Hydraulic-geomechanical Effective Stress Model: Determination of Discrete Fracture Network Parameters from a Pump Test and Application to Geothermal Reservoir Modelling

Fracture networks dominate the permeability of crystalline geothermal reservoir rocks. Insitu stress conditions have a significant impact on the flow, transport and exchange characteristics of fracture networks. Here a geomechanical model is presented which describes fracture closure under effective stress and the change in parameters such as storage, permeability, porosity and aperture. The model uses geometrical considerations based on a fractal distribution of apertures on the fracture surfaces, and applies analytical elastic deformation solutions to calculate the strain response to increases in effective stress. The model is first applied to fit laboratory scale experimental data gained on the compressive closure of a fractured sample (Durham 1997) recovered from a depth of 3800m from the KTB pilot borehole (Emmermann and Lauterjung 1997). The elastic constants for these fits were established externally, the fitting parameters applied included the initial aperture of the fracture, the minimum contact area between the surfaces and the number of allowable contacts. After accurate fitting of the laboratory scale experimental data, the geomechanical model was applied at a field scale to aid in the modelling of a long term pump test in the KTB pilot hole, the open hole section being 3850 to 4000m. Effective hydraulic parameters determined by a finite element model of the fracture systems connected to the KTB pilot borehole were analysed on hand of the geomechanical model to allow the determination of the discrete fracture geometry operating within the fracture zone. This geomechanical model takes account of the changes in the flow parameters within the fracture systems due to changes in local effective stress as a result of the groundwater extraction. Applying the geomechanical model and an iterative procedure allowed the number of fractures in the fracture zones comprising the hydraulic signal, and their average aperture to be estimated. The number of fractures predicted to be hydraulically active in the fracture zone is of the same order as in-situ field measurements and the original fracture logs. INTRODUCTION Fracture systems dominate the mass and energy transport of deep crystalline rocks (Emmermann and Lauterjung 1997). The characteristics of the three dimensional fracture systems in terms of flow, transport and heat conduction are controlled by a number of critical factors. In particular the geometry of the system in terms of the orientation of the fractures in the pervasive stress system (eg. Kessels 2000), the fracture connectivity (Bour and Davey 1998, Manzocchi 2002), fracture permeability (Nicholl et al. 1999, Wang et al. 1988), porosity (Montemagno and Pyrak-Nolte 1995) and area of fracture system available for sorption and heat exchange (Renner and Sauter 1997, Wels et al. 1996, Watanabe and Takahashi 1995) form some of the most important factors which need to be addressed. One important aspect highlighted by a number of authors in the development of long term behaviour of fracture systems is their response to stress changes which may be generated due to hydraulic alterations of the systems, long term stress field alterations, and thermo-elastic stress alterations due to a change in the amount of heat in the systems (hot dry rock heat extraction). Indeed O’Sullivan et al. (2001) particularly pointed out that long term geomechanical effects of thermal stress changes on geothermal reservoir characteristics need to be investigated. The response of a fracture network to stimulation by either extraction or injection of fluid is a time dependent integral signal comprising the individual responses of the discrete fractures (McDermott et al. 2003). The individual responses of the discrete fractures within the fracture system are determined by interaction of the fluid injected or extracted and its physical characteristics such as density, viscosity, heat capacity and temperature. Within the solid medium factors such as the elastic response of the medium and pervasive insitu conditions including temperature and pressure have a critical impact. Alterations to the contact area of the fractures, storage, effective porosity, flow channelling and permeability within the fracture system can be expected with alterations to the pervasive conditions.