Sourceless LWD Borehole Acoustics: Field Testing the Concept

Abstract

Seismic compressional and shear velocities are uniquely sensitive to the elasticity of the Earth and are used to estimate many properties of interest in oil exploration, reservoir development, and production efforts. Such properties include formation lithology, porosity, saturation, mechanical properties, presence of fractures, principal stresses, pore pressure, and formation damage. Logging-while-drilling (LWD) acoustic tools can be used when wireline acoustic logging is prohibitive and/or real-time decisions based on acoustics are needed. It has been shown that modern LWD acoustic tools with powerful, specialized acoustic transmitters can deliver most acoustic data and products with the same quality as their wireline counterparts. However, there is a lot of variation in the quality of traditional LWD acoustic data depending on the tool design. The operator’s decision to run an LWD acoustic tool is not an easy one, though, as the tool is often the most complicated and the longest part of the drillstring. In this paper, we present the field test results of a “sourceless” LWD acoustic tool that uses seismic energy generated by the drill bit to extract formation elastic properties instead of a powerful, specialized transmitter. Such a tool may consist of a receiver section of hydrophones only, which compared with traditional LWD tools, significantly simplifies the mechanical design, shortens the length of the tool, and reduces the tool cost. In the absence of an active transmitter, continuously recording the drill-bit energy as the drill bit rotates enables maximizing the signal-to-noise ratio (SNR) of the formation arrivals. In addition, the presence of azimuthally distributed receiver elements at each receiver axial position permits decomposition of the received wavefield into monopole, dipole, and quadrupole components. Processing results of the sourceless tool using time and frequency semblance are compared to those from a traditional LWD acoustic tool in the same well to demonstrate the viability of the concept. We also demonstrate that, in the case of “traditional” LWD acoustic tools, it is beneficial to place the tool transmitter below the receiver array. In this case, the drill-bit energy propagating in the formation along the borehole is “added” to the energy generated by the source and effectively increases the frequency bandwidth of the source signal and improves the SNR of the resulting formation arrivals.