Event locations and magnitudes give limited insight into the processes that control the growth and dynamics of hydraulic fractures. Our understanding of reservoir behaviour can be enhanced by considering the seismic moment tensor representations of these events, which serve as a direct snapshot of the instantaneous deformation of the surrounding rock by the seismicity.
Microseismic Moment TensorWhen an event occurs, the rock has experienced some form of failure. Seismic moment tensors are able to describe whether the failure is shearing or tearing (i.e. mode II or mode III failure), if a change in volume has occurred (i.e. opening or closing fractures) or some combination of these scenarios.
Seismic moment tensors (SMTs) are represented by “beachballs”. The pattern on the symbol indicates the type of failure which occurred and the colour represents the relative proportion of double-couple (DC), isotropic (opening or closing) and compensated linear vector dipole (CLVD) modes.
The characteristic beachball design scheme and orientation of the moment tensors represent the strain at the source. The coloured sections of the beachball are in tension while the white sections are being compressed. This deformation is also described by the tension axis (blue, outward arrow) and the pressure axis (red, inward arrow).
Benefits of Seismic Moment Tensor Inversion (SMTI) Analysis:
- Characterize microseismic event failures
- Advanced understanding of fracture propagation
- Identify which fractures are contributing to production
- Increased knowledge of fracture networks
- Provides insight into fracture effectiveness
- Characterize stimulated volumes and surfaces
- Estimate fluid flow enhancement
- Understand stress-strain field and fracture orientations
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