Summary.
Improvements in liner cementation in the North Sea are being made through optimization of cementing practices and the use of advanced technology, which includes liner rotation during hole conditioning and cementing. This paper reviews our experiences and discusses key aspects of liner rotation, including equipment selection, job design, and implementation. Comparison of cement-bond logs (CBL's) with those from nonrotated-liner cement jobs shows that liner rotation during cementing can greatly improve results.
Introduction
Liner rotation has been implemented in recognition that pipe movement can help improve mud displacement. Liner equipment is commercially available to facilitate rotation during hole conditioning and primary cementing. It is claimed that liner rotation during cementing results in more successful primary cement jobs, with little or no requirement for remedial operations. Pipe movement can be implemented through either reciprocation or rotation.
The main operational benefit of rotation over reciprocation is that the liner can be set and the running tool released before cementing. With rotation, pipe movement can be achieved with added confidence that the liner has been set successfully and that the running tool has released.
When a liner is placed across a reservoir interval, the importance of good cement bonding, liner lap integrity, and complete zonal isolation cannot be overemphasized. A poor primary cementation may adversely affect the productivity of a reservoir; early water coning is an example. Similarly, erroneous or nonanalyzable test data may result from appraisal well testing.
Zonal isolation is achieved through effective displacement of the mud by the cement. Problems with displacement can result from decentralized casing. Cementing poorly centralized casing almost always results in a channel of gelled mud being left behind, with pipe movement often the only practical means of obtaining mud mobility. Even with adequate centralization, a dependence solely on annular velocity for mud removal may not be practical or advisable. Pumping at velocities in excess of 350 ft/min [ 1.78 m/s] to obtain better displacement through turbulences may create sufficiently high friction pressures to fracture the formation or to induce fluid loss from the slurry and possible bridging. Pumping at lower velocities, i.e., 90 ft/min [0.46 m/s], may result in channeling. Laboratory and field results indicate a low displacement velocity to he a poor method of mud removal where there is decentralization. In short, although liner cementations are critical, the chances of success are not particularly good.
When pipe movement is considered, reciprocation is traditionally regarded as the simplest to implement. Where hole irregularities occur, reciprocation could be beneficial by exposing clean pipe to gauge or tight hole sections. Normally, pipe is reciprocated in strokes of 15 to 30 ft [4.57 to 9.14 m] at around 10 ft/min [0.05 m/s].
One disadvantage of reciprocating a liner is that velocity and pressure surges can occur, causing relative annular fluid velocities to pulse between small and large values. This has been considered advantageous, but on the downstroke, pressure surges similar to or greater than those when casing is run can occur, possibly inducing losses or causing excessive loss of filtrate. Where the over-balance is kept deliberately low (+/-200 psi [+/- 1.38 kPa]), there is a risk that hydrocarbons may be swabbed in during the upstroke. Another disadvantage is that the pipe may stick on the upstroke and prevent setting of the liner at the correct depth. Operators have noted such experiences as the cement is being displaced. A further disadvantage with reciprocation is that liners can be set only after the cementing operation has been completed. If for any reason the liner hanger then fails to set, serious consideration will have to be given on how to hang off the liner and to release the running tool before the cement sets.
Investigations into pipe-stretch behavior indicate that, although the pipe may be reciprocated 15 to 30 ft [4.57 to 9.14 m] on surface, there would be reduced movement at the liner. Fig. 1 illustrates the results of a computer simulation of relative downhole movement. A 3,530-ft [ 1076-m] string of 7-in. [ 17.8-cm] liner was run on 12,100 ft [3688 m] of 5-in. [12.7-cm] drillpipe in 12.6-lbm/gal [1509.8-kg/m3] mud. Because of pipe stretch, a 16-ft [4.9-m] surface stroke translates to 12 ft [3.7 mi downhole. Of interest are the long periods of liner inactivity caused by expansion and compression of the running string. Reciprocating at 18 strokes/h, as shown in Fig. 1, results in a period of liner mobility of about 35 minutes vs. 52 minutes of surface activity.
By comparison with reciprocation, rotation is continuously active downhole. Reciprocation has periods of inactivity related to the degree of stretch and drag. In summary, the implications associated with liner reciprocation should be considered carefully before the technique is used.
Liner movement can be achieved through rotation without the risk that the liner may fail to set after cement placement. The liner, with floating cone and bearings, is set just after mud circulation and before cementation. If for any reason rotation has to cease during the operation, the cement is displaced as normal and the setting tool retrieved. Rotation provides more continuous movement than reciprocation, and drag forces pull gelled mud out from the narrow side of the annulus, increasing mud mobility.
Equipment Selection
A 1985 technical survey of major suppliers of liner/completion equipment short-listed three companies having the necessary operational experience, equipment, and engineering personnel for liner-rotation jobs in the North Sea. After several minor modifications were incorporated into the original designs, the equipment of all three manufacturers is now being used in the North Sea. Both sealed-journal bearings and "unsealed" tapered-roller-bearing hanger types are used. Each hanger type has given satisfactory rotation when jobs have been properly planned and well executed.
Two options are available for setting a liner hanger:mechanical set, which involves raising, turning, and lowering the tool by preset amounts, andhydraulic set, which involves dropping a ball into a seating seat in the body of the tool, then setting the liner by applied hydraulic pressure. These setting mechanisms are similar to those used on conventional liners.
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