Repulsing clays on drill bits

Abstract

bit balling is prevented by impressing a negative electrical charge on the drill bit by the use of bi-metallic electromotive potential differences, batteries or other electrical energy sources.

Description

BACKGROUND OF THE DISCLOSURE

This invention relates to the prevention of bit balling while drilling wells through fine formation materials such as sticking clays.

A problem is encountered in drilling through earth formations which contain materials which stick to the 32 31 with teeth 34, thereon. Drill bits usually are made of a case hardened steel or alloy and drill bit teeth usually are coated with a tungsten carbide material which has a high resistance to abrasion. Located at 32 between teeth 34 are magnesium inserts to provide this material directly adjacent the drill bit teeth. Because magnesium is a fairly soft material it may be preferable to locate the magnesium material in the interior of the bit or in areas on the bit exterior which are not subject to extensive wear.

For the purposes of the following discussion, it is presumed that formation 16 contains fine formation materials such as montmorillonite clay or a large amount of other small size clay which have similar adhesive properties.

In drilling into a formation 16 having fine materials which cause sticking and/or swelling the usual problem is the hydration of the clays. As the drill bit 12 engages the formation 16 the drill bit teeth 34 tend to shear apart portions of the hydrated clay. When these clays are partially dispersed they exert their maximum plasticity and have a large negative charge. Because of this large negative charge the clays are attracted to the drill bit and drill collars.

The magnesium located on the drill bit sub 10 has a high electromotive force which impresses a negative charge on the drill bit 12. Magnesium having the greater negative voltage acts as the anode and the drill bit having the lesser negative voltage acts as the cathode. This is best illustrated by reference to FIG. 1A where the magnesium ring is shown as the anode at the left and the drill bit is shown as the cathode on the right. Drilling fluid is shown in between as an electrolyte and the metallic pathway between the drill bit and magnesium connects the anode and cathode to complete the circuit.

As illustrated by FIG. 1 current flow follows the metallic pathway of the drill string from the magnesium to the drill bit when there is drilling fluid present to act as an electrolyte. Because of the potential difference between the magnesium anode and the drill bit cathode there is migration of electrons from the magnesium to the drill bit along the metallic drill string pathway. The excess electrons received at the drill bit cathode will combine with positively charged hydrogen ions in the electrolyte to form free hydrogen. With this loss of electrons, magnesium ions at the anode react with hydroxyl ions. The formation of hydrogen at the cathode drill bit also aids in preventing clays from sticking by covering the surface of the drill bit. By this process the drill bit is protected from sticking clays with the magnesium acting as a sacrificial anode.

In the practical galvanic series commercially pure magnesium exhibits -1.75 volts while clean mild steel exhibits from -0.5 to -0.8 volts. Shown below is a list of metals showing their voltage measured relative to a saturated copper-cooper sulfate reference cell.

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PRACTICAL GALVANIC SERIES
(measured relative to a copper-copper sulfate reference
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electrode)
Metal                   Volts
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Commercial pure magnesium
-1.75
Magnesium alloy (6%, Al, 3% Zn, 0.15% Mg)
-1.6
Zinc                    -1.1
Aluminum alloy (5% Zn)  -1.05
Comm. pure aluminum     -0.8
Mild steel (Clean and shiny)
-0.5 to -0.8
Mild steel rusted       -.02 to -0.5
Cast iron               -0.5
Copper, brass, bronze   -0.2
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In the event the repelling force created by the magnesium drill bit sub on the drill bit is not sufficient to repel the clay it may be necessary to place the magnesium on the drill bit 12, itself. To prevent bit balling which is a term used for an excessive build up of clay on the drill bit, it may be necessary to place magnesium inserts between the drill bit teeth. This provides a multiplicity of sacrificial anodes immediately adjacent the cutting teeth. Another location for positioning the magnesium could be between the rows of teeth on the drill bit cone 32 31. Because the magnesium is not very resistant to abrasion, it may be necessary to locate the magnesium on the portion of the drill bit exterior which does not directly contact the formation 16. Since all the exterior of the bit may come into contact with the formation at one time or another, it may be preferable to position the magnesium in the interior bore of the bit.

In addition to protecting the drill bit, magnesium rings can be utilized to protect stabilizers, hole openers or any other area of the drill string where sticking clays, etc. disrupt the drilling operation.

Referring next to FIG. 2 there is seen a standard drill string 11 located in a wellbore 38 which has been drilled into a formation 16 having fine materials including clays. At the end of the drill string 11 is a standard drill bit 12 having teeth 14 thereon. These teeth 14 are shown engaging the bottom 36 of wellbore 38. Located immediately above the drill bit 12 is a drill bit sub 10. The sub is one of standard design and has been machined down to reduce the diameter of the mid portion. An annular ring of insulating material 44 having retaining shoulders at the top and bottom is located around the machined diameter of the sub 10. Located between the retaining shoulders of the insulating material 44 is sacrificial anode material 46 such as high silicon iron. Located at the surface 40 is the top of the well 20 and a source of electrical energy 24 such as a generator or a connection with commercial electric lines. An insulated electric line 22 extends between the source of electrical energy 24 and the sacrificial anode material 46. This electrical line 22 is shown exiting the drill string 11 at 26 and is connected with the generator 24. Generator 24 is grounded to the well pipe by electrical connector 50. The electrical line 22 has been shown in the interior of the drill string, however, if mud flow down the interior of the drill string proves troublesome it may be preferable to locate this electric line 22 on the exterior of the drill string.

