Abstract

Microseismic (MS) monitoring of hydraulic fractures using either the downhole or surface geophones has become a common practice. The same setup may be used for the multi-stage fractures. The main limitations in using these data to understand hydraulic fracturing results are: the sensor location which is far away from the source, limited azimuthal coverage and inadequate velocity models. Analysis becomes more complex when we start dealing with anisotropic reservoirs. Typically the service companies produce the spatial and temporal plots of hypocenters without estimates of uncertainty leading the engineer to believe the hypocenter locations are exact. The hypocenter location problem becomes more complex in anisotropic shale reservoirs. Hypocenter locations are often determined from the arrival times of P-wave and S-waves and a known velocity model. The difference in the velocity structure and the complex fracture network make accurate fracture mapping difficult. In this paper, we report on a series of laboratory hydraulic fracturing studies performed on sandstone and a strongly foliated metamorphic rock, pyrophyllite. Microseismic events associated with the hydraulic fracturing are dominated by shear failure; fracture orientation is controlled as predicted by applied stresses in isotropic materials. However, in anisotropic materials the hydraulic fracture direction is influenced by the magnitude of anisotropy but is predictable when the anisotropic elastic constants are known. Visual evidence supports the hypocenter locations and focal mechanism determinations. Scanning Electron Microscope (SEM) observations reveal the nonlinear and nonplanar nature of the induced fractures. The fracture trace confirms the wider aperture at the injection point and the frequent occurrence of the terminations and bifurcations farther along the fracture front.

You can access this article if you purchase or spend a download.