Permeability-specific Spatial Correlation Systematics for UtahForge EGS Stimulation Meqs


Peter LEARY and Peter MALIN

Key Words:

EGS, Meqs, crustal permeability, spatial correlation flow, modeling


Stanford Geothermal Workshop




Enhanced Geothermal Systems



Paper Number:


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1605 KB

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Recent UtahForge well 16A(78)-32 stimulation Meq XYZM (location/magnitude) data show two spatial correlation features observed in ambient crust Meqs: lognormal moment distributions and event-pair spatial correlation scaling with inter-event offset r, G(r) ~ 1/r^p, p ~ 1. Neither spatial correlation feature is compatible with the uncorrelated randomness that is typically assumed for crustal fluid flow modeling via spatially averaged heterogeneity and/or DFN distributions. Both UtahForge Meq spatial correlation features are, however, congruent with rock-fluid interaction spatial-correlation empirics attested by ambient crust well-log, well-core, and well-flow data worldwide. UtahForge Meq spatial correlation features are seen to apply equally to ambient crustal Meqs recorded at tectonic, convective geothermal, and crystalline basement EGS stimulation sites. More specifically, the UtahForge Meq spatial correlations are latent in the ambient crust poro-permeability relation κ(x,y,z) ~ exp(αφ(x,y,z)), where porosity 0 less than φ(x,y,z) less than 1 is a pink-noise random power-law scaling volumetric distribution and α a poro-connectivity parameter with empirical value range 2-3 less than mean(αφ) less than 5-6 for crustal formations with mean porosities 0.003 less than mean(φ) less than 0.3. The poro-permeability relation κ(x,y,z) ~ exp(αφ(x,y,z)) is inherently lognormal for the empirical αφ condition, with high-end permeability sites spatially correlated as G(r) ~ 1/r^p, p ~ 1. It follows that many if not most UtahForge stimulation Meqs, in line with ambient crust Meqs elsewhere, arise from low-velocity seismic slip within high permeability poro-connectivity structures rather than as high velocity slip on planar fault surfaces. Meq moment distributions thus reflect ambient crust permeability structures, while hectohertz seismic emission waveform spectra and coda scattering of crustal basement EGS stimulation Meqs provide support evidence for seismic slip on poro-permeability structures given by the κ(x,y,z) ~ exp(αφ(x,y,z)) ambient crust rock-fluid interaction empiric. Crustal flow models for EGS stimulation processes require systematic computational means for representing the power-law scaling spatial correlation properties of ambient crust permeability distributions.

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