Characterization of Fractured Reservoirs Using Continuous Self-potential Measurements

We have extended our “EKP-postprocessor” (Ishido and Pritchett, 1996) to calculate electrokinetic potentials in fractured geothermal reservoirs that are represented computationally as “MINC” doubleporosity media. In such reservoir descriptions, global mass exchange between adjacent macroscopic computational grid blocks takes place mainly through the “fracture zone”. Inter-block flow through the “matrix region” is relatively unimportant and is usually neglected in most MINC treatments. But this approximation is inappropriate when calculating the global “drag current” caused by electric charges moving with the flowing fluid due to electrokinetic coupling. Since the magnitude of the drag current density is proportional not to the permeability but to the porosity of the medium, the contribution of the drag current through the matrix region to the total global current between adjacent macroscopic grid blocks is not negligible, and in fact usually predominates under steady-state conditions. This property of the drag current brings about much more pronounced differences in the “self-potential transients” between competing “fractured/MINC” and “porous-medium” descriptions of the same reservoir than is the case for pressure transients. Combining continuous pressure and self-potential measurements may therefore provide a means for better characterizing fractured geothermal reservoirs.