With the apparatus described in FIG. 2 a negative electric charge can be impressed on the drill bit 12 by supplying electric energy from source 24 through line 22 which is connected with the sacrificial anode 46. This anode then acts in the same manner as the magnesium ring described in FIG. 1. One advantage of this procedure is the ability to impress a larger negative charge on the drill bit than is possible with the bi-metallic potential difference created by the magnesium ring 28 18 described in FIG. 1. By appropriate switching, energy can be supplied to protect the drill bit only at such times as sticking formations are being drilled. With the negative charge impressed on the drill bit, the negatively charged clay is repelled and therefore does not stick to the drill bit. In addition, the drill collars can also be negatively charged to repel sticking clays by similar apparatus at appropriate locations. One disadvantage of this system is that it cannot be effectively utilized in deeper wells due to the loss in current caused by the internal resistance of the electric line 22.

FIG. 3 describes a system which is effective in deeper formations than those which can effectively be protected by the apparatus described in FIG. 2. The lower portion of a drill string is located in a wellbore 38 which penetrates formation 16 which includes sticking materials. The drill string comprises a drill bit sub 10 below which is a drill bit 12 which is shown engaging the bottom 36 of formation 16. The drill bit 12 has drill bit cones 32 31 having teeth 14 thereon. Shown in the interior of drill bit sub 10 is battery pack 28 located along the wall of drill bit sub 10. The battery pack 28 includes a multiplicity of batteries which are resistant to high temperatures and which are well insulated such that they will not be contaminated by drilling fluid or other liquids. An electrical connection 30 connects the battery pack 28 with a sacrificial anode material 46, insulated by material 44 such as a fiberized silica-epoxy material. The insulated anode is located on a machined down portion 42 of the sub 10.

With this apparatus it can easily be seen that a negative charge can be applied to the drill bit 12 by energy supplied by the battery pack 28 to the anode 46 to protect the drill bit 12 in the same manner as described in FIGS. 1 and 2. Thus, when the teeth 14 on the drill bit 22 contact materials to the formation 16 which have a tendency to stick to the drill bit, the charge provided by the battery pack 28 acts to repel the negatively charged fine materials such as montmorillonate clay.

By impressing a negative charge directly adjacent a point in the drill string where repulsion of materials is desired, such as at a drill bit, a reamer or other cutting means or at a stabilizer, by any of the methods shown in FIGS. 1-3 as a great savings can be had when drilling through troublesome formations. Sticking shales and like materials have caused substantial losses of time and money including at times the complete loss of a wellbore. Thus a great savings can be had by preventing these materials from attaching to the drill string.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

Claims

1. process for drilling wellbores in earth formations containing materials which attach to the drill bit and drill collars comprising: positioning a metal with a substantially higher electromotive force than the drill bit at a point in the drill string directly adjacent the drill bit and between the drill collars and the drill bit for impressing a negative electrical charge on the drill bit during such time as the attaching materials are being drilled.

2. The process of claim 1 wherein the high electromotive force metallic member is an annularly shaped magnesium metal which is press fit on a conventional drill bit sub.

3. A process for drilling wellbores through a sticking shale earth formation comprising: applying a rotating drill bit to the formation; and impressing a negative charge to the drill bit while drilling through the shale formation by locating directly adjacent the drill bit a metal having a higher electromotive force than that of the drill bit.

4. Apparatus for drilling through earth formations having a high concentration of fine materials including clays comprising: a drill bit having teeth; drill pipe attached to the drill bit and extending to the surface; and means for impressing a negative charge on the drill bit, said impressing means being a metal which is located between the teeth on the drill bit , said metal including magnesium.

5. The apparatus of claim 4 wherein the metal includes magnesium and is located between the teeth of the drill bit.

A method for preventing the attachment of certain materials to a drill bit when drilling through earth formations having a high concentration of materials which attach to the drill bit comprising: positioning a metal member with a higher electromotive force than the drill bit in the drill string directly adjacent the drill bit and below drill collars in the drill string, rotating the drill string, and applying the rotating drill

bit to the formation. 7. The method of claim 6 and further including positioning the metal member in the drill string by press fitting an

annular metal sleeve on the drill bit sub. 8. Apparatus for preventing the attachment of certain materials to a drill bit when drilling through earth formations having a high concentration of materials which attach to the drill bit, including: a drill string extending to the surface and having drill collars and a drill bit at its lower end; means including a metal member having a higher electromotive force than said drill bit positioned in the said drill string below the said drill collars for impressing a negative charge on the said drill bit.

9. The apparatus of claim 8 wherein the impressing means is a metal member having a higher

electromotive force than the drill bit. 10. The apparatus of claim 8 wherein the impressing means is a magnesium sleeve press fitted on the drill bit sub.