Abstract

We develop a 1:12 scale model logging-while-drilling (LWD) acoustic tool for laboratory measurements in borehole models to investigate the effects of tool wave modes on our ability to determine formation velocities in acoustic LWD. The scaled tool is comprised of three sections: (1) the source section, consisting of four transducers mounted on the tool body that can generate monopole, dipole, and quadrupole waves; (2) the receiver section, consisting of six sets of receivers, each containing two transducers mounted on opposite sides of the tool center line; and (3) the connector section, a threaded steel cylinder that connects the source and receiver sections tightly to simulate an LWD tool. We use four borehole models to simulate fast and slow isotropic and anisotropic formations. The slow-formation models are constructed of synthetic material (Lucite® for the isotropic case and Phenolite® for the anisotropic case). The fast-formation models are made from natural rock samples (sandstone for the isotropic case and slate for the anisotropic case). Tool-wave characteristicswere measured in a water tank, followed by a series of experiments in the four borehole models to record monopole, dipole, and quadrupole acoustic waves when the tool was used with or without the connector. Without the connector in place, the tool measured formation arrivals that were consistent with wireline-logging predictions. With the connector in place, the coupling of the source and receiver sections resulted in strong tool waves that could mask formation arrivals. In general, the quadrupole mode more consistently provided correct formation shear-wave velocities because the tool modes propagated with higher velocities and could be separated from the formation shear arrivals. The dipole tool mode often could interfere with the formation flexural wave, especially for soft formations. By increasing the operating frequency of the source, tool waves could be eliminated and formation arrivals more easily measured in all cases. Based on these observations, it is important to choose the optimum working frequencies in LWD to reduce tool modes and allow formation shear velocities to be measured with dipole or quadrupole tools, particularly in anisotropic formations.

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