The spatial distribution of rock properties contains substantially more valuable information about the hydraulic fracturing process than a map of the hypocenter locations. Here, I introduce a methodology for processing microseismic data that reduces lengthy microseismic records (spanning over hours and days) to just a few seconds that contain all relevant information about the spatial distribution of the S- and P-wave differential slowness. Full-waveform inversion of this reduced data volume yields high-resolution spatial distribution of a particular combination of S- and P-wave velocities as well as accurate hypocenter locations. The method is not only computationally efficient, but also does not require any phase information of the source time functions.
The permeability of reservoir rocks can be enhanced through hydraulic fracturing, wherein high-pressure fluids and proppants are pumped into rock formations to create fractures. These fractures create pathways for the oil and gas to flow from the stimulated portions of the rock to the wellbore and then to the surface.
The above process breaks up the rock at many locations in the formation. The disintegration or fracturing of the rocks are associated with a significant drop in the P-wave and S-wave moduli. At other locations, no fractures are created; instead the pore pressure increases because of the fluids pumped at high pressure. These zones will experience a drop in P- and S-wave velocities as can be seen from Figure 1. Yet at other locations, where the formation has neither been fractured nor has experienced a pore-pressure increase (because of the lack of hydraulic communication with the well bore), the rocks might be subjected to an increase in confining stress. The rock formations above and below the hydraulic fracture zone are a prime candidate for confining stress effects only. The P- and S-wave velocities would increase in these zones (Tosaya (1982), Figure 1).
The first two zones, fractured rocks and pore-pressure-affected rocks, will result in hydrocarbon production because of hydraulic communication with the well bore. The third zone with confining-stress effects only, however, will not contribute to any production because of the lack of hydraulic communication with the well. Knowing the P- and S-wave velocity distribution can provide valuable information necessary for computing stimulated reservoir volume (SRV), planning re-fracturing operations, and in overall resource exploitation.