engleski

Numerical and analytical modelling of heat extraction rates in the coaxial heat exchanger for a retrofitted deep oil & gas wells

In the last decade there is an increase of interest to retrofit and exploit dry and abandoned oil and gas wells for geothermal power and heating. Current research focus is based upon a deep coaxial heat exchanger system. The heat energy is extracted by circulating working fluid with inflow through inner pipe, or tubing, and outflow through the annulus. This paper presents conceptual model for a development of a pilot project case study for a Pčelić-1 well with depth of 4500 m. Heat transfer process in analytical model is described by well-known Eskilson’s line source model, coupled with solution for thermal resistance by using so-called g-function. Numerical model was set up in FEFLOW™ software, used for modelling mass-flow-heat transport by solving differential equations using numerical FEM (finite element) method. The model considered isolating the tubing, by using vacuum double pipe, to determine the effect on heat extraction rates. The result of numerical model was compared with results of analytical model, during 6 months of constant load operations. Numerical model confirmed the absence of thermal short-circuit between colder fluid in tubing and warm fluid in annulus in case of vacuum insulated double pipe tubing. The phenomena of thermal short-circuit between colder and warmer fluid is pronounced in the case of tubing with no insulation, where the heat is transferred from the warmer fluid onto the colder fluid in the tubing, due to pipe’s high thermal conductivity. The effect of using insulated tubing is seen in larger available thermal energy, for the same thermogeological and fluid flow parameters. Additionally, available thermal energy modelling with insulated tubing was done, using analytical model, for two different load use during 20 years of system operation – constant and variable. The results obtained were compared with previously obtained for the system without tubing insulation. Conceptual model will provide necessary groundwork for retrofitting of oil and gas wells. This paper therefore presents groundwork in establishing model of heat extraction with different methods which can be used in future assessment of such systems in Croatia.

deep coaxial borehole heat exchanger, line source model, numerical model, retrofitting of oil and gas wells, geothermal heating, constant load, variable load

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