Apparatus and method for drilling with a flexible shaft

Field of the Invention

This invention relates to the field of investigating earth formations surrounding a borehole using a flexible shaft to drill perforations through a borehole wall and into the earth formation.

Background of the Invention

The use of a flexible shaft in drilling operations has been done for years. A number of drilling systems have been proposed where the drilling bit is driven by a flexible shaft. One such system that can be implemented in oil and gas production is described in U. S. Patent 4,658,916 (Bond). This patent utilizes a flexible drill shaft that is operable primarily from the vertical borehole when drilling in the formation in a direction that is along a generally horizontal path for a significant distance of lateral drilling away from the borehole thereby to enlarge formation contract area. U.S. patent 4,226,288 (Collins) discloses apparatus for use in a borehole for boring a side hole into the strata surrounding the borehole, the apparatus including a flexible shaft with a drill bit at its end. A worn drill bit may be replaced without selection from a magazine, the magazine being positioned to feed or drop single bits by gravity serially to a cradle which holds the bit for attachment to the shaft. U.S. patent 4,065,845 (Strom et al) discloses a device for changing bore crowns on a drill rod, the device including a rotatably mounted bore crown magazine member, and means for automatically stopping and locking the magazine member in a desired rotated position.

Generally, the motivation for using a flexible shaft is to overcome space limitations on the drilling equipment. A flexible drilling shaft will enable the drilling of a hole which is deeper than the headroom available above the hole to be drilled. For example, in the coal mining industry, roof bolt holes are drilled into the ceiling of coal seams to a depth which can reach three times the height of the coal seem itself. In oil and gas wells it is often necessary to drill holes perpendicular to the borehole wall which are deeper than the internal diameter of the borehole. This need also applies in cased wells. In these situations, to drill such holes requires a system where a flexible drilling shaft is fed around a bend into the hole as the drilling progresses. It is important to note that the available space in these cased wells is far smaller than in previous flexible drilling shaft applications. Rather than three feet of height in coal mines, inner diameters of cased wells tend to be five inches or less. Therefore, the drilling mechanism and the flexible shaft must be much smaller in scale.

For cased well applications, a flexible shaft, with fittings at both ends, is operated in a tubing of fixed curvature. The fittings are used to permit easy connection of the shaft to another assembly, such as the drive motor shaft and the drill bit. To facilitate drilling, the drill bit not only must be torqued so that it rotates about it's central axis (measured in "revolutions per minute" or "RPM"), but also it must be thrusted against the material to be drilled. This thrust is referred to as "weight-on-bit" or "WOB". In a drilling system that uses a flexible drilling shaft, both of these forces are typically applied to the bit through the flexshaft. An analysis of a flexible shaft in operation would yield an aggregate force balance of torques, moments and axial forces, each which would produce a deformation of the shaft.

During drilling of the steel casing, it has been found that the shafts experience large axial compressive forces. These forces tend to induce helixing and shorten the effective length of the shafts. Also, due to the high stress, the shaft life will be shortened. It is desirable to have a long shaft life not only for system reliability, but also to increase the allowable number of drilled holes before one must retrieve the mechanism from the well and replace the worn shaft. Thus, it is important to minimize, or eliminate, the stress elements within the shaft.

Another problem that has been recognized with such systems is the dulling of the drill bit. After perforating the steel casing, the flexible shaft must continue applying torque and thrust, albeit at lower values, while the drill bit cuts through several inches of cement. Then, in many cases, it is desirable to continue drilling into the rock, which is typically shale, limestone, or sandstone. A common component of many of these formations is quartz, a crystalline substance that is much harder than any cutting edge of typical drill bits (except for diamond, which cannot be used as it cannot drill through steel). These quartz particles dull the bit enough so that it requires higher values of torque and WOB in order to continue drilling.

