Modeling Utah FORGE 2022 EGS Hydraulic Stimulations: Tensile Hydraulic Fractures versus Fluid-Induced Dilatant Shear Ruptures

TL;DR

This study compares hydraulic fractures induced by different fluids at Utah FORGE, demonstrating how fluid viscosity influences fracture type, microseismic activity, and failure mode through analytical and numerical modeling.

Contribution

It provides a detailed analysis of how fluid viscosity affects hydraulic fracture development and failure modes, supported by combined analytical and 3D numerical models.

Findings

Cross-linked gel causes sustained microseismic activity and planar tensile fractures.

Slickwater may induce shear or toughness-dominated fractures, with immediate microseismic arrest.

Numerical models confirm the influence of stress and fracture orientation uncertainties.

Abstract

We investigate two hydraulic stimulation stages performed in April 2022 at the Utah FORGE enhanced geothermal system test site using analytical and numerical models for tensile hydraulic fractures and fluid-induced dilatant shear fractures. The two injection stages differ primarily by the viscosity of the fracturing fluid. Despite similar injection rate schedules and well-head pressure responses, the two stages exhibit markedly different post-shut-in microseismic behavior. The cross-linked gel stage shows sustained microseismic activity for several hours after shut-in, whereas the slickwater stage exhibits an immediate decrease. For the cross-linked gel stage, the located microseismic events reveal the development of a planar radial fracture and allow confident retrieval of the fracture extent evolution with time. We demonstrate that this evolution follows the scalings predicted for viscosity-storage-dominated radial hydraulic fracture by analytical models, providing strong evidence for the development of a planar tensile hydraulic fracture. We further show that leak-off is required to reproduce the fracture extent. In contrast, the immediate arrest observed during the slick-water stage suggests either a transition to a toughness- or leak-off-dominated hydraulic fracture regime, or the development of a fluid-induced shear fracture. We show that the slickwater stage could plausibly correspond to a dilatant shear fracture, provided sufficient dilatancy, whereas this hypothesis is invalidated for the cross-linked gel stage. We confirm these insights using a 3D axisymmetric fully-coupled hydro-mechanical numerical model capable of resolving both tensile and shear failure modes, and including leak-off. Finally, we propagate uncertainties in the in-situ stress state and natural fracture orientations through this numerical model to assess their impact on injection pressures.