Non-uniqueness of geothermal natural-state simulations

摘要: A numerical simulation representing the natural state of a geothermal system commonly involves finding an approximate solution to a mass and energy balance equilibrium problem; that is, solving a steady-state simulation. Solving this type of natural-state simulation problem is often problematic, and geothermal simulators regularly struggle or are unable to achieve a reasonable steady-state solution for a given geothermal model. When a simulator achieves a steady natural state, it is usually assumed that the simulated natural state is unique for the chosen model parameters. However, as we show here, this uniqueness assumption may be wrong as there might be multiple equilibrium solutions for a single natural-state model. Using a model based on a working model of the Wairakei-Tauhara geothermal field, we demonstrate how multiple equilibrium or steady-state solutions can be achieved for a fixed set of rock properties and model boundary conditions. The different steady-state solutions are found by varying the initial conditions used to initialize the transient to steady-state simulations which model the natural state. For the presented example, we were able to find four different steady-state solutions to a natural-state problem. Although the large-scale temperature distributions are similar, these four simulated natural states have temperatures which, in parts of the model, differ by up to 122 C: a model discrepancy which is substantially greater than expected temperature observation errors. This uniqueness problem, therefore, complicates natural-state model inversion or calibration.