Miscible/Immiscible carbon dioxide injection is considered to be as one of most effective technology to improve oil recovery from complicated formations, and hard to recover oil reserves, in particular. Application of this technology can increase ultimate oil recovery by 10–15% and even more1. One of the main advantage of this technology is that it can be applied in a wide range of geological conditions for production both light and heavy oils. Striking illustration of this technology high efficiency is the fact that the number of active carbon dioxide (CO2) projects worldwide sharply increased during last 20 years, from 30–35 in the beginning of 1980s to almost 100 in 2000, while oil production increased from 25,000 BPD to more than 200,000 BPD during this period2. One of the main factor restraining wide application of this technology is its dependence on the natural CO2 sources, transportation of CO2, breakthrough of CO2 to production wells, corrosion of well and field equipment, safety and environmental problems etc. Except Hungary reserves of natural CO2 for field application of this technology are not existed in Europe. This restrains wide implementation of CO2 injection not only in many European countries but also worldwide where this technology could be efficiently applied and significantly improve oil production as well as ultimate oil recovery from depleted oil formations. Carbon dioxide enhanced oil recovery may be the key to recovering huge amount of oil from highly watered, depleted, heterogeneous and other type of so called "hard-to-recover" oil reserves. One of the main advantage of proposed technology is eliminating of CO2 negative impact on the environment and strong control of the process. New understanding of the nature of miscible displacement process when behavior of CO2 can be explain as a "super critical fluid" opens a unique future of oil displacement using this technology. Results of R & D as well as new approach in estimation of the efficiency of EOR technology application are described below. Proposed technology of in-situ CO2 generation is based on exothermic chemical reaction between gas forming water solution and low concentrated active acids. Introduction Displacement of oil by CO2 in miscible/immiscible conditions to improve oil recovery allowed to determine the mechanism of oil displacement and possible positive and negative factors affecting on oil displacement process3–6. Among main positive factors affecting on displacement of oil by CO2 the following can be outlined:CO2 solved in water increases water viscosity by 20–30% and its mobility by 2–3 times,CO2 solved in oil reduces oil viscosity by 1.5 - 2.5 times,CO2 solved in oil decreases interfacial tension in oil-water contact,CO2 solved in oil increases oil volume what affects on displacement of residual oil. At the same time following negative factors accompanied displacement of oil by CO2 have been observed:Changing of thermobaric conditions results on reduction of CO2 concentration followed by coagulation and sedimentation of waxes and asphaltens from oil,CO2 breakthrough to oil producers,Well and oil field equipment corrosion,CO2 transportation problems,High cost of technology,Shortage or absence of CO2 sources in the vicinity of oil fields. It is clear that development of technology where displacement agent to displace oil from formation (CO2) is generated in-situ avoiding any surface facility and any negative impact on the environment has a high priority.

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