A hybrid discrete/finite element modeling study of complex hydraulic fracture development for enhanced geothermal systems (EGS) in granitic basements

• Published in: Geothermics Vol. 64; pp. 362 - 381
• Main Authors: Hofmann, Hannes, Babadagli, Tayfun, Yoon, Jeoung Seok, Blöcher, Guido, Zimmermann, Günter
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.11.2016; Elsevier Science Ltd
• Subjects: Basements; Complexity; Computer simulation; Crack propagation; Drilling; Enhanced geothermal systems; Finite element analysis; Finite element method; Fracture mechanics; Fractures; Geothermal power; Hydraulic fracturing; Hydraulic fracturing design; Interface between DEM and FEM; Mathematical analysis; Mathematical models; Multi-branched fracture development in granites; Networks; OpenGeoSys; Particle flow code; Permeability; Precambrian; Reservoirs; Rocks; Stimulation; Studies; System effectiveness; Two dimensional models; Viscosity; Wells; Hydraulic fracturing design; Multi-branched fracture development in granites; Interface between DEM and FEM; Enhanced geothermal systems; Particle flow code; OpenGeoSys
• ISSN: 0375-6505; 1879-3576
• DOI: 10.1016/j.geothermics.2016.06.016

•An interface combining a particle based DEM and an open source FEM code presented.•Complex hydraulic fracture development and thermal behavior was modeled for EGS.•Both wells should be stimulated to achieve a better hydraulic connection between them.•A small stage spacing (100m) should be used to increase fracture network complexity.•Wells should be drilled at an angle of 45° for the best thermal and hydraulic performance. The technical and economic success of enhanced geothermal systems (EGS) in granitic basement rocks depends on the ability to effectively develop complex fracture networks by hydraulic stimulation treatments. To study hydraulic fracture growth horizontally, two dimensional discrete element models were set up for the conditions expected in Precambrian basement rocks in Northern Alberta. The simulated fracture networks were then transferred to a finite element reservoir model to investigate the thermal and hydraulic efficiencies of the fracture networks. Natural and engineering factors influencing stimulated fracture network complexity are summarized and their significance is discussed based on the simulation results. The numerical results fit reasonably well to major field observations and led to a proposed reservoir stimulation guideline. This guideline involves drilling wells with a horizontal segment at reservoir depth, performing multiple sequential stimulation treatments alternating between two wells with an offset between the stages in both wells, and a long continuous injection of low viscosity fluid at a high rate without proppants. Favorable drilling targets for the development of complex fracture networks are brittle rocks in complex tectonic settings that are characterized by at least two intersecting natural fracture sets that are favorably oriented to the in-situ stress field, a low difference between the major principal stresses, and a low reservoir permeability. The reservoir simulations show the benefit of developing well connected complex fracture networks compared to single parallel hydraulic fractures.