In this article, the concept of Fatigue Hydraulic Fracturing (FHF) is described, and its geothermal application is discussed. The basic idea behind fatigue fracturing is to vary the effective stress magnitudes at the fracture tip to optimize fracture initiation and growth. The optimization process can include lowering seismic radiated energy and/or generating fracture networks with various geometry and permeability. Historically, we start referring to results from mechanical laboratory core testing, discrete element simulation of fluid-induced seismicity, and application of cyclic water-fracs at the enhanced geothermal system site Gnβ-Schönebeck, Germany. Then, an in situ experiment at Äspö Hard Rock Laboratory is summarized to bridge the gap between laboratory core testing and wellbore-size hydraulic fracture treatments in hard rock. Three different fluid injection schemes (continuous, progressive and pulse injection) are tested underground in naturally fractured, crystalline rock mass in terms of associated induced seismicity and permeability performance. Under controlled conditions, hydraulic fractures are extended to about 20–40 m2 in size from a 28 m long, horizontal borehole drilled from a tunnel at 410 m depth. The facture process is mapped by an extensive array of acoustic emission and micro-seismic monitoring instruments. Results from three water-injection tests in Ävrö granodiorite indicate that the fracture breakdown pressure in tendency becomes lower and the number of fluid-induced seismic events becomes less when continuous, conventional fluid injection is replaced by progressive fluid-injection with several phases of depressurization simulating the fatigue treatment. One reason for this may be that in the dynamic, fatigue treatment a larger fracture process zone is generated compared to the size of the fluid pressurized zone developing during the injection phases into crystalline rock. We see mine-scale tests with hybrid sensor arrays of importance to identify and understand the actual hydraulic fracture mechanisms in hard rock. In addition, the mesoscale data obtained underground allow downscaling to laboratory core results, and upscaling to borehole reservoir stimulation results.

1. Introduction

Hydraulic fracture growth through naturally fractured rock is an area of interest and current research for petroleum, mining and geothermal applications. In particular in the development of enhanced geothermal systems (EGS), hydraulic fracturing is used to form fracture networks connecting injection and production well for heat exchange purposes. However, hydraulic fracturing imposes environmental risks, one of which is induced seismicity associated with the permeability enhancement process [1–3]. Of particular interest are, therefore, methods that limit the number and magnitude of seismic events while the required fracture permeability is obtained. In this context, the fatigue hydraulic fracturing (FHF) concept and the multi-stage hydraulic stimulation concept have been proposed. The key point in fatigue hydraulic fracturing is the frequent lowering of the injection pressure to allow stress relaxation at the fracture tip [4]. Reducing the maximum injection pressure by alternative injection schemes will affect the damage zone surrounding the fracture and also the radiation pattern of seismic events associated with fracture growth [5]. For multi-stage stimulation, instead of massive stimulation, injection rate and pressure are controlled, and the reservoir is formed stage by stage [6, 7].

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