| Abstract |
Hydrothermal convection in porous geothermal reservoir systems can be seen as a two-edged sword. On the one hand regions of upflow in convective systems can increase the geothermal energy potential of the reservoir; on the other hand, convection introduces uncertainty, because it is difficult to locate these regions of upflow. Several predictive criteria, such as the Rayleigh number, exist to estimate whether convection might occur under certain conditions. Once a convection system is established, diagnostic measures are needed for describing the convection pattern, e.g. the likely number of upwelling regions. Of further interest is, whether likely locations of upwelling regions can be determined. We use the thermodynamic measure called entropy production to describe the influence of spatially heterogeneous permeability on a hydrothermal convection pattern in a hot sedimentary aquifer by analyzing its entropy production number. We apply this measure to a hydrothermal model of the Perth Basin, a sedimentary geothermal system, where convection is likely to occur. To this end, we set up a Monte Carlo study with multiple ensembles. Each ensemble contains several hundred realizations of spatially heterogeneous permeability. The ensembles only differ in the horizontal spatial continuity (i.e. correlation length) of permeability. In the Monte Carlo study, the assessed Rayleigh number of the system does not contain valuable information about the convection pattern in the models with heterogeneous permeability. Contrary, the measure of entropy production shows that the convection patterns in our models drastically change with the introduction and increase of a finite correlation length in permeability. An initial decrease of the average entropy production number with increasing lateral correlation length shows that less ensemble members show convection. When neglecting the purely conductive ensembles in our analysis, no change in the convection pattern is seen for lateral correlation lengths larger 2000 m. The fact that the choice of model dimension and scale influences a simulated convection pattern is known. Our findings, however, show that also the consideration of spatial anisotropy of important flow-parameters, such as permeability, should be considered if hydrothermal convection is likely to occur in a geothermal reservoir system. |