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

Abstract

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.