Benefits of heating heavy oil with medium-voltage mineral insulated cables
Many technologies have been proposed to reduce viscosity of heavy oil to allow its extraction from reservoirs. Steam heating, water flooding, solvent dissolving, in-situ combustion, direct ohmic heating of the reservoir, microwave heating, as well as resistive heating elements are some of the techniques. Heavy oils, rated less than 22.3 API gravity, do not flow easily. Extra-heavy oil and bitumen is rated less than 10 API gravity and is heavier than water. The oil’s viscosity is temperature dependent, and even an 80 C increase will lower the viscosity for easier extraction; see the real-world viscosity chart. Steam, used in a SAGD operation, is currently a solution, but the use of a significant amount of water is required and is extracted along with the oil. Between two and seven barrels of water are produced with each barrel of oil. This produced water may be considered hazardous waste because of high salinity, mud, and traces of drilling chemicals. While many technologies are possible, an electrical heater may be an optimum solution both technically and economically.
The reliable heater
Electrical heaters have been thought of as solution for more than 50 years. However, the technical capabilities of low voltage (fewer than 600 V) have limited the use because of parasitic energy loss in the overburden and short lengths of heaters in horizontal wells. These two detractors have made low-voltage heaters uneconomic for commercial applications, while numerous short-length pilots (up to 250 ft) have proven this technique of viscosity reduction to be technically feasible but not economic.
A major oil company has provided research in the past five years that has enabled the development of a medium-voltage (4,160 V) heater system. This heater technology is now available for commercial applications. An example is a heater that can produce 1,000 W per meter and have a length of 1,000 meters. This is a 1-megawatt heater that matches the horizontal-well drilling technology. The following chart compares the 600 V system with the 4,160 V system. Note the limited length of the 600 V heater and the increased weight necessary for transmission of the power to the heated section of the heater. Even with the increased diameter of the 600 V heater, the overburden energy loss is 51.9% compared with only 11.8% for the 4,160 V heater.
Long length EOR extraction
The heater shown looks a lot like a standard mineral insulated (MI) cable heater. There are two important differences:
- The increased ability for the magnesium oxide (MgO) to withstand the higher voltage without electrical breakdown, thus permitting operation at 4,160 V.
- The ability to manufacture the cable in long lengths, up to 2,000 meters without splices, is beneficial as splices have been a problem in other designs, which increased the diameter at the splice by about a factor of three times. This has caused considerable deployment issues, sometimes necessitating a larger well diameter.
While viscosity reduction is one application, preheating a SAGD operation also may be a major opportunity. In this example, a well with an initial temperature of 10 C would be heated electrically to a little more than 120 C and then be steam injected to produce the “drive” to move the oil to the producing well, located under the heater well. This might produce a 50% reduction in water injection, as well as much less heat loss in the overburden.
These new technical capabilities could enable use of medium-voltage electrical heaters in many applications for downhole heating, including viscosity reduction and flow assurance. This technology is ready to become mainstream as product reliability and deployment technologies are now proven.
– Chet Sandberg, PE, consults for MCAAA Ltd. in the UK and is an IEEE Fellow. Edited by Eric R. Eissler, associate editor, Oil & Gas Engineering, CFE Media, firstname.lastname@example.org.
About the author
Chet Sandberg, PE, received a BS degree from Massachusetts Institute of Technology, and an MS from Stanford. He consults for MCAAA Ltd. in the UK. He is a member of the NFPA, National Electrical Code Panel 17, and chair of the Technical Working Group of the Energy Storage Association. In December 1999, he was honored by the IEEE by being elevated to the highest member level of IEEE Fellow. Sandberg also is a senior member of ISA, and a member of ASME, NFPA, and SPE. He can be contacted at email@example.com.
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