MEQ ~ Permeability – Modelling High Frequency Emissions from Stimulation-Induced Seismic Activity in the Ambient Crust
Peter LEARY and Peter MALIN
[Institute of Earth Science and Engineering, Univ A, New Zealand]
Microseismic activity in magnitude range -1 less than m less than 2 induced by controlled injection of fluid into ambient crystalline rock at 6km depth and recorded by a wellbore sensor array at 2.5km depth above the injection volume shows that the radiated seismic waves are dominated by high frequencies 200 Hz less than f less than 800 Hz. The observed velocity waveform spectra to not conform to the standard slip-plane phenomenology, e.g., v(f) ~ v0f/(1 + f/f0)2) with characteristic frequency f0 ~ 1/L scaling inversely as slip surface dimension L ~ 10-m/2. We further observe that the statistics of microseismic event spatial correlation and lognormal size distribution are congruent with the statistical distributions of crustal permeability given by κ(x,y,z) ~ exp(αφ(x,y,z)) for normally-distributed spatially-correlated formation porosity distributions φ(x,y,z) with parameter α fixed by the empirical condition αφ ~ 3-5 for formation mean porosity φ. In the absence of induced microseismicity waveform evidence for seismic slip activity on planar surfaces, we use Haskell’s formalism for elastodynamic waveform radiation by arbitrary slip dislocation structures to model the observed high frequency waveforms and their spectra. Satisfactory generic waveform attributes proceed from Haskell seismic dislocation fronts moving at low rupture speeds across spatially complex localised distributions κ(x,y,z) consistent with crustal permeability empirics. The same computation performed for rapid rupture speeds on planar surfaces gives standard fault-like slip waveform spectra. Our modelling indicates that induced seismicity in ambient crust occurs largely on pre-existing crustal permeability structures of limited physical extent rather than on planar slip surfaces of potentially unlimited extent. In these circumstances is logical to expect there is essentially no hazard of run-away earth ruptures occurring during fluid injection in the ambient crust. It is further logical to expect that wellbore fluid pressurization does not induce controllable extensive stress-aligned planar flow structures as commonly speculated in the past.
|        Topic: Geophysics||Paper Number: 13032|