om Engineered Geothermal System (EGS) data acquired by the UtahForge project in 2024, we deconstruct the flow stimulation mechanics into four steps : (i) wellbore-centric fluid flow in/out of the cross-well flow pair; (ii) cross-well flow stimulation of a realistic poro-permeable crust; (iii) advective heat transport in the cross-well heat exchange volume; (iv) conductive heat transport into the cross-well heat exchange volume. Step (i) quantifies the heat energy production Q = ρCTV for fluid volumetric heat capacity ρC, crustal temperature T and wellbore volumetric flow V. Step (ii) quantifies the volumetric flow V ~ 2πr0φv0ℓ for bulk fluid flow φv0 into open wellbore interval of radius r0 and length ℓ. Step (iii) quantifies grain-scale equilibrium solid-fluid heat transfer for ambient crust poro-permeability distribution κ(x,y,z) ~ exp(αφ(x,y,z)) stimulated by EGS pressurisation as given by Peclet number Pe ~ r0φv0/D ~ 10 for thermal diffusivity D = K/ρC ~ 10-6 m2/s and K = thermal conductivity 4W/m/oC. Step (iv) approximates heat extraction as notionally carried by a central advective line-sink of strength Q/ℓ. Step(iii) is attested by microseismic emission first motions recorded by downhole sensors in which poro-permeability increases via increased poro-connectivity parameter α rather than via increased porosity αφ(x,y,z). The step (iv) central line-sink analytic advection-diffusion approximation T(r,r) gives heat reservoir lifetimes τ ~ 3 to 10 years for the UtahForge-scale 100m cross-well offset. Life-times τ treble for 200m cross-well offsets. The scale of heat depletion increases linearly with heat removal rate Q. From the present UtahForge data we can see how upscaling EGS Q requires upscaling cross-well offsets. The close association of EGS microseismicity with κ(x,y,z) ~ exp(αφ(x,y,z)) poro-permeability yields the observational means to survey/monitor the UtahForge EGS system and so quantify the future of EGS.

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