An apparatus for enhancing extraction of hydrocarbons from an underground reservoir using a well , the apparatus comprising: a power source operable to supply periodic electrical power at a first frequency; a group of impulse generator units comprising a first impulse generator unit and a second impulse generator unit , each impulse generator unit of the group of impulse generator units for location underground and having: a frequency conversion unit operable to convert the periodic electrical power at the first frequency into periodic electrical power at a second frequency; and an energy coupling unit operable to couple electromagnetic energy generated by the periodic electrical power at the second frequency into the reservoir, the second frequency being at least ten times higher than the first frequency; and a conducting cable for coupling the power source and the group of impulse generator units; the apparatus characterized by : each impulse generator unit of the group of impulse generator units having a controller unit operable to control an operation of at least one of the frequency conversion unit and the energy coupling unit of the impulse generator unit; and the controller unit of the first impulse generator unit is further operable to control the operation of the controller unit of at least the second impulse generator unit of the group; and in use, each impulse generator unit of the group is coupled to the conducting cable along a portion of the cable between a first end and a second end. a power source operable to supply periodic electrical power at a first frequency; a group of impulse generator units comprising a first impulse generator unit and a second impulse generator unit , each impulse generator unit of the group of impulse generator units for location underground and having: a frequency conversion unit operable to convert the periodic electrical power at the first frequency into periodic electrical power at a second frequency; and an energy coupling unit operable to couple electromagnetic energy generated by the periodic electrical power at the second frequency into the reservoir, the second frequency being at least ten times higher than the first frequency; and a frequency conversion unit operable to convert the periodic electrical power at the first frequency into periodic electrical power at a second frequency; and an energy coupling unit operable to couple electromagnetic energy generated by the periodic electrical power at the second frequency into the reservoir, the second frequency being at least ten times higher than the first frequency; and a conducting cable for coupling the power source and the group of impulse generator units; the apparatus characterized by : each impulse generator unit of the group of impulse generator units having a controller unit operable to control an operation of at least one of the frequency conversion unit and the energy coupling unit of the impulse generator unit; and the controller unit of the first impulse generator unit is further operable to control the operation of the controller unit of at least the second impulse generator unit of the group; and in use, each impulse generator unit of the group is coupled to the conducting cable along a portion of the cable between a first end and a second end. each impulse generator unit of the group of impulse generator units having a controller unit operable to control an operation of at least one of the frequency conversion unit and the energy coupling unit of the impulse generator unit; and the controller unit of the first impulse generator unit is further operable to control the operation of the controller unit of at least the second impulse generator unit of the group; and in use, each impulse generator unit of the group is coupled to the conducting cable along a portion of the cable between a first end and a second end. <EOS>