Title:

Dependency of EGS Development on the Alignment Between Natural Fracture Set Orientations and Regional Stress State

Authors:

Aleta FINNILA, Thomas DOE, Robert MCLAREN

Key Words:

enhanced geothermal systems, EGS, discrete fracture network, DFN, normal, strike-slip, thrust, stress, stimulation, hydro-shearing

Conference:

Stanford Geothermal Workshop

Year:

2017

Session:

Enhanced Geothermal Systems

Language:

English

Paper Number:

Finnila

File Size:

911 KB

View File:

Abstract:

This work examines how differences in the orientation of two conjugate sets of natural fractures affect the efficacy of hydraulic stimulation to create successful EGS sites in normal, strike-slip and thrust faulting regions. Many potential EGS locations require hydraulic stimulation to create connectivity between injection and production wells and/or increase the number of conductive pathways between the wells. Hydro-shearing natural fractures by increasing their pore pressure sufficient to cause shear failure is one method being actively explored. The injection pumping pressure required to cause hydro-shearing is a function of the stress state of the reservoir and the orientation of the natural fractures. By keeping lithology, thermal gradient, well geometries and both original and stimulated fracture transmissivity constant while varying natural fracture set orientation and regional stress state, this paper explores the significance of these varying factors for EGS impedance and thermal evolution. Performance using vertical vs horizontal well trajectories is also examined. Natural fracture orientation variation for normal stress conditions shows large contrasts in the percentage of critically stressed fractures for two sets both aligned maximally aligned to the current stress field (87%), one set maximally aligned (45%) and neither set well aligned (4%). Impedance is quite sensitive to natural fracture alignment but not well orientation. Thermal performance is similar for both one and two sets aligned with the stress field. Strike slip stress conditions show less dependence upon natural fracture set orientation for the number of critically stressed fractures during hydro-shearing (16-86%) but greater sensitivity for impedance and well orientation with horizontal wells out-performing vertical wells. Compressional stress conditions show a range for the percentage of critically stressed fractures of 10-89% for having zero, one or two sets being favorably aligned, a strong dependence for impedance and some sensitivity for thermal evolution. Well orientation is important for the compressional stress state with vertical wells out-performing horizontal ones.


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