Fracture Diagnostics With Tube Wave Reflection Logs

Abstract

Summary The Tube-Wave Reflection Log (TWRL) is a new fracture diagnostic method that is based on cased-hole acoustic logging. It relies on reflection and attenuation effects in tube waves resulting from fluid flow through perforations into hydraulic fractures. Information about the location, conductivity, and height of fractures can be deduced from these effects. Introduction The TWRL uses low-frequency tube waves to detect and provide information about fractured intervals behind perforated casing. Tube waves are interface waves that propagate in unperforated casing with little attenuation at a speed roughly equal to the fluid compressional velocity. The TWRL concept relies on the sensitivity of tube waves to changes in wellbore-wall permeability. An increase in permeability causes an increase in attenuation and a decrease in velocity of the tube waves owing to fluid flow in and out of the permeable zone. These phenomena are well known. A hydraulically induced fracture, in communication with a cased borehole through perforations, acts as a high-permeability zone in an otherwise impermeable wellbore. Throughout this zone, the tube waves are attenuated and slowed down owing to fluid flow into and out of the fracture. Tube-wave reflections also occur at the top and bottom of the fracture zone because of impedance contrasts.