Splice for Lead-Coated and Insulated Conductors

This invention relates to a cable splice for an electrical cable and to a method for making a cable splice. More particularly, this invention relates to a cable splice for use in high temperature, high pressure gassy oil wells adapted to prevent depressurization-caused rupture failures and to avoid corrosion and destruction of the conductors by sulfide gases, and to a method for making said cable splices.

In general, cable structures employed for oil well service have stranded metallic conductors, usually formed of copper, separately insulated with a material of high dielectric strength, cabled and surrounded by an outer armor. In joining cable sections, a splice is formed between the conductors comprising a metallic member frictionally engaging the conductors which is insulated by a section of insulant surrounding any remaining exposed areas of the conductors and the metallic member. The section of insulant typically is formed by a plurality of layers of insulating tape, which may be further protected and confined by a heat shrinkable sleeve positioned over the splice insulation and extending over the adjacent insulation to seal the splice area.

Where the well cable is to be employed in highly gassy wells containing corrosive gases, particularly sulfides, invasion of the cable structure by low molecular weight hydrocarbons and corrosive well fluids under high pressure occurs either by way of pinholes or other defects in the primary insulation of the cable or by gas permeation.

When the cable is removed from the well bore, the external pressure is decreased, creating an unbalanced high pressure condition within the splice structure which leads to blow-out or rupture failure. Further, the conductors are particularly susceptible to attack and corrosion by the sulfides, which results in destruction of the conductors and a shortened cable life.

One method for preventing sulfide attack on the conductors has been the coating of the conductor with a sulfide-impervious layer such as metallic lead, a lead-tin alloy, or tin. The coating acts as a barrier to prevent contact of the sulfide-containing fluids with the conductor.

A method for forming a cable splice between conductors coated with lead, lead-tin alloy or tin is needed which would prevent contact of the conductors in the splice area by corrosive well-fluids and which could be carried out rapidly under field conditions.

This invention provides a cable splice that is resistant to blow-out and rupture failure, and which protects the conductors from destruction by corrosive well fluids and consequent corrosive failure, and a method for making said cable splice. More particularly, this invention provides a cable splice having an adhesive layer, a foil layer, a second adhesive layer, an insulation layer formed of a thermosetting elastomeric polymer having a limited degree of porosity, a fabric layer which surrounds and confines the insulation layer and two layers of heat-shrinkable polymer, and a method for splicing and insulating an electrical cable.

The cable conductors to be joined through the splice of this invention will typically comprise a stranded conductor encased in a coating of lead, lead-tin alloy or tin, an insulation layer surrounding the conductors, and fiber braid surrounding the insulation layer. To form the splice the conductors are trimmed to expose undamaged ends, the braid layer, if present, is trimmed to expose a portion of the primary insulation, which also is trimmed to expose a portion of the coated conductor. The exposed insulation is scraped or buffed to remove dirt and roughen the surfaces. The exposed conductor ends are joined by conventional means such as with a deformable metallic sleeve, and the metallic sleeve and any remaining exposed area of the conductors is coated with an adhesive. An elongated strip of foil formed from lead or lead-tin alloy is then applied over the exposed conductors and joint area. The foil, the exposed insulation and a portion of the braid layer is then coated with an adhesive, and insulating tape formed of a thermosetting elastomeric polymer is helically wound over the foil layer and exposed insulation. An elongated strip of fabric is applied over the insulating tape in overlapped relationship and is optionally secured by application of an adhesive to the end portions and lap area. The fabric is then covered with a helically-wound layer of heat-shrinkable tape, and a sleeve member formed of heat-shrinkable tubing is positioned to extend over splice area including any braid-covered portions. Heat sufficient to shrink the heat-shrinkable tape and tubing is then applied to compress and secure the insulation, fabric and foil layers.

The splice made in accordance with this invention is suitable for service when immersed in and surrounded by corrosive well fluids under high temperatures and pressures, and is not susceptible to depressurization failure or corrosion of the conductors. Corrosive well fluids including sulfides are excluded from the conductor area of the splice structure by the lead foil, thus preventing attack on the conductors and subsequent failure. Where minor amounts of low molecular weight well fluids permeate the somewhat porous insulation layer, the fabric layer restrains the insulation layer against swell and rupture from high internal pressures, and the low molecular weight fluids are desorbed without rupture or damage to the insulation.

The FIGURE is a side view in partial section showing a pair of cable ends joined through a cable splice.

Referring now to the drawing, there is shown a pair of cable ends 10 which have metallic conductors 12 having coating 13 formed of lead, lead tin-alloy or tin, covered with insulation 14, and braid layer 15. Conductors of this construction are known in the cable art and do not form a part of the instant invention. The cable ends 10 are joined through cable splice 11, comprising a metallic sleeve 16, first adhesive layer 17, foil layer 18, second adhesive layer 19, insulating tape layer 20, fabric layer 21, heat-shrinkable polymeric tape layer 22 and heat-shrinkable sleeve layer 23.

