Fundamental study of seismic emission tomography in terms of fluid pressure fluctuations

Recent years, seismic emission tomography which utilizes seismic oscillation due to fluid flow inside fractures has drawn more attention. However, the relationship between observed seismic data and fluid behavior in a reservoir has not been revealed yet. In the present study, we conduct numerical experiments for understanding the mechanism of the induced microseismic emission in order to extract more information about fluid behavior from observed seismic data. We simulate fluid flow in a fracture using the lattice Boltzmann method (LBM). We adopt two numerical models, i) parallel plate model, and ii) pore throat model. We calculate stress changes at the fracture wall induced by unsteady flow and multi-phase flow fields. The unsteady flow is generated by cyclic pressure change at the inflow boundary. In this case, inner portion of the fracture is filled only water or oil. In the multi-phase flow, we consider migration of oil droplet in a fracture with a throat filled by water. In the parallel plate model, larger shear stress change can be observed in the case of oil. This stems from more rapid change in fluid velocity close to the fracture wall due to the high viscosity of oil. In the case of the multi-phase flow in the pore throat model, about 8 Pa of shear stress and 28 Pa of normal stress are observed at the fracture wall when an oil droplet whose diameter is 1 mm passes through the pore throat. We estimate where fluid flowing using seismic wave from that stress changes. Our results show that the induced microseismic emission by fluid flow is strongly dependent on the fluid viscosity, geometry of fracture network, etc., which influences the pattern and the flux of the flow.