Though these increased values do not pose a problem in the cement or rock (as the initial torque and thrust were very low), they do while trying to drill steel in subsequent perforations. As previously noted, the high thrust required in order to successfully drill steel greatly shortens the life of the shaft. Once the bit dulls, the required thrust gets even larger. It has been found that after drilling only a couple of inches into sandstone, the bit is too dull to start another perforation while being driven by a flexible shaft. If one attempts to generate the required thrust, the flexible shaft is often destroyed.

It is an object of this invention to alleviate this problem.

Summary of the Invention

An example of the invention includes a mechanism that allows for the removal and replacement of the used drill bit on the end of the flexible shaft.

In preferred implementations, a particular type of connector is used between the flexible shaft and the bit, a method to attach (and ultimately detach) the bit to the shaft is provided, a cartridge to hold several drill bits is used, and a mechanism that indexes this cartridge for access to all of the bits is provided.

The mechanism that connects the shaft to the drill bit needs to provide a "quick" connection between the bit and shaft. One connection mechanism is commonly referred to as a "bayonet-style" connector. Similar connections are used to attach bayonets to rifles or to connect wires to various stereo components. As the flexible shaft advances toward the backside of the drill bit (which is still held in the cartridge), the shaft is slowly rotated normally in the clockwise direction. This rotation allows the bayonet-style connector to engage. Now connected, the shaft and bit advance toward the material (usually casing) to begin drilling the hole. When the drilling procedure is complete, the flexible shaft is retracted (still turning clockwise) until the dulled bit is back inside the cartridge. At this point, the motor turning the flexible shaft is reversed, and the shaft begins turning counter-clockwise. This allows the bayonet-style connector to disengage, which leaves the used bit in its original place in the cartridge.

The cartridge that holds the bits can be of many different designs. One of the designs that fits best into the geometric constraints of the drilling system is referred to as the "revolver". Much like the cylindrical-shaped cartridge used in a "six-shooter" pistol, this revolver holds at least six drill bits aligned about a radius. After each bit is used and is disengaged from the flexible shaft, the revolver is rotated so that a new bit aligns with the flexible shaft, ready for the next drilling operation. This process can continue until all of the bits within the revolver have been used.

In order to know how many bits have been used and the number of unused bits that remain, there is a need to index the cartridge. There are many ways to index the cartridge, or in this case, the revolver. One of the designs that fits best into the geometric constraints is referred to as the "ratchet" mechanism. After each hole is drilled and the dulled drill bit is deposited into the revolver, a piston is hydraulic actuated. This piston is connected to the base on which the revolver is positioned. As the piston moves in a direction away from the revolver, so does the base and so does the revolver move in that same direction. As the revolver moves, a rotation mechanism causes the revolver to rotate. One such mechanism is a springloaded "finger" engages a saw-toothed groove (in the side of the revolver) and causes the revolver to rotate. The mechanism is designed so that the revolver rotates exactly the amount needed for the next drill bit to align with the flexible shaft. Ball detents in the base can be used to account for any tolerancing errors. In order to reset the ratchet system, the piston is moved back to the previous revolver position. This time, however, the finger slides up the ramp of the saw-tooth groove and does not create enough force to cause any counter rotation of the revolver.

Embodiments of the system of the invention are simple, robust, and can be built into the small diameter tool package capable of passing into the internal diameter of the casing. It constitutes a great improvement over previous flexible shaft drilling systems whereby a single bit was used and, due to the short life of the shaft, only a couple of successive drilling operations could be performed before failure.

Brief Description of the Drawing

Figure 1 is a schematic of a formation testing apparatus that is used in a cased borehole environment.

Figure 2 is an isometric drawing of the drill bit, bayonet-style quick connector and the end of the flexible driveshaft.

Figure 3a is an isometric assembly drawing that illustrates the interaction of the flexible driveshaft, drill bits, and revolver with the drilling system in the starting position.

Figure 3b is an isometric assembly drawing that illustrates the interaction of the flexible driveshaft, drill bits, and revolver with the flexible shaft extended.

Figure 4a is an isometric assembly drawing that illustrates how the hydraulic piston moves the base and revolver with the piston being in a more downward position.

