Modeling Borehole Stoneley Wave Propagation Across Permeable In-Situ Fractures

Abstract

The characterization of the hydraulic transmissivity of permeable fractures is a very important task in the study of ground water flow in basement rocks. Previous studies that model the permeable structure as a single fluid-filled fracture failed to explain the observed significant Stoneley wave attenuation across the permeable structure. In this paper, the structure is modeled as a permeable fracture zone. The results show that Stoneley waves can be strongly attenuated or even eliminated without significant reflection, because of the dissipation of wave energy into the permeable zone. Three field cases are modeled and the theoretical results are compared with the field data. It is shown that low- and medium-frequency Stoneley waves (1 - 5 kHz) 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 higher 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 and porosity in the fracture zone. This finding is demonstrated by a logging data set obtained using high-frequency Stoneley waves at 34 kHz.