Vibration and Rotation Considerations in Extending Coiled-Tubing Reach
- Kenneth Ray Newman (CTES/NOV)
- Document ID
- Society of Petroleum Engineers
- SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, 20-21 March, The Woodlands, Texas, U.S.A.
- Publication Date
- Document Type
- Conference Paper
- 2007. Society of Petroleum Engineers
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale, 1.6.1 Drilling Operation Management, 3 Production and Well Operations, 4.2.3 Materials and Corrosion, 1.11 Drilling Fluids and Materials, 1.7.5 Well Control, 1.6 Drilling Operations
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In recent years there has been increased usage of coiled tubing (CT) to drill1 shallow wells or to drill laterals from an existing wellbore. Often the weight on bit (WOB) which can be applied with CT is limited due to friction between the CT and the wellbore, preventing some drilling operations and methods from being possible. Two methods proposed to reduce this friction are vibration of the CT and rotation of the CT. This paper presents research that was performed to determine the effectiveness and practicality of these two methods.
The US Department of Energy (DOE) funded research in microhole drilling with CT. A portion of this funding was directed towards researching methods for mitigating downhole friction to enable CT to be utilized for extended-reach drilling applications. Small diameter CT (used for microhole drilling) helically buckles easily when in compression. Once helically buckled, additional wall contact forces2 (WCF) are caused by the buckling. These additional WCF cause increased friction between the CT and the wellbore. This friction increases exponentially with the compressive force in the CT until no additional force can be transmitted to the bit. This situation is
known as helical lockup.
Figure 1 shows the exponential nature of the increase in WCF when CT is helically buckled. The dashed lines represent the WCF due simply to the weight of the CT, in pounds per foot. The solid lines represent the WCF due to helical buckling for various compressive axial forces. When the axial force reaches the HBL, the additional WCF due to buckling far exceeds the WCF due to weight.
The following methods have been used or considered, to increase the WOB for CT drilling:
- Geometry changes - increasing the diameter of the CT or decreasing the hole diameter is an effective way of increasing the WOB. Increasing the CT wall thickness may increase the WOB, especially in vertical wells.
- Lubricants - various lubricating fluids have been used to reduce the friction coefficient between the CT and the well tubulars or the wellbore. These lubricants have been somewhat successful in cased-hole workover applications. However in drilling applications the drilling fluid must turn the downhole motor, maintain pressure control (unless drilling under-balanced) and carry the cuttings out
of the hole. It is difficult to design a drilling fluid which will perform all of these functions and also provide significant lubrication.
- Tractors - downhole tractors are available for CT drilling applications. The tractors provide the WOB, so that the CT can remain in tension. However, a downhole tractor adds additional deployment logistics, risk and expense. Currently tractors are not available in small enough diameters to perform microhole drilling, though this is one of the objectives of the DOE research.
- Rollers - various types of rollers have been developed which can be attached to the CT and to the drilling BHA. The time and logistics associated with attaching and removing rollers on the CT make them undesirable.
- Straightening the CT - CT has residual curvature due to the bending it undergoes on the reel and guide arch. A device which causes a small reverse bend to straighten the CT can be used, so the CT in the well is straight (or nearly straight). This device does not remove the residual stresses. Instead it modifies the residual stresses so that they result in nearly straight pipe. Some work has been
done which shows that straightened CT can be pushed further in a horizontal well before reaching helical lockup. Other work has shown that the maximum WOB provided by straightened CT is the same as the maximum WOB provided by curved CT. Thus the benefit of straightening appears to be inconclusive. It is clear that straightening CT exacerbates the fatigue damage.
- Vibration - downhole vibrators have been utilized to decrease friction and increase the WOB. Again, field results seem mixed. In some cases the vibrator helped, and in other cases there appeared to be little benefit.
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