A reliable evaluation of long-term performance of a heat pump coupled with a borehole heat exchanger (BHE) field is necessary to verify the stability of its heat exchange capability over the time. The BHE field pattern is often assumed to be regular (e.g., rectangular, L-shaped, T-shaped, etc.), or is assumed to be adequately approximated by one of these shapes. Moreover, a planned geothermal system is often designed regardless of the presence of other existing or planned BHE systems. In order to evaluate the validity and the possible limitations of these assumptions commonly made by the designer, a number of 25-year time span simulations have been carried out by means of 2D finite element modeling. In particular, the case of a real 28 BHE field, irregularly shaped and related to a building located in Northern Italy, has been studied together with its 7-by-4 regular grid approximation and a series of 28 BHE fields having different shapes. Besides the real annual thermal load profile characterized by quasi-balanced winter heating and summer cooling, two other profiles characterized by increasingly unbalanced operation have been taken into account. The numerical study shows that (i) the regularly shaped approximation, a common choice in BHE design, seems to be reasonable under the condition that groundwater flow is absent for all the thermal load profiles; (ii) if a strong heating/cooling imbalance occurs, the thermal footprint of a BHE field can be very extensive, preventing the installation of future nearby BHE systems.