MODELING DISSOLUTION AND FRACTURE ENLARGEMENT BY COOLING THERMAL WATERS DURING EARLY EVOLUTION OF HYPOGENE KARST SYSTEMS

The evolution of karst systems under hypogene conditions is investigated using coupled numerical models of flow, heat transfer and dissolution chemistry in a single fracture. Retrograde dissolution of calcite in the H_2O-CO_2-CaCO_3 system along a cooling flow path is investigated using both equilibrium and kinetic dissolution models.

Initially there is a positive feedback between flow, heat transfer and dissolution that results in a continuously increasing mass flow rate. This is followed by a sudden rapid increase in mass flow-rate through the fracture. We call the time when this occurs the maturation time. As the flow rate continues to increase after maturation the coupling of flow, heat transfer and dissolution creates a negative feedback which slows the rate of fracture growth.

A simple analytical model is developed to explain and reasonably predict the maturation phenomenon. The maturation phenomenon is shown to result from the cubic relationship between aperture and transmissivity in fractures. This leads to a simplified model for understanding pre-maturation behavior. The simplified model is used in an initial investigation of pre-maturation behavior in two-dimensional variable aperture fractures, which shows that preferential flow paths are amplified with time and higher aperture variability leads to less diffuse growth.