Fig. 9.24. Gyroscopic multiple shot surveying instrument (Surwel). Film shows direction by arrow, vertical deviation by cross, and time (depth) at which each picture was taken. Courtesy Sperry-Sun Well Surveying Company.
drives the chisel-shaped nose into the formation. When sufficient weight is applied, the shear pin fails, allowing the bit to rotate and drill off the face, as shown in Figure 9.22. The face is a concave groove with the desired deflection. After a few feet (10 to 20) have been drilled, a survey instrument is run to determine the angle and direction of the hole. If this is satisfactory, drilling is continued. If not, remedial measures must be taken.
The knuckle joint is essentially an extension of the drill string incorporating a universal joint at the junction. Thus rotation at different angles is possible, as illustrated in Figure 9.23. The principal disadvantage of this tool is that the angular change may be quite abrupt, and cause a dog-leg. Deviation changes of 5 to 7° in 15 or 20 ft may occur.
The most widely used of these tools is the removable whipstock. Those interested in a more detailed coverage of deflection tools are referred to Brantly's Handbook.16
Directional drilling requires the measurement of both vertical deviation and horizontal direction. This is accomplished with various devices which combine the plumb-bob or pendulum reading with a simultaneously recorded compass reading. The compass used is either magnetic or gyroscopic. The instruments used are complicated mechanisms and different designs are available from various companies. They are often classified as single shot or multiple shot, depending on the number of readings obtainable from a single run.
Instruments utilizing a magnetic compass require shielding from the magnetic disturbance caused by the drill string. This is commonly accomplished by using a special nonmagnetic drill collar made of K-monel metal, which is run just above the bit to house the instruments. This metal is permanently nonmagnetic and has physical properties equal to the best drill collar steel. It is, however, quite expensive. Another way to eliminate magnetic disturbances is to use core or trigger type bits which allow the surveying instrument to protrude below the bit in the open hole.
The magnetic type single- and multi-shot instruments may be run into the hole in several ways, depending on the situation. Some of the more common methods are: (1) Free drop or go-devil operation: the instruments are housed in special shock absorber barrels such as those used for the inclinometers previously mentioned. Single-shot types are retrieved with an overshot on a wire line, or by pulling the pipe. The latter procedure is used only if a trip is to be made anyway. Multi-shot surveys of this type always require pulling of the pipe, so that the instrument remains in the K-monel collar. A time vs. depth log (using a synchronised surface watch) is kept, so that the depth of each picture may be determined.
A unique surveying device is the gyroscopic instrument (Surwel) shown in Figure 9.24. This utilizes the ability of a gyroscope to maintain the same directional orientation over a considerable time period. Rotation at 10,000 to 15,000 rpm is induced by power from batteries contained in the instrument. The timing device is set to take pictures at the desired intervals with a record of time, vertical angle, and direction being recorded. A separate log of depth vs. time is again kept at the surface with a synchronized watch. Surveys are taken both going in and coming out of the hole. This is the equivalent of two separate sets of measurements which serve to check each other. This device may be run on drill pipe, tubing, or other standard rig equipment, and is the only instrument which can be run inside the casing on a wire line.
An interesting feature of directional drilling is the orientation of primary deflection tools. It is desirable, of course, to start the initial deflection in the proper direction. This requires that the bottom hole position of the whipstock or knuckle joint be known, before it is set. Two basic methods of orienting deflection tools are in use:
(1) Drill pipe alignment method: this consists of keeping accurate track of the drill pipe's rotation as it is run into the hole. The following procedure refers to Figure 9.25. The deflection tool is affixed to the drill pipe and faced toward the desired direction (B-B). A sighting bar is fastened to the drill pipe and aimed at some convenient point such as the derrick leg (point C). The next stand of pipe is then attached to the string. The derrick man puts a second clamp near the top of this section and aligns it with the lower clamp's sighting bar by means of a cross-hair telescope (D). After the section is aligned, the telescope, lower clamp, and sighting bar are removed and the stand is lowered into the hole. The removed clamp is sent up on the elevators to the derrick man. The sighting bar is now inserted in the new floor
Diagrammatic representation of drill pipe alignment procedure
Fig. 9.25. Drill pipe alignment method for orientation of deflection tools. Courtesy Eastman Oil Well Survey Company.
clamp and the process is repeated until all the pipe is in the hole. This procedure keeps a constant angle between the tool face and the sighting bar. When the tool reaches bottom, the desired orientation is obtained by rotating the pipe to its original position.
This method is slow and is subject to error in deep wells due to unavoidable torsional stresses in the pipe which do not allow it to hang perfectly free. Consequently, this technique is little used at this time, except in shallow wells.
(2) Bottom hole orientation methods: there are several instruments and techniques used which allow the rapid and accurate bottom hole orientation of deflection tools. Only one rather typical procedure will be discussed as illustrated by Figure 9.26. A special K-monel substitute (or sub) is inserted in the string just above the deflection tool. This sub contains permanent magnets as shown, the positions of which are known with respect to the tool face. A single shot instrument containing two magnetic compasses is run with one compass opposite the magnets in the sub. The second compass is sufficiently removed so that it records magnetic north. The photographic record superimposes these readings so that the position ttA
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