Source Radiation Patterns In Cased Boreholes

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.Massachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation Consortiu

Cement Bond Evaluation Using Early Refracted Arrivals

The cement bond evaluation tool is a device used to examine the integrity of cement bonding to the casing. A conceptual tool operating between 80-200 kHz is considered here, with a transmitter and two receivers, oriented parallel to the axis of the borehole and next to the casing. The compressional head wave in the casing, excited by the transmitter, will be the first arrival to be measured by the two receivers in most situations. With both receivers on the same side of the transmitter, the attenuation of this wave in traveling between the two receivers is dependent on the properties of the medium immediately outside the casing. The radially layered borehole was modeled as a layered plane medium for large operating frequency. A spectral integral approach (complete wave synthesis) was used to compute the response at the receiver locations, which then provided attenuations. Different parameters, such as transducer separation (1-12 in), annulus thickness (0-6 in), annulus impedance (free pipe to good cement), casing thickness (0.25-0.45 in), standoff distance (0.5-1 in) and source frequency (80-200 kHz) were varied in the evaluation of the operation of the tool. The parameter studies based on the theoretical computations revealed that free pipe could be distinguished from the presence of cement in a variety of situations. Additionally, lower bounds on receiver separations are given for reliable operation of the tool.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumGas Research Institut