Borehole Electrolytic Power Supply

Abstract

Power supply apparatus for generating electrical energy for a sustained period of time within the environment of an earth borehole filled with drilling mud. The power supply is used in a downhole well logging system for supplying electrical energy to circuitry located down within the borehole and includes an elongate magnesium electrode having a plurality of circumferentially spaced, longitudinally extending slots formed therein. The electrode is located in a special sub within the drill pipe and a potential difference is produced between the magnesium electrode and the walls of the steel drill pipe due to electrolytic decay of the metals, when a conductive drilling mud is located therebetween.

Description

BACKGROUND OF THE INVENTION

This invention relates to a system for generating electrical power at a location within an earth borehole containing drilling mud and, more particularly, to a well logging power supply which operates on the electro-chemical erosion of a steel drilling pipe and a magnesium electrode.

In the drilling of earth boreholes, for example to seek sub-surface oil, gas or minerals, it is desirable to obtain measurements of various parameters in situ, that is, measurements which are made downhole within the borehole while drilling is in progress. For example, in order to operate drilling equipment at maximum efficiency, it is useful to obtain borehole environmental parameters such as temperature, pressure, mud weight, vibration, and borehole deviation as well as parameters relating to the earth formations being penetrated by the drill bit such as electrical resistivity and spontaneous potential.

Systems have been proposed which measure various parameters downhole and then either record the information within the downhole instrument package or transmit the information back up the borehole for detection and recording of the information at the surface. In either case, electrical power is generally required at the downhole location in order to operate the measuring instrument package. A principal problem encountered in the design of borehole power supplies is that the downhole environment is relatively hostile in nature with large pressures and high mud flow rates being characteristic.

In the past, electrical power has been either generated downhole, for example by a mud driven turbine generator, or produced by various types of batteries. Mud turbine generators are relatively complex and are subject to wear and other malfunctions which may disable the entire downhole instument package until the complete drill string is pulled and the turbine replaced. Conventional batteries, on the other hand, may not produce an adequate quantity of power for a sustained period of time and further, require elaborate sealing precautions to avoid damage due to high pressures and the flow of liquid drilling mud.

In prior art systems such as that shown in U.S. Pat. No. 3,079,549 to P. W. Martin, it has been suggested that electrical power be generated downhole by making sections of the drill stem of dissimilar materials and utilizing the electromotive force between them. While this technique may be desirable in some aspects, it possesses the disadvantages of an inefficient, longitudinally spaced electrode configuration and electrodes which are subject to wear and damage due to contact with the walls of the borehole, especially if drill stem sections of a material softer than steel are employed.

One embodiment of the system of the present invention utilizes the steel drill pipe as one electrode, a magnesium rod mounted within but insulated from the drill pipe as the other electrode and the mud stream flowing therebetween as an electrolyte to generate an electrical potential. The present system comprises a highly efficient electrode configuration which is relatively safe from damage and which generates an adequate quantity of power over a sustained period of time to operate a a downhole instrument package.

SUMMARY OF THE INVENTION

The invention relates to an electro-chemical system for generating power within an earth borehole due to the potential difference between a drilling pipe and a coaxial electrode of a different material than the pipe which are separated by a flowing stream of conductive drilling mud. More particularly, the invention comprises a downhole power source for generating electrical energy within an earth borehole which includes a hollow cylindrical drilling pipe. A cylindrical electrode is positioned coaxially within the pipe to define an annular space therebetween. The electrode is formed from a material having a different electro-chemical potential than the drilling pipe. A conductive drilling mud is located in the annular space between the electrode and the drilling pipe to produce an electrical potential therebetween.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present invention and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is an elevational view, partially schematic and partially in longitudinal cross-section, illustrating the general location and arrangement of the power supply of the present invention as used in connection with a typical earth borehole drilling rig;

FIG. 2 is a longitudinal sectional view of a portion of the apparatus of FIG. 1, showing the power supply of the present invention;

FIG. 3 is a cross sectional view taken along line 3--3 of FIG. 2;

FIG. 4 is a graph of maximum output power as a function of length of the magnesium electrode used in the present invention, for various resistivities of drilling mud;

and

FIG. 5 is a graph of expected electrode life at maximum power as a function of length, for various resistivities of drilling mud.

