MODELING BOREHOLE STONELEY WAVE PROPAGATION ACROSS PERMEABLE IN-SITU FRACTURES

The characterization of hydraulic transmissivity of permeable fracture reservoirs is a very important task in the exploration of water resources and hydrocarbons. Previ-ous studies that model the permeable structure as a single fluid-filled fracture failed to explain the observed significant Stoneley wave attenuation across the permeable struc-ture. In this paper, the structure is modeled as a permeable fracture zone and synthetic Stoneley wave seismograms in the vicinity of the structure are calculated. The results show that Stoneley waves can be strongly attenuated or even eliminated without sig-nificant reflection, because of the dissipation of wave energy into the permeable zone. Several field cases are also modeled and the theoretical results are compared with the field data. It is shown that low- and medium-frequency Stoneley waves (1 kHz data from Moodus, Conneticut, and 5 kHz data from Monitoba, Canada) are very sensitive to the permeability of the fractures and can be used to assess permeability from in-situ logging data, if the fracture porosity and zone thickness can be measured. At high frequencies, however, Stoneley waves are not very sensitive to permeability but are mainly affected by the sum of the fracture openings expressed as the product of fracture zone thickness 44 Tang et al. and porosity in the fracture zone. This finding is demonstrated by a logging data set (Monitoba, Canada) obtained using high-frequency Stoneley waves at 34 kHz