This paper presents a study on the thermal simulationof a large existing borehole thermal energy storage(BTES) system located in Stockholm, Sweden. Thebore field investigated presents an uneven pattern,which comprises vertical and inclined boreholes, for atotal of 130 units. Such complex bore field geometrycannot be modeled with the current availablecommercial design tools. The test case presented isutilized to explore the influence of boundaryconditions and level of detail utilized for representingthe model geometry on the output of the simulation.Two boundary conditions and three geometricalconfigurations were studied. The results show that, inthe considered case, the results obtained with thetested models give a marginal difference, hence alsothe greatest level of simplification can be utilizedwithout loosing accuracy in the analysis.