DETAILED DESCRIPTION

Referring first to FIG. 1, there is shown a conventional earth borehole drilling rig including a lower uncased portion of the borehole 10 and an upper portion of the borehole 11 in which the usual surface string of casing 12 has been set. A conventional rotary drilling rig is shown with portions above the surface of the borehole and portions within the borehole. The latter portions include a drill pipe 13 having a drill collar 14 and a drill bit 15 attached to the lower end. The drill bit 15 may be rotated by either the drill pipe 13 or by a rotary mud motor,(not shown). A drill stem is shown within the borehole and comprises the drill pipe 13 connected at the upper end through a square Kelly Bar 16 to a swivel 17. The swivel 17 is suspended from a traveling block hook 18, a traveling block 19, drilling lines 20 and a crown block 21 located in the top of a derrick 22. The square Kelly Bar 16 passes through conventional gripping means in a rotary drilling table 25 which is supported in the customary manner upon the derrick floor supports. The rotary table 25 is rotated by means of a conventional bevel gear 26 and a pinion rotary table drive 27. The pinion drive 27 is coupled to be driven in accordance with the usual practice through a shaft 28 by the power unit of a draw works 30.

A body of drilling fluid 31(commonly known as drilling mud) is contained within a mud reservoir or pump 32. A drilling fluid circulation passage extends from the discharge connection 35 of a drilling fluid circulation pump 36, through connecting pipes 37, a riser 38, a suitable flexible connection 39, the swivel 17, the Kelly Bar 16, down the drill pipe 15, through the drill collar 14 and out an opening 40 in the drill bit 13. The fluid travels back up the lower uncased portion of the borehole 10 and through the surface casing 12. The upper end of the surface casing 12, which provides a return path for circulating drilling fluid from the open borehole, is provided with a lateral outlet pipe 42 which extends to and discharges into the drilling fluid reservoir 32. The drilling fluid circulating pump 36 takes suction through a pipe 38 from the body of drilling fluid 39 contained in the mud reservoir 40. A surge chamber 45 may be connected to the discharge connection 35 of the drilling fluid circulating pump 36 for the purpose of smoothing out or reducing the pump discharge pressure fluctuations.

Attached to the lower end of the drill pipe 13 is a special sub 51, the bottom of which is connected to the drill bit 15 through a drill collar 52. The sub 51 is preferably located in the region of the borehole 10 from which environmental parameter measurements are to be made.

Referring to FIG. 2, there is shown a longitudinal cross-section view of the special sub 51 which includes a cylindrical outer casing 53 preferably formed from a section of steel drilling pipe having a female threaded opening 54 at the upper end and a male threaded opening 55 at the lower end. A cylindrical telemetry instrument package 56 which includes d. c. to d. c. converter 57 is positioned within and in axial alignment with the casing 53. The instrument package 56 is attached to and spaced from the inner walls 58 of the casing 53 by means of a plurality of circumferentially spaced struts 59 to define an annular space 60 therebetween. The lower end of the package 56 includes an internally threaded socket 61, which is electrically isolated from the body of the package 56 but electrically connected to one input terminal (not shown) of the d. c. to d. c. converter 57. Attached to the lower end of the package 56 is a generally cylindrical, axially positioned electrode 62, formed from a material, such as magnesium, having a different electrochemical potential than that of the outer casing 53. The electrode 62 is attached to the package 56 by means of an upper threaded section 63 which engages the socket 61. The body of the electrode 62 is electrically isolated from the package 56 by an insulative disk 64. The electrode 63 also includes a lower threaded section 65 which is attached to an insulative, streamlining dome 66.

A flow of drilling mud enters the sub 51 through the top opening 54, flows through the annular space 60 in the direction of arrows 70 and exits through the bottom opening 55.

As just shown in the cross-section view in FIG. 3, the electrode 62 includes a plurality of circumferentially spaced, radially extending, longitudinally disposed slots 67. The slots 67 serve to increase the effective surface area of the electrode 62 and channel drilling fluid flowing through the annular space 60 to keep the electrode surfaces flushed and cleaned of any polarizing film that might tend to develop.

Referring again to FIG. 2, it can be seen that the power supply comprises a primary cell using the magnesium electrode 62 as a sacrificial anode, the steel of the drill pipe casing 53 as a cathode, and the conductive drilling mud in the annular space 60 as an electrolyte. Since magnesium stands at about a plus 2.40 volts in the electromotive series of elements and steel ranges from about a plus 1.15 volts to a plus 1.3 volts, the electrochemical potential of the resultant primary cell is from about 1.1 to 1.25 volts. This potential, when coupled to the input terminals of the d. c. to d. c. converter 57 produces an output voltage on the order of 12 volts, suitable for driving transistor circuitry in the telemetry instrument package 56.

