Elimination of LWD (Logging While Drilling) Tool Modes Using Seismoelectric Data

Abstract

Borehole acoustic logging-while-drilling (LWD) for formation evaluation has become an indispensable part of hydrocarbon reservoir assessment (Tang et al., 2002; Citta et al., 2004; Esmersoy et al., 2005). However, the detection of acoustic formation arrivalsover tool mode contamination has been a challenging problem in acoustic LWD technology. This is because the tool mode contamination in LWD is more severe than in wireline tools in most geological environments (Tang et al., 2002; Huang, 2003). In this paper we propose a new method for separating tool waves from formation acoustic waves in acoustic LWD. This method is to measure the seismoelectric signal excited by the LWD acoustic waves. The acoustic waves propagating along the borehole or in the formation can induce electric fields. The generated electric field is localized around the wave pulses and carried along the borehole at the formation acoustic wave velocity. The LWD tool waves which propagate along the rigid tool rim can not excite any electric signal. This is due to the effectively grounding of the drill string during the LWD process makes it impossible to accumulate any excess charge at the conductive tool – borehole fluid interface. Therefore, there should be no contribution by the tool modes to the recorded seismoelectric signals. In this study, we designed the laboratory experiments to collect simulated LWD monopole and dipole acoustic and seismoelectric signals in a borehole in sandstone. By analyzing the acoustic and electric signals, we can observe the difference between them, which are the mainly tool modes and noise. 1 In this paper, acoustic LWD measurement or signal is composed of formation acoustic waves (modes or arrivals) which are the waves propagating along the formation and tool waves (modes) which are waves propagating along the tool. 2 Seismoelectric signal refers to the electric field induced by seismic (acoustic) waves.