Detection and Characterization of Fractures from Generation of Tube Waves

Abstract

Field testing has been conducted to evaluate a model which predicts the permeability and orientation of a permeable zone, in a deep water well 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.