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In Serbia, there are about 1200 abandoned oil and gas wells, which have significant potential to produce thermal energy. Abandoned wells, depending on their location and thermal potential, can be used to produce thermal energy or electricity, or for the combined production of electricity and thermal energy. Thermal energy that is produced from abandoned wells is obtained using geothermal energy, so the electrical energy and/or thermal energy that would be obtained using such heat sources are balanced as energy obtained from renewable energy sources. The costs of equipping deep wells and applying the technology that would be used to produce thermal energy and/or electricity represent a far smaller part of the costs compared to the costs of making deep wells, so these abandoned wells represent an already existing significant material resource. The production of thermal energy from abandoned wells, which is based on pressing the working medium into the well, heating it and pushing it to the surface, is less energy intensive since the same well is used for the transfer of hot energy - through the appropriate working medium through tubing (narrow pipe) to the surface, and then the used cooled water is returned to the same well through the intermediate space. The paper presents a methodology for calculating the theoretical thermal potential of abandoned oil and gas wells, which was applied to the example of wells K-1, T-1, S-1 and M-1 located in Serbia, with a depth of 2.000 m, 2.250 m, 1.700 m and 2.102 m. It is shown that the theoretical temperature of the working fluid on the surface of the earth is in the range of 77.59 to 94.46°C.

In the Serbia there are more gas reservoirs with reserves of more than 10 million Sm3 of natural gas. One of the reasons why they have not been exploited yet is that they contain nitrogen and carbon dioxide, that should be separated during the production process. Today, there are many developed technologies for separation of nitrogen and carbon dioxide from natural gas such as cryogenic processes, membrane technology, molecular sieves, nitrogen spinners, etc. So far, none of the mentioned technologies has been applied in our country. The paper presents technologies for separating nitrogen and carbon dioxide from natural gas. In this paper is presented an analysis of the possibility of applying these technologies from the aspect of the capacity, pressure and content of nitrogen and carbon dioxide in natural gas. In particular, it should be noted that all technologies are already commercially applied in the world. Investment and operating costs for the presented technologies were determined. The economic viability of selected technologies for separating carbon dioxide and nitrogen from natural gas directly depends on the required process capacity. For low daily production rate, membrane technology and molecular sieves are excellent for gas reservoirs in our country. Investment costs are the lowest in implementing the membrane technology and molecular sieves and they are ranging from 9.89 to 11.40 $ / Sm3 , while exploitation costs range from 0.010 to 0.020 $ / Sm3 . This is very important for the reasons of the project economic sensitivity when the small gas reservoir are considered.

In the world today for the exploitation of oil reservoirs by artificial lift methods are applied different types of deep pumps (piston, centrifugal, screw, hydraulic), water jet pumps and gas lift (continuous, intermittent and plunger). Maximum values of oil production achieved by these exploitation methods are significantly different. In order to select the optimal exploitation method of oil well, the multicriteria analysis models are used. In this paper is presented an analysis of the multicriteria model's application known as VIKOR, TOPSIS, ELECTRE, AHP and PROMETHEE for selection of optimal exploitation method for typical oil well at Serbian exploration area. Ranking results of applicability of the deep piston pumps, hydraulic pumps, screw pumps, gas lift method and electric submersible centrifugal pumps, indicated that in the all above multicriteria models except in PROMETHEE, the optimal method of exploitation are deep piston pumps and gas lift.

Determining the optimal production oil wells represent the harmonization of production characteristics of a given well with the operational parameters of applied artificial methods of exploitation. This means that the optimal production well achieved when balance the amount of fluid that enters the well (defined by number, diameter, length and spatial orientation perforation) with the amount of fluid applied by artificial method of exploitation belong the well. The paper discusses the key parameters for determining the optimum production well from the application of artificial methods of exploitation such as: characteristics of fluid inflow into the well, forecasting future production well, operational parameters of artificial methods, timing of introduction of artificial methods of exploitation and the price of oil.