| Title | Hydraulic-Geomechanical Effective Stress Model: Determination of Discrete Fracture Network Parameters from a Pump Test and Application to Geothermal Reservoir Modelling. |
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| Authors | McDermott, C.I., Kolditz, O. |
| Year | 2004 |
| Conference | Stanford Geothermal Workshop |
| Keywords | stress model, network parameters, reservoir modeling |
| Abstract | 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. |