Dual-porosity model for harmonic pulse testing in fractured geothermal reservoir

• The analytical model for the interpretation of a Harmonic Pulse Test (HPT) in naturally fractured geothermal reservoirs is derived. • The radial composite effect due to the thermal front is also considered. • Differently from Pressure Transient Analysis, neither a preliminary well closure nor the closure of neighbor wells is required. • Synthetic validation is provided against established analytical and numerical models. • The HPT is suitable for thermal front monitoring during ongoing operations in naturally fractured geothermal reservoirs. Well testing and conventional Pressure Transient Analysis (PTA) are fundamental and well-established methodologies for characterizing well and reservoir parameters. However, the applicability of PTA is limited during production or injection operations, since it requires a shut-in of the tested well, and it is significantly affected by interferences from neighboring wells. In previous works, we proposed, implemented, and validated against real data a methodology called Harmonic Pulse Testing (HPT). HPT is complementary to PTA. By specifically deploying the periodicity of rate and pressure signals, it has been designed to be applied during ongoing field operations. In this work, we present a new analytical solution for HPT in naturally fractured reservoirs. The proposed solution is also applied to geothermal systems, as it is coupled with a radial composite model capable of approximating the thermal front. The model has been validated against well-established analytical and numerical models under different scenarios. The calculation steps for converting the numerical dual-porosity model into storativity ratio and inter-porosity flow coefficient are also provided. The results of a validation exercise demonstrate that our model is robust against potential interference from other wells and allows the detection of the thermal front. The methodology can therefore be successfully applied during ongoing operations in naturally fractured geothermal reservoirs.