, proven coiled tubing simulator.
The campaigns involve running long lengths of perforating guns into newly drilled and completed horizontal wells. The guns were all run on coiled tubing, the perforating conducted in the balanced or underbalanced condition, using down hole formation isolation valves to deploy the guns in and out of the wells.
Computer simulation modeling results are used to illustrate and quantify the effects seen during the actual work programs. The observations will show the following:
Small changes in down hole conditions are not reflected significantly on the surface weight indicator, when the coil is an appreciable distance away from its maximum attainable depth.
The same small changes in down hole conditions reflect significantly on the surface weight indicator, when the coiled tubing nears its final maximum attainable depth.
Refining a computer simulation model using field data from shallow depths does not permit accurate estimation of what the maximum attainable depth will be.
Estimation of chemically derived drag reduction effects, from weight readings at depths distant from the maximum attainable depth is inaccurate.
Overview of the behavior of Coiled Tubing in Compression
It is useful to have a simplified model in mind when considering what is happening when coiled tubing (or any other tubular) is pushed or placed under compression. Perhaps the simplest scenario to envisage is a perfectly straight section of coiled tubing, being pushed along a perfectly straight, horizontal well bore. This, of course, is a totally improbable scenario but is still valid for the purpose of creating a picture of the generic behavior of coiled tubing.
Initially, as the coiled tubing enters the straight well, it simply runs along the low side of the well, the drag on the coiled tubing being equal to its own weight multiplied by the coefficient of friction between the coil and the well, µ. As the coil extends deeper into the well, the total weight of coil in the well increases and so, therefore, does the force required to push it. At a critical point, the coiled tubing will buckle; it will go from being perfectly straight, to form a spiral. In the same way that a small diameter rod will bow if a large compressive load is placed upon it, the coiled tubing will bow until it hits the inside wall of the well. The coiled tubing is prevented from bowing out a significant distance because the inside surface of the well cannot move.
So our theoretical length of coiled tubing suddenly flips from straight to spiraled where the compressive load exceeds the critical buckling load for the coil. In this scenario, only the end being pushed will spiral as the compressive load at the free end remains under the critical compressive load.