Evaluation of permeable fractures by multi-offset hydrophone vertical seismic profiling (VSP): T. Kiguchi, H. Ito, Y. Kuwahara, S. Nakao & D. T. Ohminato, Geothermal Science & Technology, 5(1–2), 1995, pp 21–29

Abstract

We carried out a high frequency multi-offset hydrophone VSP experiment in the shallow wells which NEDO (New Energy and Industrial Technology Development Organization) drilled at the Fracture Evaluation Test site in Iidate, Fukushima prefecture, Japan in order to detect permeable fractures intersecting borehole and to characterize them. Although the optical borehole television (BTV) and ultrasonic borehole televiewer (BHTV) mapped numerous fractures at the wellbore, it is still difficult to discriminate permeable fractures from impermeable ones. Tube waves are observed at several depths. We calculated fracture dip, strike and permeability from the amplitude ratio of tube wave to P-wave. Our analysis is based on the model that tube wave is generated when the incident P-wave compress the fractures intersecting wellbore. Calculated permeability is of the order of 100 md. With tube wave analysis, we are able to distinguish permeable fractures from impermeable fractures observed by borehole imaging tools. For example, one tube wave is generated at the depth where high and low dip angle fractures are observed with well logging. We conclude that the low dip angle fracture is permeable on the basis of the tube wave analysis. This is consistent with the flow properties of the fractures estimated from the circulation test.