An investigation of partitioning tracers for characterizing geothermal reservoirs and predicting enthalpy production

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Wu, Xingru

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The objectives of this research included: (1) development of tracer selection criteria for geothermal reservoirs to enable quantitative analysis of tracer tests; (2) identify critical parameters that affect tracer response; (3) development of a fast and easy approach to estimate enthalpy production and thermal recovery under two-phase flow conditions and (4) perform sensitivity analysis to determine the influence of different reservoir parameters on enthalpy production. A tracer selection protocol was developed after reviewing related literature of tracer applications and doing systematic simulations of tracer injection. An important conclusion is that for superheated geothermal reservoir, the partition coefficient (the K value) of the geothermal tracer should be high in order to get early information about reservoir characterization and liquid breakthrough. An algorithm is presented for calculating the pore volume contacted by the injected tracer (the swept pore volume) using the first temporal moment of the tracer concentration data recorded at the producer. It can be used in the absence of detailed reservoir characterization data or flow and transport models since only an integration of the production data is needed to yield the mean residence time of the tracer. Once the pore volume of the fracture is known, then the thermal breakthrough can be calculated using a retardation factor that takes into account the latent heat of the water. The ultimate goal of geothermal reservoir management is to economically recover as much energy as possible from the reservoir. A semi-analytical solution to the problem of heat and mass transfer in the proposed fractured model was developed. The main features of the semi-analytic solutions are that the matrix block size is assumed to be finite and phase transition is taken into consideration. The solution was initially developed for a reservoir with a single vertical fracture and was later extended to a case with a fracture network consisting of a system of vertical fractures. High-resolution numerical simulations have been carried out to validate the solutions from the semi-analytical model. Comparisons between the analytical and numerical results show that the analytical model accurately predicts enthalpy production from qualified geothermal reservoirs.