Field estimates of fracture compliances using active & passive seismics

Fracture networks provide a major control on the permeability of petroleum reservoirs, thus the ability to remotely characterize them is of great importance. Seismic anisotropy is a useful attribute to determine fracture orientation and provide some indication of the fracture density, however it is often difficult to constrain their ability to facilitate fluid flow. A potentially useful property to provide insight into this is the ratio of the normal to tangential fracture compliance (ZN/ZT). ZN/ZT is sensitive to many properties including: The stiffness of the infilling fluid, fracture connectivity and permeability, and the internal architecture of the fracture. Although P-wave anisotropy is primarily controlled by ZN and S-wave anisotropy by ZT, we find that for waves propagating oblique to the fracture plane, both P- And Svelocities are sensitive to the ZN/ZT ratio. Thus we can exploit these sensitivities to can gain some insight into ZN/ZT of subsurface fractures. We demonstrate this by estimating ZN/ZT using: (a) Azimuthal variations in P- And S-velocities estimated from a near-surface refraction survey, and (b) shear wave splitting measurements from a hydraulic stimulation microseismic dataset. In both cases we found that the natural fracture ZN/ZT was relatively low (∼0.2-0.3), but in the case of the hydraulic stimulation ZN/ZT appeared to increase after the initial stage of the stimulation. We suggest this may be due to the increase in fracture connectivity and the generation of new clean fractures.