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Fracture Connectivity of Fractal Fracture Networks Estimated Using Electrical Resistivity (2011-2013)

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Investigator(s): Lilja Magnuadottir

This study aimed to estimate the connectivity of fracture networks using direct current resistivity measurements. In these surveys, a direct current is sent into the ground through electrodes and the voltage differences between them are recorded. The input current and measured voltage difference give information about the subsurface resistivity, which can then be used to infer fracture locations. Other geophysical surveys used commonly to find hidden geothermal resources are self-potential and magnetotelluric surveys. Garg et al. (2007) described how self-potential, magnetotelluric and direct current surveys were all used to explore the Beowawe geothermal field in the Basin and Range Province of the western USA. However, usually these surveys are performed on the surface with very low resolution when exploring deeper portions of the reservoirs, making it impossible to characterize fractures that are small-scaled compared to the size of the reservoir.  Therefore, the possibility of placing the electrodes inside geothermal wells was considered in this study, in order to measure the resistivity more precisely in the deeper parts of the reservoir.

A series of simulations were conducted on discrete fractal fracture networks using General Purpose Reservoir Simulator (GPRS) developed at Stanford University (Cao, 2002). The objective was to investigate the influence of connectivity on the time history of electric potential between well pairs to study the possibility of using changes in electric potential with conductive tracer injection to characterize fracture connectivity.

The study showed a successful attempt to use time histories of electric potential with conductive fluid injection to estimate the connectivity of a reservoir. More cases need to be studied and more simulations performed to test the reliability of this method. A sensitivity analysis willalso be performed to demonstrate the advantages of using electrical resistivity measurements instead of only using the tracer return curves at the producers.

References: 

Cao, H., (2002), “Development of Techniques for General Purpose Simulators,” PhD Thesis, Stanford University.

Garg, S.K., Pritchett, J.W., Wannamaker, P.E. and Combs, J., (2007), “Characterization of Geothermal Reservoirs with Electrical Surveys: Beowawe geothermal field”, Geothermics, 36, 487-517.