RIASSUNTO
Abstract
Weight transfer and helical buckling eventually leading to lock up is an issue that the industry has had to deal with since the outset of the use of coiled tubing. Limitations imposed by this restriction have reduced the number of available applications that can be carried out. The use of larger sizes, improved materials combined with friction reducers and better modelling techniques have gone a long way to understand and extend these limits. This paper reviews a new dynamic excitation tool, which reduces frictional drag and therefore adds to available options for further extending limits and capabilities of coiled tubing.
The paper follows the tools history from developmental testing in Norway and subsequent successful Coiled Tubing Drilling (CTD) field trials in Alaska.
Results of reverse friction factor modelling are included and parameters that affect the tools ability to create dynamic excitation are also discussed.
Tool Description
The tool comprises a short Positive Displacement Motor (PDM) section which powers a valve (Figure 1). The motor only drives the valve and there is no output drive (bit box). The drive off the short motor section oscillates an upper valve plate with a hole in it. This occurs as the rotor moves from side to side within the stator. The oscillating plate moves above a static plate, with a central hole. As the two holes align the total flow area increases (Figure 1B). As the rotation continues it partially obscures the static hole and decreases the flow area (Figure 1A and C).
The changing flow area creates pressure pulses, which in turn induces axial vibration, travelling outward from the tool breaking static friction with the wellbore. The frequency and size of the pressure pulses is proportional to flow rate.
Testing at the Ullrig Facility, Rogaland, Norway
Whilst the tool has previously been used successfully in slide drilling applications, it had never been used before with coiled tubing. To prove the effectiveness of the coiled tubing tool, a series of trials were conducted at the Ullrig test facility in Norway (Figure 2).
Ullrig Test Objectives
A number of tests were carried out with the following objective:
Prove tool compatibility with coiled tubing
Quantify average and peak axial vibrations
Compare force and drag with and without the tool
Optimise bottom hole assembly (BHA) placement of the tool
Determine effectiveness of shock tools
Compare Weight on Bit (WOB) during milling
A total of over twenty different tests were used to optimize the performance of the tool and confirm effectiveness in various BHA configurations.
Ullrig Test Results
The Ullrig tests confirmed the tools ability to be very effective when used with coiled tubing. The graphs in Figure 3 and 4 illustrate comparative performance with and without the dynamic excitation tool. In all cases significant weight transfer improvements were seen, typically in the order of 70% to 90% and on one test was as high as 98%.
Testing also confirmed that the use of a shock tool in the assembly made no difference to tool performance, as opposed to coventional drilling applications where heavier BHAs benefit from focused axial movement.
Use of the tool also improved the ability to transfer force without the normal ""stick slip"" associated with weight transfer. Milling cement with the tool resulted in smoother drilling and less stalling than previous tests where the tool was not used.