| Abstract |
This paper investigates the change of aperture of a subsurface artificial fracture during borehole pressurization with full-waveform acoustic logs (FWAL). The FWAL logs of 15kHz at wellhead pressure of 0MPa, 1MPa and 3MPa were obtained in a 365m deep borehole in the Higashi- Hachimantai Hot Dry Rock model field, Iwate Prefecture, Japan. In this field, a single fracture was induced hydraulically in intact welded tuff. Continuous core samples 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 borehole. Significant P-wave attenuation and Stoneley wave attenuation both were observed at the depth of the artificial fracture, and slight increase of Stoneley wave reflection was observed. By use of the time-frequency analysis, we were able to obtain the rate of attenuation and reflection in the frequency domain. Based on the depth range of the P-wave and the Stoneley wave attenuation, it is found that the artificial fracture is embedded in a permeable zone about 1.4-1.5m wide. The permeable zone width increased 10% by raising wellhead pressure from 0MPa to 3MPa. In addition, the increasing of the wellhead pressure results in increases of the P-wave attenuation and the Stoneley wave attenuation and reflection. The P-wave attenuation rose from 76% to 85%, the Stoneley wave attenuation rose from 12% to 16% and the Stoneley wave reflection rose from 1.5% to 2.2%, respectively. A parallel plate fracture model was examined to estimate the fracture aperture using the Stoneley wave attenuation and reflection. The estimated fracture aperture from the Stoneley wave attenuation increased from 2.5mm to 3.5mm and that from the Stoneley wave reflection also increased from 0.5mm to 0.7mm, as wellhead pressure increased. The difference in fracture aperture given by the Stoneley wave attenuation and reflection is associated with the permeable zone in the artificial fracture vicinity. |