Technologies for extraction
The basic concept for the recovery of non conventional oils is that the dense not very mobile hydrocarbons must be moved towards the extraction well. This is obtained in different ways: by increasing the permeability of the rock of the reservoir, by artificially creating pressure gradients in the oilfield, or by increasing the mobility of the oils by decreasing their viscosity. Various methods are used, and can be summarized as follows:
- “cold “extraction technologies that use physical and mechanical methods to increase the pressure in the oilfield and the permeability of the rock of the reservoir, while the viscosity of the oils is decreased by injecting chemical solvents.
- “thermal” methods that, instead, utilize heat to increase the mobility of the hydrocarbons in the reservoir.
Some technologies require wells, similar to the extraction wells, while others use mining techniques for the extraction (i.e. the collection) of the rocks (for example in the case of tar sand and shale, these are extracted or dug out and then processed at a later stage). With the use of the more modern technologies the percentage recovered from the reservoirs may increase to 70%. Some methods, such as gravity drainage extraction, and mining and extraction of tar sands, date back to 100 years ago. However these methods have recently been reviewed and refined, and in the future it is believed that there will be a possibility of also using resources that are unheard of at present, such as methane hydrates or tar clay. These technologies, however, have some disadvantages: great energy consumption, the need to dispose of residues ( such as shale and sand contaminated by the hydrocarbons produced in large quantities), a high level of CO2 emissions and the production of sulphur and mud with a high content of toxic substances.
The first techniques utilized to produce viscous oils on a large scale date back to 1950 approximately, and these used steam. The countries that first started to study this technology were the USA, Canada, Indonesia, Romania, Russia, China and Kazakhstan, and at present with the thermal steam extraction technique, approximately 4-5 106 bbl/day are produced. Another technique that was initially used more often was in-situ combustion, in which combustion of a part of the hydrocarbons of the oilfield was used to heat and liquefy the remaining hydrocarbons, also the water flooding technique was used, and injection of solvents, polymer displacement and injection of inert gas (such as CH4 or N2) and other techniques that use a high pressure gradient to displace the oils and direct them towards the wells. A curiosity: the techniques that utilize repeated pressure impulses in order to make the hydrocarbons move towards the extraction wells were born in California, where it was observed that, after the vibrations of very strong earthquakes, the productivity of some of the oilfields increased spontaneously for some weeks. In the Eighties new concepts were born that developed highly productive techniques, for example simultaneous use of the Cold Heavy Oil Production with Sand (CHOPS) method, to increase the thrust of the gas that is dissolved in the viscous oils, and Horizontal drilling techniques and gravity drainage extraction, that greatly increase the recovery coefficient. At present these are the more productive and more used techniques : with the CHOPS technique, 0.7 Mbbl/day are produced, in Canada alone; with the horizontal drilling technique 0.7 Mbbl/day are produced in Venezuela. Total production amounts to 4 Mbbl/day.
The more immediate development for the future is the thermal technique, using SAGD (Steam Assisted Gravity Drainage) that combines the steam injection thermal technique with recovery by means of horizontal drilling: a very efficient method that enables a recovery of up to 80% in 5-8 years, however it is still expensive. There are many possible methods, however these are not always suited for all the occasions, therefore it is indispensible to begin accurately planning the interventions, according to the characteristics of the oilfields. Often a number of technologies are used, as a sequence or combined with one another. For example, after having used the CHOPS method that simultaneously extracts sand and hydrocarbons, creating a remarkable increase in the permeability of the reservoir, it is possible to further increase the recovery coefficient by using a thermal method such as VAPEX or SAGD. The recovery of oils from oil shale or tar sand is more complicated and involves the extraction of the material that cannot be processed in situ by mining. The production methods use heating processes (retorting, in special equipment called “retorts”) and destructive distillation, that destroys the rock leaving the hydrocarbons contained in it as residues. In some cases retorting can be carried out in situ, but prior to this the rock must be broken up by means of explosions. After thermal treatment, the extracted material is “washed” with hot water and emulsifiers in order to separate the hydrocarbons from the rock. The main disadvantage is the production of a large quantity of residual material, that must then be appropriately disposed of.
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