The two new general procedures for the bore drag prediction, based on the borehole friction factor concept were compared. The procedures employed iteration over the directional survey stations, numerical integration between the stations and mathematical models of the axial loads within a moving pipe in the borehole. The models considered several new effects such as hydrodynamic viscous drag, contact surface, and the bearing angle component of dogleg severity.
The study addressed the extent and conditions under which the two-dimensional and the three-dimensional procedures diverged significantly. The method was based on the computer calculated values of the borehole friction factor from the measured hook loads. The field data used included four casing runs from the offshore locations in the Gulf Coast area. In addition, the systematic theoretical study was performed with over 100 computer-simulated directional wells.
The study revealed a good agreement between 2-D and 3-D procedures for most common drilling conditions. Two 2-D model's accuracy was mostly affected by the bearing angle component of shallow doglegs. In addition, the reliability of the borehole friction factor field assessment was mainly controlled by the inclination angle and the length of the slant hole section of a well.
The complex spatial configuration of directional wells engender an additional axial load (drag) when pipe or bottomhole tools are run in the boreholes. Traditionally, the frictional effects were not computed but were accounted for by the design factors. Such an approach resulted in the overestimated design and high operational uncertainty. The example here might be reluctance of many operators to reciprocate casing strings, despite a beneficial effect of this operation on the cement bond.
The knowledge of the frictional loads will improve the design criteria, and will help to optimize the design for the minimum cost.
In their interesting study, Johancsik et al., developed a simplified model to predict torque and drag for the drillstring. They also used the model to find the sliding friction coefficient. The model was tested in three directional wells with a significant length of the cased hole section (70%, 83%, and 99%). No distinction was made between cased hole friction and the open borehole friction. Also, the hydrodynamic effects were not considered which, for the drillstring movement, might have been an adequate simplification.
Sheppard, et al. investigated the advantages of planning an undersection trajectory (steady buildup) to reduce torque and drag. In the one field case studied, they evaluated friction factor values of 0.36 only within the shallow depth interval 1900 to 2400 ft.
Bratovich, et al, investigated problems of running logging tools in high-angle wells. The field tests and the laboratory tests were performed. For the open hole, they reported a friction factor value 0.36 for the stand-off tool, and 0.40 for the wireline. The open hole tests were conducted in lignosulfonate water-base muds with densities varying between 9.7 to 12.5 lbm/gal.
To date, very few tests have been performed with the actual field data. The concept, that a single value of the friction factor represents borehole conditions has not been verified.