9:00 AM - 9:15 AM
[SSS09-01] Estimation of the elastic wave velocity in rock fractures revealed by the digital rock physics and energy approach
Keywords:digital rock physics, elastic wave velocity, permeability, S-wave splitting, Lattice Boltzmann Method
Recent advances in seismic velocity monitoring have enabled us to detect velocity changes not only in events with large stress changes (i.e., earthquakes), but also in geothermal reservoirs with smaller stress changes. Such velocity changes are generally interpreted by changes in porosity or crack density; however, velocity changes will also be triggered by the aperture in a fractured rock mass (e.g., seismogenic faults and geothermal reservoirs). Since the aperture will affect fluid flow behaviors in fractured rocks, velocity changes in fractures must have a correlation with hydraulic properties (e.g., fracture permeability). Although the relationship between the aperture and fracture permeability is well known, their relationships with elastic wave velocity are still unknown. It would be beneficial if changes in the aperture and fracture permeability are correlated with the seismic velocity that can be remotely monitored. In this study, we develop a method to estimate the elastic wave velocity in fractured rocks, and establish a model relating it to permeability changes. Since aperture changes are difficult to be measured in experiments, we employed an approach using digital data of real rock samples (i.e., digital rock physics).
We first analyzed the surface roughness characteristics of a single natural fracture (35 mm x 70 mm) retrieved from a geothermal area to create digital rock models. Based on the roughness characteristics, three synthetic fractures (24 mm, 48 mm, and 96 mm square) are generated by using fractional Brownian motion. We prepared digital rocks that have 10 mm thickness with 0.1 mm resolution, varying their mean aperture ranging between 0.05–0.2 mm. We then calculated the elastic moduli of the models by finite element analysis. In this analysis, the elastic energy was calculated under the isostrain condition, and the elastic moduli were calculated by the second-order derivative of the energy with respect to the strain. We also conducted fluid flow simulation using the lattice Boltzmann method on the same digital fracture model to investigate the relationship between permeability and velocity.
The numerical simulation of a single fracture shows that the P-wave velocity decreased up to 0.2 km/s and the S-wave velocity decreased up to 0.15 km/s as the aperture increases under our conditions. We also found that these velocity changes are less sensitive to the fracture size, and modeled the permeability-velocity relationship regardless of the fracture size. On the other hand, in the multi-fracture models, the velocity change depends on the fracture density, and it is necessary to estimate the fracture density in order to link it to the field observation data. In addition to elastic wave velocity direction perpendicular to the fracture plane, we also confirm the similar relationship between permeability and S-wave splitting.
We first analyzed the surface roughness characteristics of a single natural fracture (35 mm x 70 mm) retrieved from a geothermal area to create digital rock models. Based on the roughness characteristics, three synthetic fractures (24 mm, 48 mm, and 96 mm square) are generated by using fractional Brownian motion. We prepared digital rocks that have 10 mm thickness with 0.1 mm resolution, varying their mean aperture ranging between 0.05–0.2 mm. We then calculated the elastic moduli of the models by finite element analysis. In this analysis, the elastic energy was calculated under the isostrain condition, and the elastic moduli were calculated by the second-order derivative of the energy with respect to the strain. We also conducted fluid flow simulation using the lattice Boltzmann method on the same digital fracture model to investigate the relationship between permeability and velocity.
The numerical simulation of a single fracture shows that the P-wave velocity decreased up to 0.2 km/s and the S-wave velocity decreased up to 0.15 km/s as the aperture increases under our conditions. We also found that these velocity changes are less sensitive to the fracture size, and modeled the permeability-velocity relationship regardless of the fracture size. On the other hand, in the multi-fracture models, the velocity change depends on the fracture density, and it is necessary to estimate the fracture density in order to link it to the field observation data. In addition to elastic wave velocity direction perpendicular to the fracture plane, we also confirm the similar relationship between permeability and S-wave splitting.