To carry out remedial work in oil wells through the production tubing string, a method using a continuous length of steel coiled tubing is used. In horizontal wells substantial friction forces are generated which resist the motion of the tubing as it is pushed into the well. As the penetration increases, the friction forces arising from the contact of the tubing with the inner casing wall increase too, and the tubing buckles. The buckling is initially sinusoidal but eventually transforms into helical. At this point the force required to push the tubing rises dramatically, and the maximum penetration is then rapidly reached. Considerable economic benefits could be gained if the limits on the maximum penetration of coiled tubing in horizontal oil wells could be increased. This article discusses ways of increasing the penetration of coiled tubing in horizontal oil wells by managing the geometry of the coiled tubing. It is shown that the best strategy is to keep the external diameter of the coiled tubing constant, and to make up the coiled tubing from several sections, each with uniform, but increased wall thickness. It is shown that with two sections the maximum penetration can be increased by about half, and that with three sections the maximum penetration can be increased by about two-thirds.
Simulation of temperature-pressure profiles and wax deposition in gas-lift wells