Optimisation of doublet well spacing in low-enthalpy geothermal systems is addressed by defining a novel objective function that is based on the Coefficient of Performance (CoP) and energy sweep efficiency. The definition of objective function that separates performance-based criteria from economic factors, allows us to better observe the effects of heterogeneity on optimisation. A checkerboard pattern of two doublets (two injection wells diagonally placed and two production wells diagonally placed over corners of a rectangle) is considered for a range of homogeneous to heterogeneous (spatially correlated and fluvial) synthetic low enthalpy reservoirs. Optimal length and width of this rectangle are sought in order to (a) maximise heat recovery from a conventionally-chosen licence area around the rectangular domain, (b) minimise heat recovery from outside this licence area, and (c) maximise CoP. We define fixed (15 years and 30 years) and varying life times of operation (between 15 and 30 years). For optimisation, in addition to a simple-search procedure of optimisation across a mesh of simulation nodes, we also utilise a surrogate response surface model to computationally solve the optimisation problem. Our results consistently show that for a fixed life time of 15 years and a discharge rate of 250 m³/hr, 400 m is the optimal well/doublet spacing. Increasing the life time and the discharge rate will increase the optimal well/doublet spacing. The results show while CoP is sensitive to the heterogeneity, adding energy sweep to the objective function makes the distances found for the homogeneous cases also consistent solutions for the heterogeneous cases.