BACKGROUND OF THE INVENTION
This invention relates to electrical cable, and particularly to electrical cable utilized to deliver electrical energy to submersible motors adapted for use in high temperature, high pressure oil wells.
Submersible pumps used in oil, mineral and water wells normally include a prime mover in the form of an electric motor directly coupled to the pump and disposed deep within the well. It is necessary to provide an electrical connection between the motor and a source of electrical energy at the surface as by the use of an electrical conducting cable which extends between the source of electrical energy and the motor.
In many instances, the motors operate at relatively high power levels, in some case exceeding 200 horsepower. Normally, the motors used are of the three-phase type and the associated cable includes three separate electrical conductors.
The electrical cable must have adequate current carrying capability and must be of sufficient dielectric strength to prevent electrical losses even under the adverse environmental conditions usually found within the well. The environmental conditions of the well vary generally depending upon geographical location. In some cases the well fluid is highly corrosive and in many instances well temperatures exceed 275°F. Most oil well fluids include brines containing dissolved H2 S gas, carbonates and salts, and large volumes of oil. The fluid pressure in wells may be quite high and in many instances exceeds 4,000 psig. Additionally, the wells are quite deep, averaging 8,000 to 10,000 feet. The electrical cable must possess sufficient physical strength to allow insertion of the motor and cable to these depths and the outer surface of the cable must resist the abrasion associated with insertion. Since the cable is normally wound upon storage or transportion reels, it must possess the additional property of flexibility, so that it will resist physical damage caused by reeling.
Typical cable construction presently being utilized includes three conductors of copper separately insulated are helically wound to form a single unit. The conductors are insulated with a material of high dielectric strength such as polyethylene or polypropylene. The helically wound and insulated conductors are sheathed in an extruded jacket of nitrile rubber surrounding the insulated conductors.
One common form of jacketed cable is covered with an outer armor in the form of a continuous wrapped band of metallic material. This band is lapped as it is wound. The armor provides abrasion resistance. Usually, the armor is formed of steel or bronze; however, in many special applications, such as wells which are excessively corrosive, stainless steel or exotic metals such as monel metal must be used.
Polyethylene has also been employed to a limited extent as the outer armor, but it has been found that the same does not stand up under severely high temperatures.
Proper material selection for the cable armor has always presented difficulties. Many different armor materials must be utilized depending upon the well conditions and no single cable construction has been found suitable for universal application. This is especially true for the deep, high pressure and high temperature wells.
Electrical power cables constructed as previously described which have been used in high temperature, high pressure oil wells, fail because of temperature distortion of the thermoplastic cable components, corrosion of the armor, or chemical and solvent attack of the elastomer jacket. Since most oil wells contain dissolved H2 S gas, carbonates, water, salts and large volumes of oil, no single material has heretofore been found which has the resistance to solvents, heat and pressure to operate for prolonged periods in such an environment.
An additional problem encountered by cable in such an environment is deformation under load. The cables are subject to both compressive and tensile forces and, under high temperatures, there is a marked tendency for the thermoplastic insulation to deform resulting in dislocation of the conductors and phase to phase or phase to ground short circuitry.
Rupture of the armor due to swell of the jacket is another example of deformation which occurs in such an environment. Rupture of the total construction also occurs during retraction of the cable from the well as a result of the depressurization of fluids which have permeated the cable.
These and other associated difficulties have clearly dictated the need for an improved impermeable, environment-insensitive cable construction.
SUMMARY OF THE INVENTION
The present invention relates to an improved multi-component electrical cable for submersible motors adapted for use in high temperature, high pressure oil wells. A cable constructed according to this invention comprises an outer armor of metal, an inner jacket of epichlorohydrin rubber, and high temperature, synthetic, organic insulators surrounding the electrical conductors.
The insulators, according to this invention, are useful in high temperature, high presure oil wells. One of these insulators is a high temperature, high molecular weight, extrudable thermoplastic fluorocarbon polymer, which is an excellent electrical insulator at elevated temperatures when unaffected by oil and brine. While the thermoplastic fluorocarbon is suitable for use in high temperature and pressure oil wells, it is very expensive and, accordingly, a very thin insulation layer is employed. Due to this factor, it is desirable to provide further insulative protection by using, in combination with the fluorocarbon insulator, another insulation in the form of a high temperature thermosetting elastomer rubber, such as ethylene-propylene copolymers and terpolymers, having the required heat resistance and electrical properties necessary for use as commercially manufactured oil well cable insulation.
The epichlorohydrin rubber used for the jacket is compounded for the minimum of oil and water permeability and swell. The epichlorohydrin rubber jacket is oil and water insensitive and impermeable providing a barrier which protects the insulators from any loss of dielectric strength and electrical protection to the conductors. Furthermore, the insulators, being temperature insensitive, will not deform and change electrical insulation thickness. The impermeability of the jacketing further insures that there be no permeating fluids to plasticize or soften the insulators and thus reduce their insensitivity to temperature deformation.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmentary perspective view of a section of electrical conducting cable for submersible motors illustrating various features of the invention;
FIG. 2 is a cross-sectional view of the cable of FIG. 1 taken generally along the lines 2--2 of FIG. 1:
FIG. 3 is a fragmentary perspective view of a section of a second embodiment of electrical conducting cable, similar to FIG. 1; and
FIG. 4 is a cross-sectional view of the cable of FIG. 3 taken generally along the lines 4--4 of FIG. 3.
