Interpreting the Structural Characteristics of Underground Natural Flows to Determine the Productivity Potential of Hydrothermal Reservoirs

摘要

Saturated aquifers (including hydrothermal reservoirs) are all naturally leaking and are part of a global flow. The remarkable fact about these leaks is that they are located at one or more points (i.e. they do not form a line of fracture and they do not produce a wide filtration area). The flow arising from those natural leaks produce stable structures, called water vessels because of their form. The interest of those flows lies in the fact that they are detectable and that they provide a unique signature to locate the optimal collecting point of the hydrothermal reservoirs from which they originate. As not all hydrothermal reservoirs produce the same outgoing enthalpy flow rate (kJ/s), it is useful to collect detectable signs of the productivity potential. These signs come in the form of the overall morphology taken by the flow arising naturally from the hydrothermal reservoir. This paper covers the hydrodynamics of the underground natural flow from the leakage point where they leave hydrothermal reservoirs (called the exurgence point) to their multiple emergence points (i.e. the end point of their path). Deep water vessels almost always end up in aquifers and only exceptionally on the surface. In the latter ease they produce geothermal manifestations like hot springs, geysers, mud pools, fumaroles, solfatara, ... A tree-like structure of flow channels - or water vessels - materializes the outgoing flow. Based on the model outlined in this paper, the most salient features of that flow structure are defined and interpreted to assess the productivity potential of the hydrothermal reservoir from which it originates. The paper starts from the laws applicable to saturated flows under a high hydraulic gradient. The equations capturing those laws define the flow hydrodynamics as well as the structural characteristics or the 3D shape of the flow. The flow complexion or morphology is expressed in function of its state variables and in function of the parameters defining the properties of the litho-spheric context and of the geofluid. The reasons why these flow structures are extremely stable over geological periods, both with respect to their internal properties and to their shape are explained in detail. The relationships between geometric characteristics of the flow morphology and its hydrodynamic properties are defined, so that gathering data over the former provides useful information over the latter.