Oil shale is sedimentary rock containing up to 20 % kerogen, a solid bituminous material that is a precursor of crude oil. Kerogens are formed in the absence of oxygen from dead plankton, sea- and freshwater algae and bacteria. Amino acids and chlorophyll degradation products are also found. Over a long period of time, the material turned into oil shale as a result of heat and pressure effects. The process is similar to the way crude oil originates, except that in the case of oil shale the pressure and the heat were not extreme enough to liquefy the oil.
Elaborate technology for the exploitation of oil shale deposits
To use the oil shale, it is therefore necessary to heat the rock after mining. The kerogen content of oil shale is mainly extracted by pyrolysis, which turns it into shale oil (synthetic mineral oil) and oil shale gas. The oil shale is mined above ground. The rock is then heated and the discharged mineral oil collected and further processed. The kerogen decomposition process begins at 300° and runs best at temperatures between 450 °C and 530 °C. In most techniques, pyrolytic decomposition takes place in an oxygen-free environment. As well as the condensable synthetic oil, this process also produces oil shale gas and solid residues. Some of these are disposed of, others can be put to further use. To minimize the use of land, a large number of techniques have already been devised for heating the rock underground and then extracting the kerogen in liquid form. All these methods are still in the test phase.
Shale gas is embedded in tiny rock pores
Oil shale deposits are also raising great expectations with regard to gas production. Whereas natural gas is normally found in huge bubbles that only need drilling and tapping, shale gas is embedded in tiny rock pores. The problem here is that the gas only escapes from the pores in small quantities by natural means, so elaborate technology has to be applied to improve the gas flow. Horizontal drilling accesses a circular site into which a chemical is pressed to break up the rock and expose the pores containing the gas.
Oil shale is one of the so-called “unconventional raw material deposits“ and will bring radical changes to the global energy market. America is already meeting a large part of its demand from unconventional sources, i.e. solid rock, which accounted for 15 per cent of the total US natural gas consumption in 2010. There are also hopes of finding oil shale deposits in Sweden, Denmark, the UK, France, Hungary, Romania, Spain, Italy, Switzerland, Turkey and the Ukraine. First of all it is a matter of locating the shale and other rocks containing gas. However, the exploitation of these raw materials is still extremely complex and expensive, and has a massive impact on the environment. Exact investigations of oil shale deposits are therefore extremely important for identifying their potential in advance and optimizing mining methods.
Success in analyzing oil shale using scanning electron microscopy (SEM)
Analysis focuses primarily on the distribution, the concentration and the properties of the organic materials, as it is these that determine the energy properties of the rock. Oil shale is difficult to analyze because of its high density and nanometer-sized pores.
Using scanning electron microscopy (SEM) it is possible to examine shale samples with a high degree of precision and therefore gain valuable insights into the potential oil and gas reserves. The preparation of the samples plays a key role for the success of this method. When shale is subjected to mechanical treatment, some of the organic constituents and clay particles are removed, leaving deep holes in the material. This problem can be avoided by applying ion beam etching techniques, which yield valuable information on the microstructure of oil shale, including the concentration, morphology and porosity of kerogen.
Ion Beam Slope Cutting as the ideal method of preparing oil shale samples
A special method that completely dispenses with mechanical processing of the oil shale samples is ion beam slope cutting. This method can achieve very flat regions of material combinations consisting of hard and soft areas: perfectly suited for oil shale which is a fine grained material with organic and inorganic compounds and very brittle.
The Triple Ion Beam Cutter Leica EM TIC 3X (Figure 1) allows production of cross sections of hard/soft, porous, heat sensitive, brittle and heterogeneous material for Scanning Electron Microscopy (SEM) and surpasses conventional slope cutting instruments.
The assembly is arranged perpendicular to the sample surface, so the sample (mounted on a holder) does not require an oscillating movement to reduce shadowing/curtaining effects. Also, it enables an efficient heat transfer from the sample. Three ion beams intersect at the center edge of the mask, forming a milling sector of 100° cutting the exposed sample (~20 to 100 μm above the mask) until the area of interest is reached (Figure 2).
A new design of the ion guns develops a milling rate of 150 μm/hour (Si 10 kV, 3.0 mA, 50 μm from edge). The unique triple ion beam system optimizes the cross-section quality and reduces working time with its ability to cut broad and deep at high speeds. This unique technique produces a vast preparation area of >4 × 1 mm at a very high material removal rate with a high quality finish. For processing, each gun can be controlled and switched separately. This enables the user to set different gun parameters depending on the application, e.g. for contrast enhancement or gentle milling.
Figure 3 shows two images of oil shale prepared with the Leica EM TIC 3X. The surfaces are extremely smooth and allow a high-quality view of all components. Pyrite and kerogen are clearly visible.