1. Field of the Invention
The invention relates generally to oil production, and more particularly to systems and methods for transmission of power to downhole equipment such as electric submersible pump systems.
2. Related Art
Oil is typically extracted from geological formations through the wells that extend far below the earth's surface. These wells may be drilled on dry land, or they may be subsea wells. Often, the naturally existing pressure in the wells is insufficient to force the oil out of the wells. In this case, artificial lift systems such as electric submersible pumps (ESP's) are used to extract the oil from the wells. ESP's are also commonly utilized when operators want to increase the flow rate of the fluid being extracted, such as when the water cut (percentage of water versus oil) increases.
An ESP system includes a pump and a motor that are lowered into a producing region of the well. The pump is connected to a conduit through which oil is pumped to the surface. A power source at the surface of the well is connected to the ESP motor via a submersible power cable. Because the power cable must extend along the entire length of the well, the cable represents a significant portion of the system, both physically and in the cost of the system. As it is necessary to drill deeper and deeper wells to reach oil-bearing formations, the power cables of ESP systems used in these wells must be longer and longer. As a result, the power cable represents an increasing portion of the system cost, particularly in light of the increasing cost of copper, which is typically used to form the conductors of the cable.
There are also significant physical limitations associated with longer cables. For instance, increased length of power cables increases the amount of power loss in the cables. Additionally, there is a maximum amount of power which is deliverable at conventional voltages. This is because, with a limited amount of space available in the wellbore for power cable conductors, there is a maximum current that can be drawn before heating in the cable causes the maximum operating temperature of the cable to be exceeded. This maximum temperature is normally the temperature at which the cable's insulation begins to break down, potentially allowing arcing between conductors.