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Engineered Geothermal System (EGS) cross-well flow V ~ 30L/s @ Q ~ 20MWth for 30 days was observed by the Utah-Forge project in 2024. The cross-well flow doublet measured 400m in length at 100m offset. A first-order question is how sustainable is the doublet’s heat extraction Q. Deconstruction of the UtahForge flow data answers the sustainabil-ity question in terms fundamentally different from what has been long supposed. EGS stimulation fluids do not absorb heat via pipe-like cubic-law flow along induced fault-scale planar heat exchange surfaces. Rather, heat exchange oc-curs at the grain-scale along disseminated volumetric flow paths controlled by the ambient-crust poro-permeability dis-tribution κ(x,y,z) ~ exp(α(x,y,z)). More specifically, EGS deconstructs into four sequential steps : (i) stimulation of am-bient crust poro-permeable medium κ(x,y,z) ~ exp(α(x,y,z)) as α α’ > α in the cross-well volume; (ii)wellbore-centric flow V in/out of ambient crust poro-permeability distributions at wellbore radius along the 400m length of open well; (iii) heat advection in the cross-well volume with stimulated poro-connectivity parameter α’; (iv) sustainability-specific heat conduction into the stimulated cross-well volume from the surrounding unstimulated crust. EGS stimulation process step (i) is attested by microseismic emissions registered on downhole sensors. Step (iv) crustal heat conduction sustainability is approximated by a central line-sink analytic computation that yields heat reservoir sustainability of ~ 3-10 years for the UtahForge 100m cross-well offset. Greater sustainability implies larger cross-well offsets. The inti-mate relation between fluid flow and seismic emissions enables downhole seismic sensor data to image EGS flow stim-ulation activity. As the heat reservoir sustainability scales inversely with heat removal rate Q, UtahForge data show how upscaling EGS Q requires upscaling cross-well offsets. The association of EGS microseismicity with κ(x,y,z) ~ exp(αφ(x,y,z)) poro-permeability gives observational means to systematically monitor the future of EGS.