Reconstructing the geothermal reservoir model and estimating the structural permeability variation in the metamorphic terrane in Ruisui, Taiwan

    This study identifies the key geothermal features in the metamorphic terrane within the Yuli belt (a metamorphic mélange) in Ruisui, eastern Taiwan, including fault-related pathways for hot fluids, cap rocks, and naturally fractured reservoirs, and builds a structural permeability distribution model. A preliminary geothermal reservoir model is created for a depth of 3 km, by integrating geological field analysis, magnetotelluric (MT) surveys, hot spring geochemistry, and data from 3 exploration wells. The low resistivity zones shown in MT results are beneath a unit of amphibole-albite schist, containing numerous ultra-mafic blocks at different sizes. We tentatively interpret the four MT low-resistivity zones as four possible reservoirs, which are associated with high-density jointed and faulted quartz-mica schists, underneath cap rocks of amphibole-albite schists, which reveal poor development of joints. The geochemical analysis of hot spring water indicates a high concentration of sodium ions, potentially originating from the amphibole-albite schist. We also observe surface exposures where water up to 50-60^oC flows up through NW-SE trending sub-vertical faults with the downhole temperature up to 200^oC at a depth of 0.9km.

    To determine whether the NW-SE vertical fault zones are suitable for open structures, which appear to be the major geothermal up-flows, we measure the orientation of faults and 3 sets of joints, most of which are sub-vertical in the field. The principal stress orientations are adopted as σ1 vertical, σ2 N120^oE, and σ3 N30^oE, by combining GPS observation, focal mechanisms of shallow earthquakes, and fault slickenlines measured in exposures. We conduct fault-slip inversion and obtain the stress ratio phi=0.51. Utilizing the Mohr-Coulomb failure criterion coupled with selected parameters, such as in-situ principal stresses, fluid pressures, and rock mechanical properties, our model indicates that steeply-dipping NW-SE trending (N120^o-130E^o) joints and faults are mechanically prone to open as fluid infiltrating or injecting during thermal events.

    We furthermore measured the fracture length and density at the aforementioned hot spring exposure where 50-60^oC hot fluid flows out from an NW-SE trending sub-vertical fault. The measuring result shows that the density of the fractures (or joints) decreases away from the fault core, thus we anticipate the joints tend to form on the microfractures created by the fault, which explains an increase of the permeability toward the fault. As for estimating the structural permeability along the NW-SE fault and open joints, the fracture aperture is calculated using linear elastic fracture mechanics, and then the permeability is estimated by using the cubic law for fluid flow in rock fractures. By doing so we obtain the structural permeability in the NW-SE fault zone, which exponentially decreases away from the fault core, and the permeability value ranges from 10^-10 to 10^-13 m² at a distance of 10 m.