Title:

Fault-Controlled Damage and Permeability at the Brady Geothermal System, Nevada, USA

Authors:

Roselyne C. LABOSO and Nicholas C. DAVATZES

Key Words:

fractures, stress, slip, Brady Geothermal Field

Conference:

Stanford Geothermal Workshop

Year:

2016

Session:

Modeling

Language:

English

Paper Number:

Laboso

File Size:

2689 KB

View File:

Abstract:

Identifying and locating permeable zones in geothermal fields is a critical step in determining reservoir potential for energy production. Despite a general association with active faults, geothermal systems typically display heterogeneously distributed fracture density, connectivity, and attitude – all of which influence permeability. We postulate that permeability in a geothermal system is favored in volumes of enhanced coulomb stress and reduced compression that promote high fracture density. Such volumes develop along a large active fault where slip locally perturbs the remote stress. Conversely, permeability can be inhibited in locations where coulomb stress is reduced, or where the faults are poorly oriented in the stress field and consequently slip infrequently. This paper explores the relationship between fault geometry and associated stress perturbations that result from fault slip driven by a remote stress. Our approach to predicting permeability from these local variations in stress is applied to the Brady normal fault system of Nevada, where the permeable volume is well-documented by the distribution of successful and marginal geothermal production wells, surface hydrothermal features, microseismicity, and surface deformations associated with the reservoir. The analysis is conducted using Poly3D, a boundary element program based on the three-dimensional fault geometries constructed by Jolie (2014) in a homogeneous linear elastic, half-space. The faults are loaded with remote stresses measured as part of the Brady EGS experiment and are also generally consistent with the tectonic setting and the geologic stress tensor inverted from the fault geometry and slip indicators by Jolie (2014). The simulations demonstrate a complex pattern of stress perturbation, influenced by the detailed fault geometry. Preliminary results show a correlation between regions of enhanced coulomb stress, reduction of least compressive principal stress, and successful wells, whereas low permeability wells lie outside these volumes. We further explore the promotion and suppression of fracturing observed in the models of the detailed fault geometry using idealized faults to quantify the role of segmentation, large-scale roughness, relative attitude, and stress state.


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