This paper studies the helical buckling of pipes (drillstring and tubing) in extended reach and horizontal wells, theoretically and experimentally, resulting in new equations to correctly predict and effectively prevent the helical buckling of pipes in such wells. The theoretical study shows that the so-called helical buckling load that appears in the current literature is only the average axial load in the helical buckling development process. The laboratory experiments confirm the theoretical analysis. The new helical buckling load equations are formulated by combining the theoretical analysis and the experimental results, thereby resolving the existing assumption-and result inconsistency in the current literature. The new equation predicts the true helical buckling load to be about 1.3 times the so-called helical buckling load in the current literature, and about 1.8 times the critical buckling load that predicts the onset of sinusoidal buckling. Consequently, larger bit weights or packer setting loads can be applied to increase the drilling rate or to ensure a proper seal, before the helical buckling of the pipes can occur.
This paper discusses helical buckling and frictional drag of coiled tubing in drilling and completing horizontal wells. Analytical equations are presented to predict helical buckling of coiled tubing, actual bit weight or packer load, and maximum horizontal length that may be drilled if helical buckling occurs. These equations can also be used for other operations with coiled tubing in horizontal wells, such as coiled tubing wireline logging, coiled tubing well stimulation, and for conventional drilling using a joint-connected drill string. The results are compared with previous results in the literature. Introduction Drilling and completing horizontal wells using coiled tubing are currently developing technologies in the oil industry. These technologies bring about significant saving, but they also raise new problems, such as buckling and "lockup" of coiled tubing in the wellbore. This paper will analyze, in detail, the nature of coiled tubing buckling and frictional drag in horizontal wells (including vertical, buildup, and horizontal sections), and presents the equations to predict bit weight/packer load, maximum horizontal section that can be drilled, and coiled tubing "lockup". When coiled tubing is run into a horizontal section, it is subjected to compressive load due to frictional drag, The axial compressive load is highest at the end of the build section and lowest at the far end of the well. When the compressive load exceeds the critical buckling load, the coiled tubing will initiate a sinusoidal buckling. Further increase of the compressive load can result in a helical buckling of the coiled tubing. The helical buckling is developed simply because of the wellbore's confinement to the buckling deflection. For tubulars confined in the wellbore and under large axial compressive load in excess of the helical buckling load, the helix becomes the buckling shape with minimum potential energy. Coiled tubing buckling can also occur in the vertical section of a horizontal well when "slacking-off" weight at the surface to apply bit weight or packer load and to push the coiled tubing into the horizontal section. With conventional drilling, heavy weight drill pipe or drill collars are often used above the "kickoff" point to prevent buckling. This is not possible with the continuous coiled tubing, so there is a high probability that the coiled tubing will buckle in the vertical wellbore section, and adversely affect the well plan. Buckling of Coiled Tubing Coiled tubing buckling may occur in any wellbore section (vertical, horizontal and buildup sections). However, the axial compressive load to initiate the buckling is different for different wellbore sections.
This paper studies helical buckling, and also critical (sinusoidal) buckling, of pipes (tubing and drillstring) in horizontal wells. The large frictional drag of helically buckled pipes is also studied.Helical buckling is developed from critical (sinusoidal) buckling as the axial load keeps increasing. But fully developed helical buckling of pipes will not occur in horizontal wellbores until the axial load becomes very large, about 1.8 times the critical buckling load that predicts the onset of sinusoidal buckling, and about 1.3 times the so-called helical buckling load that appears in the current literature. The so-called helical buckling load in the CUIrent literature is actually the average axial load in the helical buckling development process. This means larger bit weight or tubing packer load may be applied to increase the drilling rate or to ensure a proper seal, before helical buckling of the pipes can occur.However, once fully developed helical buckling occurs, the frictional drag may become much larger than it was before the onset of helical buckling. The pipe.. could even become "lockedup" so that the bit weight or packer setting load can not be increased any more by slacking off weight at the surface. .
This paper presents a modified two-phase well-control model to simulate two-phase kick behavior after comparing a dynamic twophase well-control model and a single-phase model. The model is reliable and easy to simulate without any of the numerical problems that might occur in a dynamic two-phase model. The simulator calculates gas influx rate from the formation assuming an infinite-acting reservoir.The paper also introduces computer applications for IBM-compatible personal computers for use as a well-control training and educational tool. The simulator has enhanced graphical presentations with random access for all menus and options by use of a computer mouse. The simulator is written in Visual Basic™3.The simulator can handle vertical wells, directional wells, extended-reach wells, and horizontal wells with two different buildup rates for onshore or offshore wells. The simulator also provides the theoretical kill sheet for any selected well geometry. The simulator is a user-interactive well-control program with an automatic wellcontrol option. It demonstrates the basic concepts of well control and shows the pressure and volume responses of the kick with time.
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