Source Radiation Patterns In Cased Boreholes

Abstract

Source radiation from open and cased boreholes are well documented. The effect of an unbonded casing on the radiation patterns of volume, radial stress and axial stress sources in a borehole has received less attention and is modeled and analyzed here in the context of performing single well imaging. Radiation patterns were evaluated using a global matrix approach and wavenumber integration. 'While a borehole with a bonded casing has a single propagating mode at low frequencies, the stoneley mode, the borehole with unbonded casing has three propagating modes. Of the two additional modes that are due to the unbonded casing, one is present mainly in the cross-section of the casing and has a phase speed close to the speed of longitudinal waves in steeL Because of its large phase speed (~ 5400 m/s), this mode radiates into almost all formations and influences the radiation pattern of all source types. Test data and the predicted radiation patterns were used to identify the annulus material behind the casing.