| Abstract |
The estimation of fluid flow behavior (permeability and water saturation) in the fracture is essential for the sustainable development of geothermal resources. Recently, fluid flow in the geothermal reservoir has been attracting attention to succeed in EGS development. The fluid flow and its distribution under the ground could be detected by geophysical explorations (seismic and electromagnetic methods); however, the quantitative interpretation of these data is still difficult due to the lack of experimental data in fractured rocks. In this study, to discuss the effect of fluid flow behavior on these geophysical parameters, we measured and calculated them by using fractured rock samples that have different apertures and fluid distributions. For the numerical simulation, we firstly digitalize real rock fractures that are also used for laboratory experiments. The fluid flow simulations were then performed by using the Lattice Boltzmann Method. After analyzing the fluid distribution in the fracture, we conducted resistivity and elastic wave simulations by finite-element and finite-difference method, respectively. As a result of single-phase (water) flow analysis, permeability decreases with pressure increase in our experiment, and calculated permeability also shows the decrease as the aperture decreases. This agreement suggests that fracture permeability is constrained by aperture closure due to the pressure change. Our digital rock simulations reveal that changes in permeability and resistivity are controlled by disconnection of fluid connectivity, whereas velocity change is constrained by porosity variation due to fracture roughness. We also upscaled the relationship between fracture permeability, resistivity, and elastic wave velocity in a laboratory specimen to larger fracture dimensions and formulated their relationships regardless of the fracture sizes. Our empirical formula directly predicts changes in fracture permeability from geophysical properties, which may make it possible to monitor subsurface hydraulic activities through geophysical observations. In terms of the effect of fluid distributions under two-phase (water/gas) flow analysis, elastic wave velocity changed with the heterogeneity of fluid distribution while resistivity increases with higher connectivity of water even at the same water saturation and aperture conditions. Thus, elastic wave velocity or resistivity analysis has a difficulty to estimate the water saturation due to the complexity of fluid distribution. However, in contrast to this, our results suggest that the heterogeneity of fluid distribution can be expressed by aperture and connectivity from our digital rock fracture approach. Our results shed light on the potential of estimating fluid distribution by integrating seismic and electromagnetic explorations. |