Referring now to the FIGS. 1 and 2 of the drawings, there is shown a multi-component electrical conducting cable for a submersible motor designed for use in high temperature, high pressure oil wells which is illustrative of the principles of the present invention.
FIG. 1 shows a cable section which includes conductors 11, a resilient jacket 13 and an outer armor 15.
Each conductor 11 is illustrated as being formed of stranded wire 17 helically wound to prevent separation of strands. These separate strands may be tinned to minimize chemical interaction between the conductor and the insulating material. Solid conductors may be used without departing from the spirit of the invention.
In the illustrated embodiment each conductor includes seven strands. The number of conductors, the diameter of the conductor and number of wires is, of course, dependent upon the load carrying capabilities required for a particular cable application. It should additionally be noted that any suitable conducting material may be used, such as, for example, copper, aluminum.
Each wound set of wire strands forms a single conductor and is separately insulated by an insulation layer 19. The conductor insulation 19 is formed of a high temperature, organic, synthetic material of high dielectric strength. High temperature, high molecular weight, extrudable fluorocarbon polymers have been found to be satisfactory for this purpose. The thickness of the layer 19 may have a different relationship to the dimensions of the other elements of the cable than what is inferred by the drawing. However, the drawing illustrates the various elements which make up the cable without reference to actual dimensions.
A preferred embodiment of a high temperature, high molecular weight, extrudable fluorocarbon polymer is a 1:1 copolymer of ethylene and chlorotrifluoroethylene insulating material, commercially available under the trade name HALAR from Allied Chemical Co. This material has a formula
(CH2 CH2 CF2 CFCl)n
and has been found to possess the following physical properties:
Tensile Strength at 23° C. 7000 psi at 340° F. 400 psi Elongation at 23° C. 200% at 340° F. 350% Melting Point 460° F. Dielectric Strength Initial 18.7 KV Dielectric Strength, 4 days at 410° F. 15.0 KV Cut Through, 1000 g., at 120° F. 137 hrs.
It will be apparent that the extrudable ethylene-chlorotrifluoroethylene copolymer is particularly suited for use in cable constructions intended for applications to oil well environments. Despite the fact of being a thermoplastic and therefore capable of melting and flowing under high temperature and pressures, it retains physical strength at elevated temperatures and dielectric strength at temperatures as high at 410°F. The resistance of the ethylene-chlorotrifluoroethylene copolymer to high temperature and high pressure fluid environments can also be improved by irradiation and by sheathing in an oil-and-brine resistant jacket. The comparative properties of irradiated and unirradiated films of the ethylene-chlorotrifluoroethylene copolymer are:
Solvent Swell** High Temperature After Exposure Deformation 4500 psig, 275° 50 psi, 275° F. 7 Days ______________________________________ ______________________________________ ASTM1-A oil BRINE ______________________________________ HALAR, unirradiated 50% elongated, 2.0% -2.5% broke in 1 day HALAR, 10 megarad 50% elongation, 0.5% -0.5% exposure* no break, 5 days HALAR, 50 megarad 10% elongation, -0.5% 2.0% exposure* no break, 5 days High Temperature Polyproplylene 10% elongation 40% 1% broke first day ______________________________________ *Exposure to Co60 irradiation source. **Per cent change in volume
It will be apparent that for use under very stringent well conditions, wire insulation will be most suited if irradiated, and the irradiated insulation is a preferred embodiment of this invention. It will also be apparent that other extrudable thermoplastic fluorocarbon polymers may be employed for this purpose. Such compositions as DuPont's commercial PFA polymers Teflon 9704 and Teflon 9705, the known FEP polymeric materials, and the commercially available FEP/PE copolymers, such as Dupont's Tefzel products and other extrudable fluorocarbon polymers and copolymers which possess the necessary extrusion characteristics are considered to be equivalent materials for this particular purpose, and therefore within the scope of the present invention.
While the above-described thermoplastics are exceptionally qualified for oil well cables in view of their extrudability and resistance to a very high temperature and high pressure fluid environment, they are very expensive and, accordingly, in use must be necessarily restricted to a required minimum quantity to provide economical manufacture of the cables. Therefore, a further protective layer is necessarily present.
To provide the further protective layer, a layer of an insulating material having the needed heat resistant and electrical properties, such as ethylene propylene copolymers and terpolymers, which is suitable for oil well cable insulation is employed to cover the thermoplastic insulation layer 19 as the conductor insulation 21.
An example of a suitable class of such an insulating material is heat-stablized ethylene-propylene copolymer rubber, including ethylene-propylene terpolymer rubber. One example of such materials is found described in U.S. Pat. No. 2,933,480.
