含气孔裂隙地层的声波测井响应特征

本文以径向分层的孔、裂隙地层多极子井孔声场理论为基础，利用孔、裂隙介质弹性波动统一理论，数值模拟了井壁上有侵入时，单极和偶极声源激发的井孔声场随原状地层含气饱和度变化的特征和规律。结果表明：当原状地层裂隙发育时，相较于饱含水，饱含气时纵波幅度衰减严重，横波波至提前，波幅增大，斯通利波幅度变化不明显，低频时弯曲波到时提前，幅度增大；当原状地层裂隙不发育时，单极子、偶极子波形都对流体性质没有明显响应。可见，原状地层中裂隙的存在加剧了含气对纵波、横波的幅度以及到时的影响，而侵入带的存在使得斯通利波和高频弯曲波对原状地层流体性质变化不敏感。数值计算结果为利用声波响应特征识别致密气层提供了参考。

Acoustic response characteristic of tight gas formation with cracks

We use the cracked porous medium elastic wave theory and simulate the characteristic of wave propagation with the change of gas saturation in the original formation based on borehole wave propagation theory in radially layered isotropic cracked porous media, and the invaded zone is water saturated and with abundant cracks. Then we discuss the acoustic response characteristics of tight gas formation. When cracks are introduced into the original formation, the results show that as follows, the compressional waveform with gas saturation is substantially delayed and attenuated relative to that with water saturation. In comparison, the shear wave simulation results show only small changes between the two saturation conditions, with wave amplitude for the gas saturation being slightly higher and arrival time being slightly advanced than that of the water saturation. The amplitude of the Stoneley wave changes little, and the flexural waveform in low frequency with gas saturation is slightly advanced and higher relative to that with water saturation. When the crack density is zero in the original formation, the monopole and dipole waveforms both have no obvious response on the fluid property. Therefore, we can conclude that the existence of cracks in the original formation increases the effect of the gas on the amplitude, the arrival time of the compressional wave and the shear wave. However, due to the invasion zone, the mode waves are not sensitive to the fluid property of the original formation. And we can use the acoustic response characteristics to recognize tight gas.