Abstract
To evaluate fluid, heat or chemical species transport in fractured rocks, we have to characterize hydrological properties of the medium. Generally, the fracture region plays an important role for fluid flow, whereas the matrix region works as a major storage of fluid and/or heat. Conventional well testing methods such as pressure transient/interference and tracer tests are applied to characterize fractured rocks. However, it is usually difficult to characterize separately each of the fracture and matrix regions by using these methods. Ishido and Pritchett (2003) carried out numerical simulation of electrokinetic phenomena in fractured rocks and showed that features of fractured rocks appear much more clearly in the “self-potential (SP) transients” than in the pressure transients. Combining continuous pressure and self-potential measurements is thought to therefore provide a mean for better characterizing of fractured rocks.
In order to study this prediction experimentally, we have carried out continuous SP monitoring using multi Ag-AgCl electrodes installed within KF-1 and KF-3 wells at the Kamaishi Mine, Japan. The observed ratio of streaming potential change to the pressure change due to wellhead valve opening showed different behaviors between intact host rock and fractured rock regions. A double-porosity behavior observed in the fractured region suggests that the time required for pressure equilibrium between the fracture and matrix regions is in the range of 1000 to 2000 seconds. Fracture spacings were estimated to be 1 to 4 meters assuming 1 to 10 micro-darcies of permeability of the matrix region.
