Power sources for downhole electrical heating

An electrical heating power source for a heating system for heating a zone in a subterranean formation, the heating system including the power source, a main electrode positioned in the zone to be heated, at least one sensor in the well for sensing a parameter relating to fluid pressure, fluid temperature, fluid level, fluid flow rate, or fluid constituency in the well or in the deposit, and a return electrode, the power source comprising:

• A.C. heating current generator means for generating a high amplitude A.C. heating current, of at least fifty amperes, at a heating frequency in a range of 0.01 Hz to 35 Hz;
• the A.C. heating current generator means including A.C. to D.C. conversion means for developing an intermediate D.C. output of predetermined amplitude from a conventional 50/60 Hz power input, input connection means for connecting the power source to a 50/60 Hz supply, an input transformer coupling the input connection means to the conversion means, and switching means, connected to the conversion means, for repetitively sampling the intermediate D.C. output of the conversion means at the heating frequency to develop the high amplitude A.C. heating current at the heating frequency, the conversion means and the switching means being combined in one circuit comprising a plurality of gated rectifiers;
• heating control means, connected to the heating current generator means and having an input derived from the sensor, for controlling the heating frequency and the energy content of the A.C. heating current in accordance with changes in said parameter, the heating control means including at least one tapped winding on the input transformer and comprising means for varying the relative duty cycles of the gated rectifiers by applying a predetermined sequence of gate signals to the gated rectifiers at the heating frequency;
• and
• output connection means for connecting the A.C. heating current to the electrodes.