Engineering Aspects of LWD Quadrupole Measurements and Field Test Results

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents the principle and engineering aspects of an LWD quadrupole shear wave tool. Extensive theoretical study and a series of laboratory and field experiments have been conducted to verify that the quadrupole measurements can determine the true shear velocity of the formation. Experimental results based on the actual laboratory and downhole while drilling measurements are presented in this paper. A multipole (i.e., monopole, dipole, and quadrupole) LWD acoustic tool has been constructed. This tool allows downhole recording of raw and stacked signals acquired by individual receiver elements of the tool. This tool consists of a universal source, which can operate at different frequencies in all three modes, i.e., monopole, dipole, and quadrupole. The six-station receiver array, with four individual receiver elements per station, permits signal acquisition in all these modes. In addition, a triaxial accelerometer system is used to monitor drilling dynamics while drilling.By making multiple snapshots of the signals in different modes downhole for various drilling conditions, we demonstrate that, when operating in the quadrupole mode, the acoustic signals acquired by the LWD tool possess the true quadrupole characteristics. Comparison of the dipole, monopole, and quadrupole signatures clearly demonstrates how common mode noise is efficiently suppressed in the quadrupole mode. The result shows that even in the case of severe lateral vibrations the combined quadrupole signal provides satisfactory shear wave measurements. The data processing/analysis results are presented. Field-measured quadrupole LWD shear data are evaluated for different drilling scenarios.

Development of a Low-Frequency Quadrupole Shear-Wave Technology to Improve Quality of LWD Shear Velocity Measurement

“…4) consists of a segmented transmitter and an array of four orthogonal spaced receivers. 1,4 The transmitter source consists of four sectors, denoted by A, B, C, and D, respectively. When electrical pulses are applied to the source, each sector will expand or contract in a radially outward or inward manner.…”

“…1,2,4 The advantages of using LWD quadrupole waves are (1) the tool quadrupole wave is absent when operating in the low-frequency range, and (2), the quadrupole wave in a slow formation travels at formation shear velocity at low frequencies. Because of these advantages, a multipole (monopole and quadrupole) LWD acoustic tool has been built.…”

“…Because of these advantages, a multipole (monopole and quadrupole) LWD acoustic tool has been built. 1,4 An engineering benefit of using the quadrupole design is that no acoustic isolation device is needed for the quadrupole wave because the tool quadrupole wave does not exist when operating in the low-frequency (<10 kHz) range.…”

“…Results in many situations proved remarkably sucessuful. 1,2,4 The measurement practice, however, also identified issues that need to be resolved with new developments. One common issue is the dispersion effect in the measured 4-kHz quadrupole data.…”

Frequency-domain simulation of logging-while-drilling borehole sonic waveforms

“…The second generation of LWD sonic logging instruments attempted to excite flexural modes (Varsamis et al, 1999;Althoff et al, 2000;Leggett et al, 2001) for assessment of S-wave velocity. However, the successful solution to this problem came with next-generation tools operating with quadrupole modes, which propagate at lower frequencies with the formation's shear slowness (Tang et al, 2002a(Tang et al, , 2002b(Tang et al, , 2006Dubinsky et al, 2003). The reason for this behavior is that the collar quadrupole mode, which arises only at higher frequencies, does not contaminate the formation quadrupole mode.…”