American Petroleum Institute API classifies crude oil into different types as follows:
- Light Crude Oil : its API is greater than 31.1°
- Medium Crude Oil: its API is between 22.3° and 30.2°.
- Heavy Crude Oil: its API is between 22.3° and 10°.
heavy crude oil has a high viscosity, high density. and the heavy oil itself is classified into many types according to the Canadian Center for Energy according to its API as follows:
1. Heavy oil: which API is more than 10° and viscosity is less than 10,000 cp , and flows under reservoir conditions.
2. Extra Heavy Oil: which API is less than 10° and viscosity is less than 10,000 cp , has some fluidity under reservoir conditions.
3. Natural Bitumen: exists with tar sands, its viscosity is more than 10,000 cp.
heavy oil is characterized with low hydrogen and high amounts of impurities such as: sulfur, nitrogen, organic acids, vanadium, nickel, silica and asphaltene, therefore the heavy oil upgrade processes is achieved either by carbon removal or hydrogen addition.
heavy crude oil was formed as a result of bacteria effect on traditional crude oil, it has different physical and chemical properties, such as high viscosity, high asphaltene content and high sulfur content.
these properties require special technical solutions, for containing high carbon content so it will release carbon dioxide during its production, the thing that affects badly on environment because it release 3 times the amount released by light crude oil.
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the real problem with heavy oil is that it is hard to flow under reservoir conditions, so many technologies were developed to extract it, the main concept of heavy oil extraction depends the heavy crude to decrease its viscosity, and that will ease its outflow, many techniques have been developed , some of them are in application stage, while others are still in test stage, in general; we can classify the heavy oil recovery techniques into the following:
this technique is also named ” Steam Flooding”, it depends on drilling many wells, some of them are used for steam injection into the reservoir to raise the crude oil temperature and displace towards the production as shown in the figure.
in this method , a large space of the reservoir is recovered, and the production rate is increased, but there is a big lose in thermal energy because of the large area recovered, and the time of displacement.
this technique was applied in California successfully, the extraction rate exceeded 50%, it was also implemented in Venezuela but with lower success, this technology can not succeed in highly-fractured reservoirs.
Steam Assisted Gravity Drainage SAGD:
because of high technology used in horizontal drilling, this technique was used successfully.
in this technology, steam is injected to the reservoir by a horizontal well parallel to another horizontal production well under the first one as shown in figure. The steam is circulated, then the oil is outflow by gravity in the lower well.
in this technique the production rate exceeds 60%, and has required steam/crude oil, this technology is effectively used in bitumen extraction from tar sand in Canada. but it has high energy requirements.
there are many attempts to improve this technique through field experiments, and there is an attempt to modify it by using one horizontal well used as an injection and production well in the same time in which steam is injected through tubing string.
this technology is used only for the extraction of sand tar sands that have 40-60 m thickness.
the high capital and operation budgets for this technology made the industry develop 50 m3 bulldozers and 350 tons huge tracks to transfer these tars to a cracker to break them and mix them with water to form a soft mixture can be pumped to tar extraction site.
in this way, extraction can be made far away from the working site, then treated with hot water , caustic soda and steam, the bitumen will be separated as a foam, then treated with naphtha (What is Naphtha) to separate water and solids completely.
in the last years, many modifications been made in this technique, by adding extra separation unit and a naphtha recovery unit, that changes raised the extraction rate to 90%. the tar sands extraction technology had many enhancements to match the environmental regulations, these enhancements included rehabilitation and greening the mining areas, right disposal of extraction residuals, water treatment and recycling, and soil preservation.
this technology is used for relatively low viscosity heavy oil (1500-3000 cp), it is applied by drilling vertical and horizontal wells, and applying an ESP “electrical submersible pump” to raise the oil to the surface, this technology’s disadvantages are that is produces sands with oil, and has a low production rate “5-10%” , while it has low capital and operation costs, low CO2 emissions, but it is not suitable for the reservoirs with ground water.
this technique was applied successfully in Canada and Venezuela.
Cyclic Steam Injection:
this technique, oil is produces consecutively, this cycle starts by injecting a certain amount of steam, closing the well for some time, then oil producing from the same well till the production rate is low, then the steam is injected again as shown in figure.
this technology is one of the main technologies for heavy oil recovery in Venezuela ,recovery factor in this technology is low “10-20%” , it was also used as a first recovery stage before conventional steam injection, it was also used with horizontal wells in Venezuela and Canada, and with heavy oil reservoirs in Kuwait.
although this technique is used since the 1960s but it is applicable on light oil reservoirs, it needs a lot of studying and field developing to be applied on heavy oils. it extracts heavy oil and convert it to lighter oil, and has a high recovery rate “more than 60%”.
in this technology a mixture of water and air is injected to the reservoir for days or weeks, then burning oil in the reservoir by inflammable materials, then shifting the burning by injecting more air, this will raise the crude oil temperature, making it flow easily and shift it to the production well as shown in figure.
Solvent Injection NAGD:
this technology which is known as “VAPEX” also is still in field developing stage, it is very similar to SAGD technology in general, but either propane or naphtha is injected instead of steam, solvent is heated to convert it to vapor, then pumped in the injection well to the reservoir, the solvent is mixed with bitumen, raise its temperature, dilute it, decrease its viscosity and make easy-to-flow; after getting the diluted bitumen, the solvent is recovered and used again.
this technology’s advantages are as follows, there is low energy and water consumption, low CO2 emission, that make it environmentally acceptable, the high-quality produced bitumen because of asphaltene settling in the reservoir.
the disadvantages of this technology is the loss of solvent and the solvent high cost, but there are many efforts to raise the solvent recovery rates to 90%.