Investigation of Geothermal Power Generation on Abandoned Hydrocarbon Wells

In Hungary there is a big opportunity to utilize geothermal resources. The oil industry has left many hydrocarbon wells abandoned, which could be prepared to produce geothermal energy. A feasible way to produce electricity if these wells are used as heatsources of an enhanced geothermal power generation system (EGS). The average heat flux density in our country is significantly higher than the average value in the whole European continent. More than 3000 wells exist, thanks to the industrial and experimental drilling excavations. Removing these wells would be a significant cost for the oil companies. The aim of this work is the neglection or reallocation of the above mentioned extra cost by the installations of geothermal power plants. This paper deals with the opportunities of geothermal power generation from a fictive 2200 meter depth oil well. Some construction changes were needed on the wells for the power generation. The bottom of the well has to be settled and another pipe is needed to fit in as a coaxial construction. This paper focuses on small-scale applications, so the possible use of organic Rankine-cycle (ORC) was considered, which enables the power generation from low temperature heat sources and operates also in lower power scales. The possibility of single and double loop ORC systems were investigated. In case of an ORC cycle with a double loop system the medium warmed up in the well while evaporating the working fluid of the ORC in the heat exchanger. In case of the single loop version, supercritical pressure was used to avoid the back streaming of working fluid caused by the density changing. The medium warmed up supercritically expands directly on the turbine. Due to the need of temperature difference in the heat exchanger the efficiency was lower in the first case. Based on our calculations with the single loop system a significantly higher efficiency can be achieved than with the double loop systems, therefore this solution was investigated more in details. Different working fluids were investigated. We pointed out the choosing aspects of the best medium. The optimization of certain parameters were also performed in order to achieve the best efficiency and thereby the best performance. Finally the economic implications of the technically best solution were presented and some suggestions to short the payback time of the investment. The scaled model can provide near continuous output, making the technology a renewable, zero-carbon option for supplying baseload electricity generation.