The characterization of hydraulic properties of reservoir fractures is one of the key tasks for HDR/HWR geothermal heat extraction. This paper investigates the permeability and aperture change of a fracture from Stoneley wave attenuation in full waveform acoustic logs during wellbore pressurization. The full waveform acoustic logs at wellhead pressure of 0 MPa, 1 MPa and 3 MPa acquired in a wellbore of Higashi-Hachimantai geothermal model field. In this field, an artificial single fracture was induced hydraulically in intact welded tuff. Continuously drilled core and impression packer data confirmed that the artificial fracture is a single fracture. The fracture aperture can be controlled by injection of water into the wellbore. At the depth of the artificial fracture, Stoneley wave attenuation is observed. The attenuation increases as wellhead pressure and frequency increase. It is difficult to explain such an attenuation behavior in terms of a parallel-plate fracture theory, and therefore the procedure uses a permeable fracture zone theory. A least-squares fit to the transmission coefficient of Stoneley wave is used to evaluate the effective fracture permeability and aperture, and the effective fracture aperture are estimated by comparing the results of a transmissibility test and a parallel plate fracture model. The present result is larger than that of the transmissivity test, and close to the result from Stoneley wave reflection with a parallel plate fracture model.
- Borehole Stoneley wave
- Fracture aperture
- Fracture permeability
- Transmission loss
ASJC Scopus subject areas
- Renewable Energy, Sustainability and the Environment
- Energy Engineering and Power Technology