Deconstructing the Crustal Heat Extraction of UtahForge Engineered Geothermal System

Abstract

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.