含孔隙、裂隙致密介质中多极子声波的传播特征

非常规油气藏（如致密性地层及蕴藏油气的页岩地层）的重要特征是低孔、低渗，但裂隙或裂缝比较发育.为满足非常规勘探的需求，本文将孔、裂隙介质弹性波传播理论应用于多极子声波测井的井孔声场模拟，重点研究了致密介质中裂隙发育时多极子声波的传播机理以及衰减特征.井孔声场的数值计算结果表明裂隙的存在明显改变了弹性波和井孔模式波的频散、衰减和激发强度，尤其是井壁临界折射纵波的激发谱的峰值随着频率的增加逐渐降低，这与应用经典的Biot理论下的计算结果相反，且裂隙的存在也使得饱含水和饱含气时临界折射纵波激发强度的差异变大.井孔模式波的衰减与地层横波衰减和井壁流体交换有关，井壁开孔边界下致密地层裂隙发育还使得井孔斯通利波和艾里相附近的弯曲波对孔隙流体的敏感性增强，在井壁闭孔边界条件下引起井孔模式波衰减的主要因素是裂隙引起的地层横波衰减造成的，且在截止频率附近弯曲波的衰减与地层的横波衰减一致.数值计算结果为解释非常规油气地层的声学响应特征提供了参考.

Propagation characteristics of multipole acoustic logging in cracked porous tight formations

The important characteristics of unconventional hydrocarbon reservoirs (such as tight formations and shale formations) are characterized by low porosity and low permeability with abundant cracks or fractures. In this work, a unified theory for elastic wave propagation in cracked porous media is applied to model multipole acoustic wave propagation along a borehole surrounded by a cracked porous formation. The simulation results show that the presence of cracks in tight rocks can give rise to significant changes of dispersion, attenuation and excitation of elastic waves and borehole mode waves. Especially the excitation peak values of the critical refracted compressional wave propagating along the borehole wall gradually decrease with frequency increasing, which is different from the calculated results by applying the classic Biot theory. The attenuation of mode waves is related to the shear-wave attenuation and fluid exchange at the borehole wall. The existence of cracks in tight formations also increases the sensitivity coefficient of borehole mode waves to pore fluid acoustic property for the permeable borehole wall. For the impermeable borehole wall, the attenuation of the borehole mode waves is mainly caused by the shear-wave attenuation caused by cracks. At the cut-off frequency, the attenuation of the flexural waves is consistent with the shear-wave attenuation in the formation. The numerical results above provide references for explaining acoustic response characteristics of unconventional hydrocarbon reservoirs.