| Abstract |
Permeability is one of the most important factors which controls the occurrence of the geothermal resource because represents the capacity of a rock to store and transmit hot fluids. When primary porosity governs the system, the approach to understand the hydraulic behaviour of the permeable media is usually based on the Darcy’s Law; however, fractured media is more complex to analyse due to the interaction between fractures and faults that can mainly allows the pathway for the fluid flow, among other aspects. Furthermore, these discontinuities can be conditioned by both intrinsic properties of host rocks (e.g. crystals size, mineralogy, preferential orientation of crystals) and external factor (stress field, temperature, pore pressure). In this sense, previous studies have shown that the problem related to characterizing fracture permeability can be partially solved by laboratory experiments under in-situ pressure and temperature conditions (geo-mechanical response) and under saturated conditions (hydro-mechanical response) complemented with the use of X-Ray microComputed Tomography (XR-ïCT). This non-destructive method allows to examine the evolution of some petrophysical properties and also measure the porous system (e.g fracture network). These results can be modelled and interpreted by different numerical model used in the computational fluid dynamic (CFD) to solve problems related to the permeability. In recent years, Lattice Boltzmann Methods (LBM) turned into a fundamental tool in the simulation including both simple to complex multi-physical problems (e.g. heat transfer, fluid dynamics, multicomponent interactions). We selected the area surrounding Liquiñe, located at Southern Volcanic Zone (39 ºS, Chile), because it represents an excellent case of study of a fractured geothermal system hosted in crystalline rocks (mainly granitoids) affected by active faults of the Liquiñe Ofqui Fault System (LOFS) and Andean Transverse Faults (ATF), and the hot springs presented in this area are spatially controlled by the interaction of these faults. Because, there are not drill-holes data in this zone, to improve the knowledge about the circulation of hot fluids in the Liquiñe fractured geothermal system, petrophysical studies and modelling of fracture permeability were carried out in this study. For this, “healthy†samples of granodiorite from the outcrop were collected and were characterized by XR-mCT. After, samples were heated at 25 ºC (room conditions), 150 ºC and 210 ºC, and mechanically tested in the laboratory to generate fractures. The temperatures were selected in based on the estimated value by previous authors for this geothermal system. A digital analysis was carried out to measure the percentage of mineralogy and porosity and their distribution along the cylindrical samples and the main parameters related to fractures, such as their length, aperture and tortuosity. The hydraulic parameters were established from both the digital analysis and digital rock model by means of the Cubic Law, Kozeny-Carman relation and Lattice-Boltzmann method. The results obtained mainly shown that when temperature is increased the mean aperture decrease, but the tortuosity remains similar. About the value of the fracture permeability, it increases by several orders of magnitude with respect to the "healthy sample" but not between the non-heated and heated and fractured samples. Finally, this aspect may assume that the circulation of hot fluids in depth may occur under test conditions similar to those natural to this geothermal system. ACKNOWLEDGEMENTS This research was supported by projects FONDECYT Regular project #1180167 and FONDAP-CONICYT project #15090013 “Centro de Excelencia en Geotermia de los Andes†(CEGA). |