Abstract

Thermal-Hydrological-Mechanical (THM) processes during the 2014 stimulation at Newberry Volcano Enhanced Geothermal Demonstration Site were simulated using TOUGHREACT-ROCMECH [1], employing initial thermal-hydrological properties calibrated to previous injection tests [2,3]. Thermoporoelastic effects as well as simultaneous shearing on one, two or three planes was allowed, using Mohr-Coulomb criteria. The minimum regional stress is E-W [5]. Permeability changes on fracture dilation followed a cubic law. Initial reservoir porosities and permeabilities were 0.03, 5×10-18 m2E-W, and 10-17 m2N-S and vertical, with higher permeability in known fracture zones. Fracture dilation angle, cohesion, and initial fractions of fracture porosity and permeability were varied to reproduce observed flow rates. The best parameters were fracture porosity equal to 0.1% of initial porosity, 2 MPa cohesion before fracture re-activation, and 2° dilation angle. Negligible cohesion results in more shearing and flow at initial low pressures. A 3.3 % initial fracture porosity per bulk porosity resulted in negligible permeability enhancement and underestimated flow. A smaller dilation angle (0.6°) and lower fracture porosity proportion (0.0007), resulted in intermediate permeability enhancement and flow. Elements close to open well undergo simultaneous shearing on two shear surfaces, keeping two principal stresses equal, while their difference from the third principal stress lessens

1. INTRODUCTION

Injection of large volumes of fluid at high pressure allows changing effective stresses to levels where shearing occurs on pre-existent fractures, relieving differences between regional stress levels in different directions (principal stresses). Assuming that fractures are somewhat nonplanar, shearing results in some fracture opening, and an increase of fluid flow or fluid permeability. Such opening is expected to remain after pressure decreases at the end of injection, resulting in easier circulation of fluids and an increased permeability. When permeability is increased in large volumes of rock at elevated temperatures, this may allow sustained removal of heat and render geothermal exploitation economic. We model conditions considered likely at an Enhanced Geothermal System Demonstration site at Newberry Volcano in Oregon. As precise values of some parameters are unknown, we model a few possibilities, and note some differences of their behaviour

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