Heating rate variant elongated electrical resistance heater

In a process in which subterranean earth formations within an interval more than 100 feet long are heated to a temperature of more than 600.degree. C., so that heat is injected substantially uniformly into that interval, an improvement for constructing and installing a heater having an electrical cable heating section which is free of splices, comprising:

• constructing said heating cable section by compressively swaging at least one portion of a junction-free electrical heating cable to reduce its size at said at least one portion, said cable is at least as long as the earth formation interval to be heated and comprises an axially aligned, malleable, electrically conductive core surrounded by granular mineral insulation within a metal sheath, so that swaged portion generates heat at a rate higher than the unswaged portion;
• correlating the location of said swaging with the pattern of heat conductivity in the earth formation interval so that at least one compressively swaged portion of the cable is located along the cable in a position such that, when the cable is extended along the earth formation interval to be heated, the compressively swaged portion is adjacent to a portion of the earth formation interval in which the heat conductivity is relatively high;
• connecting said selectively swaged heating cable section to at least one power supply cable and spooling the interconnected cables;
• and
• unspooling the interconnected cables into a wellbore along with a weight-supporting metal conduit while periodically attaching the cables to the conduit and extending the cables and conduit to a depth at which the compressively swaged portions of the cable are positioned adjacent to the earth formations having a relatively high thermal conductivity.