Feasibility Study on Monitoring Enhanced Geothermal Systems Using Electrical Methods

摘要

The DOE report on "The Future of Geothermal Energy (2006)" defines Enhanced Geothermal Systems (EGS) as engineered reservoirs that have been created to extract economical amounts of heat from low permeability and/or porosity geothermal resources. Critical to the success of EGS is the successful manipulation of fluids in the subsurface to enhance permeability. Knowledge of the change in volume and location of fluids in the rocks and fractures (both natural and induced) will be needed to manage injection strategies such as the number and location of step out wells, in-fill wells and the ratio of injection to production wells. The key difficulty in manipulating fluids has been our inability to reliably predict their locations, movements and concentrations. To address the problem, several types of geophysical measurements can be used to image the subsurface and infer fluid concentrations. In context of EGS stimulation experiments, micro earthquake (MEQ) methods have been the favored geophysical technique. However, it is also well known that MEQ data are much less sensitive to fluid saturation than electrical data, but that electrical methods are lower resolution methods, compared to geophysical methods employing seismic wave fields. Nevertheless, there is good reason to believe electrical methods can add significant value. Electrical methods have a long history in geothermal exploration, particularly magnetotellurics (MT). Here we consider such methods, including controlled source electrical measurements (CSEM) for EGS moni-rntoring. Electrical resistivity is primarily a function of fluids filling rock pores and fractures as well as clays in geothermal environments. Electrical methods are also known to be highly sensitive to temporal changes in fluid saturation, porosity and permeability and have been and continue to be exploited for monitoring injected fluids in enhanced oil recovery (EOR) processes.