MODELING GEOTHERMAL DISSOLUTION OF KARST FRACTURES

Most previous studies of dissolution growth in karst fractures have focused on meteoric systems. Calcite has a retrograde solubility with respect to temperature, therefore different behavior would be expected in hydrothermal systems. As deep geothermal waters rise through karst formations, they cool and maintain their dissolutional aggressiveness. We have developed several models to investigate the growth of calcite fractures in an effort to understand hypogene cave development. Our models include three-way coupling between flow, heat and mass transfer. We considered several dissolution models, including temperature controlled gradient reactions and three kinetic reactions. We will present computational results contrasting meteoric and geothermal cases. Growth of meteoric systems is commonly measured in terms of the breakthrough time. In these systems both the overall fracture aperture and flow rate increase causing calcite-unsaturated water to occupy the entire fracture. The time when this first occurs is considered the breakthrough time and typically appears at the onset of turbulent flow. Preliminary results from some of our simulations indicate two breakthrough-like phenomena. One is similar to that observed in meteoric systems, but it does not correspond to the laminar-turbulent flow transition point. This occurs because a thermal gradient along the fracture length combined with retrograde solubility allows growth along the entire fracture. When the aperture increases over the length of the fracture, a rapid increase in flow rate occurs. As the flow rate increases there is a switch from conduction dominated heat transfer to convection dominated heat transfer which reduces solubility in the fracture and slows growth. The second, lesser breakthrough is sometimes observed at the laminar-turbulent flow transition where the flow rate again increases, however this is relatively small compared to the first breakthrough.