Figure 4b is an isometric assembly drawing that illustrates how the hydraulic piston moves the base and revolver with the piston being in a more central position.

Figure 4c is an isometric assembly drawing that illustrates how the hydraulic piston moves the base and revolver with the piston being in a more upward position.

Figure 5 is a top view (cross-section) of the assembly that illustrates how the ratchet system causes the rotation of the revolver.

Figure 6 is an exemplary flow diagram of the sequence which may be employed.

Figure 7 is a schematic of a revolver used in a plugging embodiment.

Detailed Description of the Preferred Embodiment

Fig. 1 shows an example of the invention in the context of a downhole formation tester that perforates a cased borehole, takes a formation sample and reseals the borehole casing. This cased hole tester is described in a patent application docket number 20.2634 filed concurrently with the present invention. The present invention is described in the context of drilling multiple holes through the casing material of a cased borehole. However, the focus of the present invention is on improving the perforating function.

In Fig. 2, a drill bit, 1 is shown in line with the flexible driveshaft 2. This drill bit has a length somewhat greater than the thickness of the casing to be drilled and a diameter somewhat greater than the diameter of the flexible driveshaft 2 and coupling 4. To connect the driveshaft 2 to the drill bit 1, the driveshaft 2 must be rotated in a clockwise direction as the two elements (2 and 4) come together. Pins 3 will eventually insert into grooves 5 which locks the drill bit 1 to the driveshaft 2 (as long as the driveshaft 2 maintains a clockwise rotation while drilling). Driving the drill via a flexible shaft allows drilling a hole to a depth greater than the diameter of the drilling apparatus. A translating drive system which can apply both torque and thrust to the flexible driveshaft which is needed and shown in Fig. 1.

In Fig. 3a, the top assembly drawing shows a cut-away view of the block 6 with the drilling system in the starting position. The flexible driveshaft 2 is forced to bend ninety degrees by the two guide plates 8. The coupling 4 is in slidable contact with the base 9. The revolver 10 is attached to the base 9 via a screw and bearing (not shown). This screw and bearing allow the revolver to rotate relative to the base 9. In this version of the assembly, there is room for six drill bits (1 and 12 shown) aligned about a radius around the center of the revolver. Note that drill bit 1 is aligned with the coupling 4, ready for attachment. Drill bit 12 is not currently aligned with the coupling.

Figure 3b shows a cut-away view of the block 6 with the drilling system in the process of perforating the casing. The flexible driveshaft 2 turns in the clockwise direction while the coupling 4 mates with the drill bit 1 as previously described. Then, using a motor-driven system (see Fig. 1), the flexshaft is advanced out into the casing, cement, and rock while creating the hole.

In Fig. 4a, the top cut-away view of the block 6 shows the drilling system back into its starting position. Drill bit 1 has just finished the perforation and is now disconnected from the coupling 4. It is now required of the system to replace bit 1 with a new sharp bit (in this case bit 12).

In Fig. 4b, the piston 7 is shown to be sliding along a bore within the block 6. This movement is accomplished by using hydraulic fluid and proper and conventional valve techniques. As the piston slides from down to up, the plates 8 (which are rigidly connected to the piston) must also slide in the direction of the piston movement. The plate movement causes the base 9 to move upward as well. Because the revolver 10 is attached to the base 9, it must also slide. In addition to this linear motion, the revolver also rotates about axis 11. This rotation of the revolver is caused by a ratchet mechanism, which will be described in Fig. 5.

In Fig. 4c, the sliding motion of piston 7 through the bore within block 6 is complete. Note that the revolver 10 has also completed its rotation, whereby drill bit 12 is now aligned with the coupling 4. Hidden behind drill bit 12 (and not shown in this view) is the used dull drill bit 1, which is no longer aligned with the coupling 4. In order to ready the system for the next perforation, piston 7 must be fully reset back to its position shown in Fig. 4a.

