Shallow gas formation is characterized by large well diameter (larger than 12.5 in), high porosity (about 30%), and very low velocity (The compressional-wave velocity is less than 2500m/s.).Therefore, in shallow gas, it is hard to extract accurately the formation compressional-wave slowness and shear-wave slowness by traditional acoustic logging processing methods. Based on Biot porous medium model, this paper simulates the monopole and dipole waveform in the slow formation under the circumstances of different porosities and gas saturation. The numerical simulation results and the field case indicate that when the porosity are higher than 25% and the gas saturation are higher than 70%, a complete attenuation of monopole leaky-P wave (leaky mode compressional wave) which is excitated by monopole source in fluid-filled borehole appears; while a smaller amplitude of dipole leaky-P wave which is excitated by low frequency dipole source occurs. Since the monopole acoustic well logging in unconsolidated slow formations is unable to measure the formation compressional slowness, the paper puts forward dispersion slowness analysis method which can be used to extract compressional slowness from dipole leaky-P wave. In addition, a field case of a Shallow gas, which is related to the processing of the formation compressional and shear slowness, is discussed. Finally, with the synthetic seismogram, the formation slowness extracted by using this paper methods is proved to be accurate and reliable.
A current trend in oil and gas exploration and production is that more and more wells are drilled through shallow, unconsolidated formations into deep water reservoirs.In this drilling environment, the borehole is usually very large, varying in the range of 12-18 inches; the formation compressional-wave slowness varying in the range of 140- 220 us/ft.This shallow gas formation presents a challenge to the acoustic measurement. The traditional monopole acoustic logging tools designated to operate in the frequency range above 10kHz often fail to excite the desirable acoustic propagation and to correctly measure the formation acoustic slowness. The need to determine formation acoustic properties for this environment suggests an important topic for acoustic logging studies.
The slow formation environment has been a subject of study for acoustic logging. Using a theoretical analysis, Paillet and Cheng (1986) studies the acoustic wave phenomena in slow formation. They described borehole P wave in the environment as “leaky-P” wave. The waves are leaky because they lose energy by converting to shear, which radiates into the formation and carries wave energy away from the borehole P waves. Because of the energy loss, the Leaky-P waves are attenuation and dispersion. Hornby and Pasternark (2000) found the only effective way to measure formation P-wave velocity from the leaky-P waves is to induce and measure the waves in the lowfrequency range (~2kHz). Yoneshima and Tang et. al. (2003, 2004), found the LWD acoustic waveform data are dispersive and applied a theoretical dispersion correction in their calculation of P-wave slowness in unconsolidated slow formations. Tang et. al. (2004) has suggested that a new capability is added in the LWD tool to acquire low-frequency compressional waves down to 3 kHz in slowformations.
