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A method and apparatus for forming an electrically conductive externally insulated casing for an oil well of the type in which the casing carries electrical current to a primary heating electrode positioned downhole, using multiple prefabricated casing segments; each casing segment is a long steel p

A method and apparatus for forming an electrically conductive externally insulated casing for an oil well of the type in which the casing carries electrical current to a primary heating electrode positioned downhole, using multiple prefabricated casing segments; each casing segment is a long steel pipe having a female thread coupling on one end and a male thread on the other end. Each segment has an insulator covering, over substantially all of its length, that has a figure of merit (erl)/D8 so that the shunt impedance of the casing to ground can be kept substantially greater than the spreading resistance of the primary heating electrode. The preferred casing segment insulation is in two layers, including a hard, durarable inner layer subject to water degradation and an outer water-impervious layer usually applied as tape. The uninsulated portions of joints between segments are covered withan insulator cement in the field and then further covered with a water-impervious tape that overlaps the water-impervious layers on two segments.

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A method of forming a casing in an oil well comprising an externally insulated electrically conductive casing employed as a conductor carrying electrical current to a heater electrode positioned downhole in the well in alignment with an oil producing formation, comprising the following steps: (A) pr

A method of forming a casing in an oil well comprising an externally insulated electrically conductive casing employed as a conductor carrying electrical current to a heater electrode positioned downhole in the well in alignment with an oil producing formation, comprising the following steps: (A) pre-assembling a plurality of casing segments, each casing segment comprising an elongated metal pipe, each casing segment having an electrical insulator covering on substantially all of its external surface; the insulator covering having a figure of merit (erL)/D4×108, after extended immersion in water, wherein er=reaative dielectric constant of the insulator covering at 60 Hz, DrL)/D4×108, after extended immersion in water, wherein er=relative dielectric constant of the insulator covering at 60 Hz, Dc=conductance of the insulator covering in mhos; Cc=capacitance of the insulator covering in farads; Rp=spreading resistance of the primary electrode in ohms; and m2p