Coiled Tubing is used increasingly to service highly deviated and horizontal wells. A limitation on horizontal displacement occurs because of the frictional forces between the coiled tubing string and borehole while running in coiled tubing. This causes helical buckling and can lead to lockup of the coiled tubing string, thereby limiting reach.
This paper will address the various techniques currently in use and propose new techniques to achieve additional reach. The topics that will be discussed are buoyancy reduction, friction reducers, optimal taper and pipe size of coiled tubing, straightening coiled tubing, downhole tractor simulation, flowing fluid, and pumpdown.
A tubing forces model has been developed that models these techniques. Results showing additional reach attained will be presented in the form of the surface weight prediction for commercial and conceptual wells.
Extended reach technology (ERT) has allowed the petroleum industry to reach and capitalize on reserves inaccessible to conventional drilling and completion methods. "The application of extended reach technology has resulted in extended field drainage radii, increased production rates, improved reservoir management, a reduction in required platforms and well counts and improved field economics…".
Extended reach technology has developed rapidly in the U.K. and Norwegian sectors of the North Sea. Given the type of well profiles, conventional wireline techniques are not appropriate to convey tools and manipulate completion hardware Due to the high angles of the long tangent sections combined with the TVD of the reservoir, such wells are known as extended reach wells. Coiled Tubing is well suited to conduct such operations. The definition of an extended reach well is a well with a measured depth to true vertical depth ratio (MD/TVD) greater than 2.0. A mega-reach well has a MD/TVD ratio greater than 3.0.
Furthermore, services conducted with drillpipe or conventional tubing are often performed with coiled tubing to reduce rig costs. The services currently being performed in highly deviated/extended reach wells include perforating, setting and retrieving plugs, opening and closing sliding sleeves, fishing, removing fill, drilling, matrix acidizing and acid washing.
A limitation on the horizontal displacement occurs because of the frictional contact forces between the coiled tubing and the wellbore When axial compression forces over a critical value are applied to coiled tubing, the coiled tubing will first buckle into a sinusoidal wave shape.