As previously mentioned, the rotation of the revolver is caused by the ratchet mechanism shown in Fig. 5. In this cross-sectional top view of the revolver and ratchet system, piston 7 attached to the revolver base 9, not shown, via guide plates 8. The piston moves back and forth causing the guide plates, base and revolver to move in the same direction as the piston. As the revolver 10 begins the linear motion as indicated by the arrow, the saw-toothed groove 15 is contacted by the finger 17. The finger 17 is attached to mount 18, which is rigidly attached to the block 6 via the probe 19. As the revolver 10 continues the linear motion, this contact between groove 15 and finger 17 forces the revolver 10 to rotate about axis 11. This rotation moves drill bit 1 (which is shown to be directly over the unseen coupling 4) counter-clockwise. In addition, it moves all the drill bits through the same rotation. This rotation allows the new drill bit 12 to ultimately align with the coupling.

As shown in Fig. 5, there can also be another finger 20 positioned at the bottom of the slot in probe 19. When this finger 20 is added to the ratchet mechanism, the design constraints are somewhat simplified. That is rather than relying on finger 17 to fully rotate the revolver 10, this upgraded system only requires finger 17 to rotate the revolver 10 halfway. On the return linear motion (from the right to the left), finger 20 contacts another saw-toothed groove, and finishes the counter-clockwise rotation so that the new drill bit 12 is ultimately aligned with the coupling 4.

Figure 6 is an example of the sequence of the drilling operation performed. To begin the operation, driveshaft 2 and attached quick connector 4 are rotated in a clockwise manner block 30. The driveshaft is advanced toward the drill bit cartridge until the quick connector engages a drill bit in the cartridge that is aligned with the connector block 31. After the connector engages the drill bit, the RPM's (revolutions per minute) of the driveshaft are increased to prepare for the actual drilling procedure block 32. The drilling procedure then occurs as indicated in block 33. At the completion of the drilling procedure, the RPM's of the driveshaft are decreased to prepare for the detachment of the drill bit block 34. While still rotating in the clockwise direction, the flexible shaft and drill bit are retracted until the bit is back in original position in the cartridge 35. Now that the drill bit is in its original position, the rotation of the driveshaft is reversed until the drill bit detached from the shaft 36. The next step is to retract the flexible shaft into the tool 37 to permit the rotating of the revolver 38. The revolver is rotated via a hydraulically activated piston 7. The revolver is rotated as shown in Fig. 5. Once the revolver is rotated and the next drill bit to be used is aligned with the flexible shaft 2, the hydraulically actuated piston is returned to its original position 39. Now the system is ready to repeat the process and drill another hole 40.

Another embodiment is shown in Fig. 7. This isometric drawing shows a revolver 10 with the usual barrels 14 for the six drill bits. As previously described, these barrels are aligned about a radius around the central axis 11. In addition to this, another concentric series of six barrels 16 have been added. These barrels 16 contain the plugs that are used to reseal the perforations as needed by the tool shown in Fig. 1. However, it is important to note that the inventors recognize that the revolver can house more that just drill bits and the rotation motion can be used to index a multitude of operations.

Although a revolver type cartridge embodiment is described herein, there are types of cartridges that can be used in this invention. One such cartridge can have alternating bits and plugs stacked consecutively in a magazine. Appropriate means can be connected to the magazine to align bits and plugs for desired drilling and plugging operations.

In addition, the revolver concepts can be implemented in embodiments other than those described herein. The revolver has applications in any operation or drilling system where multiple drilling operations occur during a single borehole run of a tool.

The method and apparatus of the invention provide a significant advantage over the prior art. The invention has been described in connection with the preferred embodiments. However, the invention is not limited thereto. Changes, variations and modifications to the basic design may be made without departing from the inventive concept in this invention. In addition, these changes, variations and modifications would be obvious to those skilled in the art having the benefit of the foregoing teachings contained in this application. All such changes, variations and modifications are intended to be within the scope of the invention which is limited by the following claims.