In forming a cable splice according to the instant invention, cable ends 10 are trimmed to remove a section of the insulation to expose lengths of the conductors 12 and coating 13, and further trimmed to remove a section of braid layer 15 to expose lengths of the insulation 14. The exposed conductors are joined by inserting in a metallic sleeve 16, which is then deformed to frictionally engage the conductors.

The metallic sleeve and any exposed areas of the conductors and coating are then coated with an adhesive 17. Adhesives suitable for the purposes of this invention generally are liquid, preferably solvent-free, adhesives such as, for example, liquid epoxy resins, liquid urethanes, and the like, which cure without heat to form a strong, flexible, water- and oil-impervious bond.The adhesive serves to form a tight bond between the metallic sleeve and the foil layer subsequently applied over the adhesive layer. A layer 18 of foil formed of lead or lead-tin alloy is then applied to the metallic sleeve and any exposed areas of the coated conductors over the adhesive layer. The foil, the exposed areas of the insulation and, preferably, a portion of the braid are then coated with a further layer of adhesive 19.

One or more layers of insulating tape 20 is then applied over the second adhesive layer 19 in the splice area with a helical wrap to cover the exposed insulation. In general, it will be desirable to first apply sufficient insulating tape over the foil-covered metallic sleeve area to fill the area to the level of the adjacent cable insulation, then to complete the layer with a single helical wrap extending over the exposed insulation at each end. The insulating tape is formed of a thermosetting elastomeric polymer which, when cured, is an excellent electrical insulator at elevated temperatures and virtually impervious to attack by oil and other well fluids. An essential property of the polymer is a limited degree of porosity in that it is capable of imbibing very low molecular weight hydrocarbons when under high pressure and then readily desorbing the imbibed gassy hydrocarbons when external pressure is removed. One such material useful for these purposes is a modified EPDM (ethylene-propylene-diene monomer terpolymer) blend such as is disclosed in U.S. Pat. No. 3,926,900. When employed for the purposes of this invention in the form of an insulating tape, the polymer is uncured, the requisite curing being carried out when construction of the splice is completed.

Over the layer of insulating tape 20 is placed a single wrap of fabric 21. The fabric may be applied in the form of an elongated strip wrapped to overlap along the length of the splice. Fabric 21 may alternatively be applied in the form of a fabric tape helically wound along the length of the splice. A number of woven fabric materials may be used for the purposes of this invention including, for example, nylon, polyester, rayon, fluorocarbon, cotton, fabrics formed from glass filament or fiber and the like. It is essential that the fabric withstand exposure to the well fluids without degradation or substantial loss in tensile strength. A liquid adhesive may be applied to the lap area and to the end areas.

Over the fabric layer is applied a single, helically-wound tape layer 22 formed of heat-shrinkable polymeric material such as PTFE, PFE and the like.

A sleeve 23 of heat-shrinkable tubing is then positioned to surround the splice area, extending over a portion of the braid at both ends of the splice area. Heat is then applied to the entire splice to shrink the sleeve 23 and the heat-shrinkable tape layer 22, thus compressing and tightly confining the insulation and fabric layers. Multiple wraps of heat-shrinkable tape are to be avoided. For the purposes of this invention, the combination of the heat-shrinkable tape and sleeve are intended only to confine and compress the insulation and fabric layers, and not to form a complete seal against the intrusion of corrosive well fluids.

The splice may then be further heated to effect curing of the insulation layer. This curing step is optional since the temperatures encountered in service in most well-bore applications will quickly effect the requisite curing.

A splice was constructed according to the principles set forth above, employing nylon fabric as the fabric layer, 4 mil lead foil to form the foil layer, PFE to form the heat-shrinkable tape layer and PFE heat-shrinkable tubing to form the sleeve layers, and cured at 325.degree.-350.degree. F. The splice was placed in a pressure vessel containing 5% aqueous ammonium sulfide, heated 5% to 300.degree. F. and held for one week. On removal from the test vessel, the cable was dissected and the splice examined. No erosion of the conductors or insulation had occurred.

A splice was constructed for comparison purposes essentially according to the principles set forth above, as in Example 1., but the lead foil layer was omitted. After exposure to aqueous ammonium sulfide at 300.degree. F. for one week, upon subsequent removal the insulation had split, the conductors had eroded to a flakey, crystalline material, and the metallic sleeve had corroded and split. It will thus be apparent that the lead foil layer effectively prevents contact of the conductors by the corrosive sulfide fluid and consequent destruction of the conductors.

The invention will thus be seen to be a cable splice which is resistant to attack by corrosive well fluids and to depressurization-caused blow-outs and rupture, and a method for forming a cable splice.