Edinburgh Napier University

  To carry out remedial work in the oil producing wells through the production tubing string, a method using a continuous length of steel tubing or coiled tubing is used, that allows quick access to the well. Coiled tubing can also be used for drilling and extending existing wells and shows significant cost savings over traditional joined drill pipe methods. As the coiled tubing is being pushed into a horizontal well it lies initially straight on the bottom part of the casing of the well. As penetration increases, the friction force arising from the contact of the tubing with the inner casing wall increases and the tubing buckles with low amplitude and long pitch length. As penetration increases further, the frictional force increases and buckling amplitude increases and the pitch length decreases. The tubing thus moves further up and round the inside of the casing. This process continues until the tubing reaches the top part of the casing and it moves from sinusoidal buckling into helical buckling. At this point the force required to push the tubing further rises dramatically, with subsequent locking of the tubing in the casing with no further penetration possible.

The work to date on this important industrial problem can be divided into two areas: (i) the theoretical approach backed by laboratory experiments in which the end of the containing tube is obstructed, so as to allow buckling to develop and (ii) full scale tests that meet with the practical problems but do not provide as much information and difficulties of scale can also extend the time required to complete satisfactory tests. The laboratory tests have demonstrated the buckling phenomenon but have not re-created the conditions existing in a well.

In this work a novel experimental apparatus is developed using tubular members of very low flexural rigidity and high value of friction between the tubular and a transparent containing cylinder. This allows limits on penetration of the tubulars to be observed in under 5 m of insertion. The experimental apparatus enables the simultaneous measurement of the insertion forces and the inserted length. A suitable control and data acquisition system has also been designed. Experimental results indicate clearly all important parameters which limit the penetration of the tubulars in horizontal containing cylinders.

A theoretical model of penetration has been developed, which is in excellent agreement with the experimental data. The model can be used to determine the importance of various design parameters on the maximum penetration of tubulars in horizontal wells. Finally, the model suggests how to maximise the penetration of coiled tubing in horizontal oil wells.