Microseismic monitoring
Learning objectives
- Explain the source of microseismic events during hydraulic fracturing
- Describe the P-S arrival-time inversion for event location
- Quote typical event counts and magnitudes per frac stage
- Link event-cloud shape to Stimulated Reservoir Volume (SRV)
Hydraulic fracturing injects 10-20 MPa of fluid pressure into a rock formation through a horizontal well. Above a critical stress threshold, the rock cracks. Each micro-crack radiates small P and S wavelets, a “microseism”. Nearby receivers pick them up; picking arrival times and inverting locates each event. The cloud of located events maps the rock volume actually stimulated by the frac, the Stimulated Reservoir Volume (SRV).
Event physics
Magnitudes typically M -3 to M -1 (occasionally M 0-+1 if a fault reactivates). That is 1 to 10 million times smaller than a felt earthquake. The radiated signal is a small wavelet 10-20 ms long; the pattern is a double-couple focal mechanism characteristic of a micro-fault slipping. Multi-stage horizontal fracs produce 5-20 thousand locatable events over 5-10 days of pumping.
Location inversion
Pick P and S arrival times at every receiver. P travels at vₚ, S at vₛ ≈ vₚ/1.7. Time difference Tₛ-Tₚ gives hypocentre distance. Multi-receiver intersection gives direction. The ray-traced, velocity-model-driven inversion typically locates each event to 10-30 m horizontally and 15-40 m vertically, resolution limited by picker SNR and the near-far offset mix of the receiver array.
Decisions from the cloud
Three operational decisions depend on the map. (1) Stage-to-production correlation: which stages produced stimulation clouds that match later production from the well? (2) Frac-height containment: did the cloud stay in the target formation or did it grow into the seal / aquifer / neighbouring producer? (3) Fault-reactivation screening: do events align on a planar feature larger than a typical stage cloud? If yes, a fault is being re-activated and injection pressure should be backed off.
Monitor-well geometry
Typical monitor wells sit 200-1500 m from the injection. Too close: signal clips the receivers. Too far: location error explodes because radii becomes near-parallel. Industry sweet spot is 500-1000 m. Surface microseismic arrays exist too, but with 100-1000× worse SNR than downhole, typically used when a monitor well is not available.
References
- Aki, K., Richards, P. G. (2002). Quantitative Seismology (2nd ed.). University Science Books.
- Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
- Yilmaz, Ö. (2001). Seismic Data Analysis: Processing, Inversion, and Interpretation of Seismic Data (2 vols.). SEG Investigations in Geophysics 10.