The internal resistance of the primary cell is inversely proportional to the exposed areas of the anode and cathode and directly proportional to the resistivity of the electrolyte in contact with both electrodes. The effective life of the primary cell is directly proportional to the quantity of magnesium in the anode electrode 61 which may be six months or more in normal operation. Power outputs on the order of 15 to 20 watts may be achieved with the system of the present invention.

There are a number of factors, such as particle size, particle density, impurities present, etc. which affect the resistivity of the drilling mud. Preferably, a mud having a resistivity on the order of 10-15 ohm-cm or less is used for the electrolyte in the power supply of the present invention in order to achieve usable power levels on the order of 10 watts or more.

FIG. 4 is a graph of maximum power output in watts as a function of the length of the magnesium electrode for various resistivities of drilling mud. These curves were calculated for a magnesium-iron cell with an anode electrode having a diameter of 3 inches. The curves are further based on an open circuit output voltage from the cell of 1.15 volts and a surface resistance of the magnesium electrode as calculated by the following modified Dwight's formula:

R=(0.012) (P) (Log.sub.10 [2L/q])/L

wherein, R = contact resistance of anode in ohms;

P = resistivity of electrolyte in ohm-cm;

L = length in feet; and

q = radius in feet (assumed 0.25 ft.).

FIG. 5 is a graph of expected cell life in years for a cell operating at maximum output power as a function of the length of the magnesium electrode for various resistivities of drilling mud. These curves were calculated for a magnesium iron cell with anode electrode 3.1416 sq. in. in cross-sectional area and a specific gravity of 1.74. Also used was a rule of thumb that 17 pounds of magnesium will last 1 year as a sacrificial anode with a 1 amp drain. As can be seen from FIG. 5, for mud resistivities on the order of 10 ohm-cm, a life expectancy of 6 months or more can be anticipated.

Thus, in accordance with the invention, it can be seen how an efficient down-hole power supply can be constructed by arranging a cylindrical anode coaxially within a steel drilling pipe, serving as the cathode, and flowing drilling mud through the annular space therebetween. A potential is produced from the cell depending on the relative electrochemical potentials of the anode and cathode materials. The output voltage is preferably amplified by a d. c. to d. c. converter to bring it to usable voltage levels.

Having discussed the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.

Claims

1. A downhole power source for generating electrical energy within an earth borehole, comprising: a hollow cylindrical drilling pipe; a cylindrical electrode positioned coaxially within said pipe to define an annular space therebetween and having a plurality of circumferentially spaced, longitudinally extending slots formed therein, said electrode being formed from a material having a different electrochemical potential than said drilling pipe; a plurality of radially extending, circumferentially-spaced struts between said pipe and said electrode to hold said electrode in position; means for insulating said electrode from said drilling pipe; and a conductive drilling mud located in the annular space between said electrode and said drilling pipe to produce an electrical potential

2. A downhole power source for generating electrical energy within an earth borehole as set forth in claim 1 wherein said drilling pipe is made of

3. A downhole power source for generating electrical energy within an earth borehole, as set forth in claim 1, which also includes:

means for moving said conductive drilling mud through said annular space to

4. A downhole power source for generating electrical energy within an earth borehole, as set forth in claim 1, which also includes:

a d. c. to d. c. converter having its input terminals connected between said electrode and said drilling pipe to amplify the potential

5. A downhole power source for generating electrical energy within an earth borehole, as set forth in claim 1,

wherein, the resistivity of said conductive drilling mud is less than 15

6. A special telemetry sub for inclusion in the drill pipe string of an earth borehole drilling apparatus, said sub comprising:

a section of hollow steel drill pipe having threads at either end for engagement with drill pipes within a string;

an elongate cylindrical telemetry instrument package positioned coaxially within said pipe to define an annular space therebetween, said package being held in position by a plurality of radially extending, circumferentially spaced struts;

a cylindrical electrode attached to and insulated from the end of said coaxial instrument package, said electrode being formed from a material having a different electrochemical potential than said drill pipe;

means for connecting to said instrument package the potential generated between said electrode and said drill pipe when a conductive drilling mud

7. A special telemetry sub as set forth in claim 6 wherein:

said telemetry instrument package includes a d. c. to d. c. converter having the input thereof connected across said drill pipe and said electrode to power said package from the potential difference

8. A special telemetry sub as set forth in claim 6 wherein:

9. A special telemetry sub as set forth in claim 8 wherein:

said cylindrical electrode includes a plurality of circumferentially spaced, longitudinally extending slots formed therein to increase the effective surface area thereof.