Detection and Characterization of Fractures from Generation of Tube Waves

Field testing has been conducted to evaluate a model which predicts the permeability and orientation of a permeable zone, in a deep water weil in crystalline rock. Tube waves are generated by seismic P-waves incident on a fracture intersecting the borehole, a process observed in vertical seismic profiling (VSP). The behavior is explained by fluid exchange between the fracture and borehole, and the observed efficiency of conversion is theoretically related to fracture permeability. Additionally, fracture orientation may be obtained from multiple-source-offset VSP surveys. Conventional temperature, caliper, resistivity and televiewer logs show the presence of fractures and their orientation, and provide indirect evidence of associated flow. Open fractures are simultaneously sampled over the full borehole depth by hydrophone VSP methods. The tUbe wave generation model produces results which compare favorably to independent estimates of fracture parameters. INTRODUCTION Increasing importance of natural and artificial fractures in resource recovery and in site characterization for waste storage has created the need for methods of identification and for quantifying fracture parameters: permeability and orientation. It has long been known that sonic and full waveform acoustic well logs exhibit effects of fractures (Paillet, 1983), and more recently VSP has been demonstrated to be useful. Huang and Hunter (1981) observed tube waves originating at distinct depths, when conducting checkshot VSP surveys using a hydrophone streamer. They attributed tube wave generation to the presence of open fractures intersecting the borehole. In a similar survey In Hamilton, Massachusetts we also observed the generation of tube waves (Figure 1). These and other field studies have shown that the tube wave generation response is commonly observed in hard rock geological settings, and is observable in fractured reservoirs of sedimentary origin. The motivation for this study is to demonstrate the application of the tube wave generation model of Beydoun, et al. (1985) for several fractures in a single well, and if possible to evaluate the model assumptions of laminar flow and the low frequency approximation. The setting for the field study is optimal since the relative homogeneity of the granite formation prevents scattering of the original P-wave 205