Torque and Drag-Two Factors in Extended-Reach Drilling
- Thor Viggo Aarrestad (Statoil A/S)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- September 1994
- Document Type
- Journal Paper
- 800 - 803
- 1994. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 1.6.10 Running and Setting Casing, 1.6.1 Drilling Operation Management, 4.3.4 Scale, 1.1 Well Planning, 1.6 Drilling Operations, 1.6.6 Directional Drilling, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.14 Casing and Cementing
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This paper addresses the various aspects of torque and drag problemsencountered in drilling extended-reach wells. It discusses how to use torqueand drag calculations and measurements to plan long-reach well profiles, toexecute drilling operations that minimize torque and drag effects, to monitorhole cleaning, and to plan jarring operations.
In extended-reach drilling, a limitation on the horizontal displacementoccurs because of frictional forces between the drillstring and the boreholewall. Drag is measured as the difference between the static weight of thedrillstring and the tripping weight. Similarly, a difference between the torqueapplied at the rig floor and the torque available at the bit occurs owing tofriction. 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 lossexist: differential sticking, key seating, hole instabilities, poor holecleaning, and the general frictional interaction associated with side forcesalong the drillstring. Therefore, drag and torque measurements may be used tomonitor operations to optimize performance. In extended-reach drilling atStatoil, torque and drag problems have initiated use of more sophisticated wellprofile and use of torque as an indicator of hole-cleaning problems.Understanding of torque and drag problems has been applied to the well planningprocess. As a result, problems are often not found in wells with horizontaldisplacements up to 5000 m. Another interesting implementation of dragknowledge in operational procedures is described in a paper on the influence ofdrag 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 whatextent the planning phase can help avoid operational problems. Although alwaysreferring to extended reach, the same principles are valid for horizontal,`S'-shaped, and designer wells.
Optimizing well profiles to minimize torque and drag problems has beendiscussed in many publications (e.g., Refs. 1, 4, and 8 through 10). Sheppardet al. thoroughly discussed the catenary curve principle for well drilling.Alfsen et al. discussed a modified catenary principle; Banks et al. includedthe concept of tortuousity and reached the important conclusion that making asmooth well path is key for successfully drilling extremely long-reachwells.
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 withmeasurements. The torque and drag program used in the work described here hasbeen used extensively at Statoil together with measurements of actual data.Confidence in the calculations has been achieved, and they have been used tomonitor and improve operational practice. Minimizing dogleg severity and evenmaking changes in dogleg severity have been implemented in our procedures.
Several papers have been published on long-reach well drilling from theStatfjord C platform. After a 6000-m horizontal displacement was reached inWell 33/09-C03, it was recognized that the well profile would need to beoptimized to reach the planned depth for Well C02-7200-m horizontaldisplacement. The catenary curve, proposed as a possible solution to the torqueand 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 componentof the force at Point P is in the opposite direction of the force at PointA.
The solution to the above problem is given in the x-y plane as
An interesting feature of the catenary curve is the zero contact forcebetween the drillstring and the borehole wall. Consequently, the catenary curvecould theoretically give zero friction between the borehole wall and thedrillstring.
Several difficulties exist in using this approach for drilling a well.First, the effective force at the bottom of the well results in drillstringcompression as opposed to the tension given in the theoretical curve.Furthermore, the catenary curve will lead to a much longer well path than moretraditional well profiles. Thus, a slight modification of the catenary curvemust be made.
An important feature of the catenary curve was kept in the well plans forWells 33/09-C24 and 33/09-C02 in the Statfjord field: the very slow build ratein the shallow part of the well with a slowly increasing build rate as welldepth increases. The sailing angle of 80 to 84 is therefore much higher thanthe traditional 60 .
Figs. 1 and 2 describe the well-path planning process with the resultingtorque calculations. The catenary curve is compared with traditionalconstant-build curves with 1.5 /30- and 2.5 /30-m build rates. A much lowersailing angle is achieved with the traditional curve design. As a result, asFig. 2 shows, the measured depth (MD) of the actual well path is longer thanwith 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 wasreported in Ref. 4 and is shown in the simulations of comparison of wallcontact 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 partsof 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 highertorque loss.
The well profile used in Statfjord Wells C24 and C2 may lead to enhancedproblems with formation stability and differential sticking owing to the highsailing angle. However, wherever these problems can be handled, the modifiedcatenary curve will give a lower friction than traditional well profiles.
Monitoring Hole Cleaning
The confidence in torque and drag simulation programs may give unexpectedbenefits. When long-reach wells are drilled, the torque and drag simulationcurves may be used to monitor hole cleaning. Deviations from properly modeledtorque and drag simulations may indicate hole-cleaning problems.
Fig. 4 shows torque simulations in Well 33/09-C02 and actual measured torquein the 12 1/4-in. section. The three smooth curves are the acceptable, planned,and actual torque simulations, respectively. The marked change in simulationcurves at about 2600 m was caused by a bit change. An aggressive bit must besimulated with a higher torque on bit than a less aggressive bit.
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