Fracture Detection And Characterization

Abstract

The effects of fractures on full waveform acoustic logs are studied on the basis of field observations, available theoretical models, and a series of ultrasonic laboratory experiments. Results from diffusion models applicable to fine microfractures and finite difference models of isolated fractures are reviewed. Laboratory experiments are carried out with fine microfractures around the borehole in a Lucite model, and isolated single fractures in aluminum models. Cases of horizontal and inclined (45°) fractures are studied as a function of fracture aperture and frequency of Stoneley waves. A vertical fracture model is also studied. Results indicate that the effect of different fractures are manifested differently on P, S, pseudo-Rayleigh, and Stoneley waves. Micro-fractures surrounding a borehole attenuate Stoneley waves most strongly. Vertical fractures attenuate Stoneley waves more strongly than other phases in the wave train. Horizontal and inclined fractures have a greater effect on P and S waves than on Stoneley waves.