Torque and Drag-Two Factors in Extended-Reach Drilling
Authors: Aarrestad Thor Viggo 
Published: December 2007

Summary
This paper addresses the various aspects of torque and drag problems encountered in drilling extended-reach wells. It discusses how to use torque and drag calculations and measurements to plan long-reach well profiles, to execute drilling operations that minimize torque and drag effects, to monitor hole cleaning, and to plan jarring operations.

Introduction
In extended-reach drilling, a limitation on the horizontal displacement occurs because of frictional forces between the drillstring and the borehole wall. Drag is measured as the difference between the static weight of the drillstring and the tripping weight. Similarly, a difference between the torque applied at the rig floor and the torque available at the bit occurs owing to friction. Torque and drag problems are often associated with each other and maybe profound in extended-reach and horizontal wells.
As Sheppard et al. stated, a variety of sources of drag and torque loss exist: differential sticking, key seating, hole instabilities, poor hole cleaning, and the general frictional interaction associated with side forces along the drillstring. Therefore, drag and torque measurements may be used to monitor operations to optimize performance. In extended-reach drilling at Statoil, torque and drag problems have initiated use of more sophisticated well profile and use of torque as an indicator of hole-cleaning problems. Understanding of torque and drag problems has been applied to the well planning process. As a result, problems are often not found in wells with horizontal displacements up to 5000 m. Another interesting implementation of drag knowledge in operational procedures is described in a paper on the influence of drag on hydraulic jar efficiency.
In this paper, we discuss torque and drag problems in extended-reach wells, how knowledge of torque and drag is used in operational procedures, and to what extent the planning phase can help avoid operational problems. Although always referring to extended reach, the same principles are valid for horizontal,'S'-shaped, and designer wells.

Well Profiles
Optimizing well profiles to minimize torque and drag problems has been discussed in many publications (e.g., Refs. 1, 4, and 8 through 10). Sheppard et al. thoroughly discussed the catenary curve principle for well drilling. Alfsen et al. discussed a modified catenary principle; Banks et al. included the concept of tortuousity and reached the important conclusion that making a smooth well path is key for successfully drilling extremely long-reach wells.
To reduce friction in any well, a good mud program design is important. Friction factors down to 0.16 simulations have proved to give a best fit with measurements. The torque and drag program used in the work described here has been used extensively at Statoil together with measurements of actual data. Confidence in the calculations has been achieved, and they have been used to monitor and improve operational practice. Minimizing dogleg severity and even making changes in dogleg severity have been implemented in our procedures.
Several papers have been published on long-reach well drilling from the Statfjord C platform. After a 6000-m horizontal displacement was reached in Well 33/09-C03, it was recognized that the well profile would need to be optimized to reach the planned depth for Well C02-7200-m horizontal displacement. The catenary curve, proposed as a possible solution to the torque and drag problems, is the solution to the following problem.
A cable with weight per length, W, has a horizontal force at left Point A, FH, and a tangential force at right Point P(x, y), FT. The horizontal component of the force at Point P is in the opposite direction of the force at Point A.
The solution to the above problem is given in the x-y plane as
where
An interesting feature of the catenary curve is the zero contact force between the drillstring and the borehole wall. Consequently, the catenary curve could theoretically give zero friction between the borehole wall and the drillstring.
Several difficulties exist in using this approach for drilling a well. First, the effective force at the bottom of the well results in drillstring compression as opposed to the tension given in the theoretical curve. Furthermore, the catenary curve will lead to a much longer well path than more traditional well profiles. Thus, a slight modification of the catenary curve must be made.
An important feature of the catenary curve was kept in the well plans for Wells 33/09-C24 and 33/09-C02 in the Statfjord field: the very slow build rate in the shallow part of the well with a slowly increasing build rate as well depth increases. The sailing angle of 80 to 84 is therefore much higher than the traditional 60 .
Figs. 1 and 2 describe the well-path planning process with the resulting torque calculations. The catenary curve is compared with traditional constant-build curves with 1.5 /30- and 2.5 /30-m build rates. A much lower sailing angle is achieved with the traditional curve design. As a result, as Fig. 2 shows, the measured depth (MD) of the actual well path is longer than with traditional shapes. The friction along the drillstring is lower, however, and a higher torque at the bit is a welcome result.
The success of reducing wall contact and thereby the total friction was reported in Ref. 4 and is shown in the simulations of comparison of wall contact force in Fig. 3. Well 33/09-C03 has a standard profile; Well 33/09-C02has a modified catenary profile. Note the difference in scale in the two parts of Fig. 3. The very high normal force in Well 33/09-C03 compared with the33/09-C02 profile will give similar marked higher friction and thus higher torque loss.
The well profile used in Statfjord Wells C24 and C2 may lead to enhanced problems with formation stability and differential sticking owing to the high sailing angle. However, wherever these problems can be handled, the modified catenary curve will give a lower friction than traditional well profiles.

Monitoring Hole Cleaning
The confidence in torque and drag simulation programs may give unexpected benefits. When long-reach wells are drilled, the torque and drag simulation curves may be used to monitor hole cleaning. Deviations from properly modeled torque and drag simulations may indicate hole-cleaning problems.
Fig. 4 shows torque simulations in Well 33/09-C02 and actual measured torque in the 12 1/4-in. section. The three smooth curves are the acceptable, planned, and actual torque simulations, respectively. The marked change in simulation curves at about 2600 m was caused by a bit change. An aggressive bit must be simulated with a higher torque on bit than a less aggressive bit.
P. 800^