The UtahForge EGS Stimulation Volume as an Aquifer with Engineered Transmissibility and Storativity

Abstract

The UtahForge 2024 EGS stimulation of a 400m horizontal cross-well volume of nominal 100m x 100m cross-section at 2.5km depth in Θ ~ 185oC crystalline crust culminated in well-to-well advective flow V =30 L/s for 30 days at heat energy production Q = ρCΘV ~ 20MWth. Abstracting the 400m cross-well stimulation volume as a thermal energy line-sink of radius R ~ 50m shows that crustal heat conduction recharges the stimulation volume so that the 20Mwth heat energy extraction reduces the crustal heat store temperature by perhaps 10% in ~ 3 to 10 years. Engineering the breakthrough line-sink cross-well advective flow volume requires our immediate and close attention. Accordingly, we note the UtahForge EGS stimulation was planned/executed as a series of hydrofrack-like fluid injections at a dozen plus sites along the 400m well-pair reach. Significant well-to-well flow occurred, however, only when all hydrofrack hardware was drilled out of the stimulation reach and the 400m extent of the injection well was pressurised for flow into the 400m open production well. It follows that the UtahForge EGS cross-well stimulation flow is advective heat transport through an engineered aquifer-like cross-well crustal volume. Eliminated from heat engine consideration are conduction-specific discrete cleavage flow planes. UtahForge cross-well flow involves instead fluid seeping through ambient crust poro-permeability distribution κ(x,y,z) ~ exp(αφ(x,y,z)) controlled by pink-noise porosity φ(x,y,z) at cm-km scales as attested by well-log, well-core, and well-flow data worldwide. While hydrofrack-like stimulation from both the injection and production wells conditioned the cross-well volume, post-stimulation open-hole cross-well advection flow resembles aquifer pump test diffusion between two wellbores. Well-to-well fluid diffusion is given by T [∂2h/∂2r + 1/r ∂h/r] = S ∂h/t + Qδ(r1) - Qδ(r2), free of cumbersome aquifer boundaries. Coefficients T and S function as unit aquifer transmissibility and storativity representing effective permeability and porosity in the EGS stimulation volume. Time-dependent UtahForge well-to-well flow data provide a pump-test-like constraint on the ratio T/S; wellbore-centric Darcy flow constrains the effective transmissivity T. Microseismicity associated with fluid flow in κ(x,y,z) ~ exp(αφ(x,y,z)) media can serve to image/monitor and survey cross-well aquifer flow. Future UtahForge data can provide more comprehensive pump-test-like constraints on the EGS stimulation process