Further protection at the insulated wire is necessary in order that it be usefully employed over the great lengths necessary to extend to the bottoms of oil wells. In order to minimize entangling, rupture and similar damage while being installed down a well casing together with the pump motor, pump, ancilliary equipment and the production pipe, it is necessary to further jacket the insulated wire with an oil and brine-resistant outer covering. While many materials have been employed in the past for such purposes, including nitrile rubber and neoprene, and they have found general application in shallow, low temperature well environments, they have been generally unsuitable for the high temperature and high pressure deep well environments. For the practice of this invention, it is essential that the flexible jacketing material resist oil and brine under the bottom hole conditions and that the wire insulation be protected to avoid permeation of gases and fluids which would cause rupture on depressurization and to avoid permeation by oil to plasticize or by water to reduce electrical resistance.
The wound conductor unit is thus disposed within the jacket 13 which is comprised of a high molecular weight epichlorohydrin rubber compounded for the minimum of oil and water permeability and swell. This jacket may be extruded about the wound conductors and preferably is formed to fill interstices 23 between adjacent conductors.
One preferred embodiment is a formulation of epichlorohydrin compounded of the materials and in approximately the ratios as follows:
TRADE NAME MATERIAL PARTS/100 PARTS OF RUBBER AVAILABLE FROM __________________________________________________________________________ Herclor H High molecular weight epichlorohydrin rubber 100.0 Hercules, Inc. Span 60 Surface active agent comprised of partial esters of hexitol anhydrides 1.5 Atlas Chemical Industries Dyphos XL Di-Basic Lead Phosphite (heat stabilizer) 10.0 National Lead Company N B C Nickel dibutyl dithiocarbamate (anti-oxidant) 1.0 Du Pont Cumate Copper, dimethyl dithiocarbamate (accelerator) 0.125 R. T. Vanderbilt Company Phenothiazine Phenothiazine 1.0 Fisher Scientific Company Vulcan Carbon Black (filler) 30.0 Cabot Corporation Hi Sil 233 Silica (filler) 10.0 P.P.G. Industries TE-70 Plasticizers 0.5 Technical Processing, Inc. TP-95 Plasticizers 1.0 Thiokol Chemical Corp. Azelaic Acid Dispersant 4.0 Eastman Organic Chemicals NA-22 2 mercaptothiazoline (accelerator) 1.0 Du Pont __________________________________________________________________________
The extruded epichlorohydrin jacket completely fills the voids formed about the separately insulated conductors. This precludes exposure of the insulation to well fluid and further prevents flow of well fluid along the cable length in the event that a rupture occurs at some point along the outer armor 15 and the outer periphery of jacket 13.
The jacket 13 of the cable is surrounded by an outer armor 15 such as metal as previously stated.
Use of a multi-component cable such as that described provides a cable construction which is abrasion resistant, impervious to well fluids, flexible and unaffected by corrosive well environments and high temperature.
A typical well cable, constructed according to the present invention, includes three seven-wire stranded copper conductors, each of which is surrounded and covered by a layer of high temperature, high molecular weight, extrudable 1:1 copolymer of ethylene and chlorotrifluoroethylene insulation 19 having an average thickness of 0.010 inch to 0.080 inch. The insulation layer 19 is covered by a layer of a high temperature thermosetting rubber in the form of an elastomer, such as ethylene propylene copolymers and terpolymers, having needed heat resistance and electrically protective properties, having an average thickness of 0.10 inch to 0.80 inch. The separate conductors are helically wound to form a single unit. The insulated conductors are helically wound to form a single unit. The insulated conductors are jacketed with an epichlorohydrin rubber. The jacket thickness is 0.040 inch minimum average. The jacket is provided with additional protection by surrounding it with a metal armor which may be wound in a conventional manner.
Another and alternate embodiment of the cable, according to this invention, is illustrated in FIGS. 3 and 4. In this embodiment, the parts are identified by a suffix A; they are the same as described with reference to FIGS. 1 and 2. It will be observed that the thermosetting rubber or elastomer insulation layer 21A surrounds and covers the three seven-wire stranded copper conductors 11 and provides the primary insulation cover, and the fluorocarbon insulation layer 19A surrounds the layer 21A and provides the secondary insulation cover. Because of the cost of the extrudable fluorocarbon, a minimum quantity of the insulation layer 21A is used. The layer 21A is surrounded by the jacketing layer 23A of epichlorohydrin rubber. The insulation layer 24A is extruded onto the insulated conductors 11A.
As can be appreciated, the cable construction of the present invention provides an efficient and durable conducting unit for use in the adverse environment associated with high temperature, high pressure oil wells.
While the epichlorohydrin rubber utilized for the jacket 13 and 13A is described as HERCLOR H (Hercules, Inc.), an epichlorohydrin homopolymer [poly (alpha - chloropropylene oxide) ], other homopolymers of epichlorohydrin such as HYDRIN 100 (B. F. Goodrich) are suitable for this application. Also epichlorohydrin rubbers prepared from epichlorohydrin and ethylene oxide are suitable for the jacket 13. These copolymers are sold under the trade name HERCLOR C (Hercules, Inc.) and HYDRIN 200 (B. F. Goodrich).
The cable is illustrated as being substantially round in section; it should be understood that this invention also comtemplates a flat cable configuration in which the conductors are in side-by-side relationship.