A modelling study of open-hole single-well seismic Imaging

Single-well, or uni-well, imaging uses an acoustic source and an array of receivers located in the same borehole to image local geological structure. Due to the intrinsic attenuation of the formation it is likely that a source emitting frequencies in the typical cross-well range would be necessary to illuminate structure at distances above 100 m from the borehole. At these frequencies a significant proportion of the source energy is converted into tube-waves which are, for the purpose of these surveys, noise. This paper reports the results of a modelling study designed to assess the feasibility of using existing cross-well hardware, i.e. a piezo-electric source and hydrophone array, modified to run in a single borehole, to perform single-well surveys. In particular we study the case of an open borehole in a gas-filled, low-permeability sandstone reservoir. Our results suggest that the amplitudes of the tube-wave reverberations generated by calliper variations are such that reflections of interest arriving in the time window after the first tube-wave arrival will not be visible. However, reflections may be visible in the time window preceding the first tube-wave arrival provided tube waves from previous shots are not still present and the long source-receiver offsets required to make observations in this window can be incorporated into the tool design.