With the shortage of energy and the rise of crude oil in the world, the development of oil shale is gaining more attention globally. To solve the problem of traditional heat conduction with low efficiency and high cost for oil shale, a new technology called radio frequency (RF) heating (microwave heating) is introduced in this paper. Radio frequency electromagnetic (microwave) can do work directly on medium molecule of oil shale and change into heat energy, the transmission of which allows both inner and outer molecules heating simultaneously without heat conduction.
Yi Pan, Can Chen, Shuangchun Yang, Guiyang Ma
Liaoning Shihua University, Fushun, China
Received February 20, 2012; revised March 18, 2012; accepted March 22, 2012
Meanwhile, oil shale is a poor microwave absorbing material and microwave absorbents must be added to reach pyrolysis temperature. By this means, shale oil could be heated at a higher speed and kerogen will gradually be cracked into gas and oil. Then shale oil and gas will flow into the production wells through fractures generated by heating and be pumped up to the surface.
Oil shale is a significant fossil resource with substantial reserves, which is as high as 400 billion if all be converted into shale oil ,and it is equivalent to 5.4 times of the world’s proven crude oil [1,2] .China is one of the richest countries in oil shale resources over the world, with 31.567 billion tons of proven oil shale reserves, which mainly distributes in MaoMing, HuaDian, FuShun, etc., ranking the fourth followed by America, Estonia and Brazil in the world [3,4].
As a potential important resource, many of the world’s companies have been working on the research and development of the shale oil. In a report of the US Department of Energy, it is confirmed that more than thirty American enterprises actively involved in the project of the oil shale. With the world’s energy demands growing, it’s worthwhile seeking for the effective development and economic utilization.
Oil shale, which mainly composed of mineral and kerogen, is a kind of low permeability rock. Kerogen is the parent material of oil and natural gas. Although the world has a large oil shale reserves, a large portion of which is buried relatively deep (under 500 m). So, the open and tunnel retorting system can’t be utilized. Only by means of heating the oil shale formation, the shale oil can be extracted effectively, which is commonly called in-situ retorting technique. Currently, three heating methods have been adopted, namely conduction heating, convection heating and radiation heating respectively. Compared with the first two means which need long retorting time and have low extraction rate , the microwave heating can greatly make up for the deficiencies.
2. Microwave Heating of Oil Shale
Microwave has dielectric thermal effect on materials, by which the migration of ion and the rotary of polarity molecules that make the molecules moving. That is, the relatively static transient of polarity molecules changes into dynamic and produces thermal effect by the dipolar high-speed rotating of molecules. The change happens in the inner part of the rolled substance that is internal heating. For oil shale is a poor conductor of heat, the utilization of conduction heating and convection heating will lead to low speed. While the adoption of radio frequency heating overcome the difficulties of heating underground oil shale, which makes the retorting easier and more effectively.
2.2. Mining Process
Lay nine wells in the single level oil shale seam, with the thickness of 40 m and the buried depth of 300 m. Well 4 to well 9 locate on the intersection of squares with the distance of 16 m, showing a positive hexagon grid distribution. Well 1 to well 3 are heating wells, the distance of which is 3 m (Ordinarily, the distance between wells is designed as the frequency of wave. The higher the frequency, the shorter the distance will be. The lower the frequency, the longer the distance between wells .) and drill each well into seam 10. Then place a microwave generator in well 2 and radiate microwave to heat oil shale formation as the predetermined direction (See Figures 1 and 2).
Figure 1. Well network layout of microwave heating.
Detailed steps are introduced as follows:
- Dill pinnate well 1 and 3 in the upper and lower seam of oil shale respectively and inject microwave absorbents, forming symmetrical microwave absorbing surface.
- Drill horizontal well 2 that radiate microwave and perforate 20 holes at the wall of cased hole. Thus two pinnate wells and a horizontal well make a heating well group.
- Drill vertical well 4 to 9 as producing wells.
- Turn on the microwave generator and radiate microwave under the power of 1700 kw. Oil and gas will be extracted through producing wells after 60 days of pyrolysis of oil shale formation. To guarantee that the underground oil and gas will flow in gas phase, the power of microwave generator and the closing time of producing wells can be adjusted to control the underground pressure and temperature. When fluids exist in gas phase and the temperature of released oil and gas is below 200 centigrade, oil and gas can be continuously released and get separated on the ground.
- Later in the retorting process, turn off the microwave generator and convert pinnate wells and horizontal wells into injection wells when little oil and gas flow out of the producing wells. Inject cold water, drive the residual oil and gas, also it will lower the temperature of subsurface formation. The cold water heated by the subsurface stratum is recovered through other wells, which can be used another well network after disposition .
Figure 2. Well network profile of microwave heating.
2.3. Technique Characteristics
Three ways can be adopted to transfer the heat to oil shale, namely conduction heating, convection heating and radiation heating . Compared with other heating techniques, microwave heating technique has advantages of strong penetration, high heating speed and efficiency, Real-Time heating, integrity, etc.
- The Real-time represents it heats rapidly when taking microwave to heat materials. Microwave radiation promote polarity molecules to move severely, making molecules collide and rub. The heating process takes place in the whole object simultaneously with quickness in temperature rise, temperature uniformity and low temperature gradient.
- The high efficiency of energy consumption mainly reflects that medium materials can absorb microwave and change it into heat energy directly. Microwave heating requires no heat conduction.
- Microwave heating has good selectivity, and it can play different roles in different materials. With turning off the microwave source, microwave energy is unavailable, which can control the temperature of oil shale accurately.
- The crack of oil shale by microwave heating will increase the fractures, which contributes to the transmission of microwave in oil shale, expanding heating area and the discharge.
- Heating shale oil by microwave, heating rate can be easily adjusted. Conventional methods require a quite long time to reach the predetermined temperature and longer cooling time when stop heating. With microwave heating process, the status of oil shale will change as the output power of microwave regulated.
3. The Present Situation of Radio Frequency Technique
3.1. RF/CF Technology Introduction of Raytheon
As the expert in RF field, Raytheon has made a perfect combination of his RF and CF which is from Hyde Park, the other expert in CF field. By this in-situ technology, oil shale is heated by CF, and critical CO2 drives the liquid and gas out (as shown in Figure 3).
Figure 3. Process of RF/CF technology.