含孔隙、裂隙地层随钻多极子声波测井理论

储层岩石中普遍存在孔隙与裂隙，对钻井中的测井声波产生重要影响.基于孔、裂隙介质弹性波理论，导出了随钻声波测井的井孔声场表达式.据此考察了地层裂隙密度与含气饱和度的变化时井孔内随钻多极子模式波（斯通利波、弯曲波和螺旋波）的速度、衰减与灵敏度以及地层纵、横波的响应特征.裂隙密度与含气饱和度对模式波的速度频散与衰减都有影响，且两参数的值越大，影响越大.具体来说，速度对裂隙密度更敏感，而衰减对含气饱和度更敏感.具有"艾里相"特征的随钻偶极和四极子波在地层含气时产生强烈衰减，可以作为判断地层含气的一个明显指示.理论模拟与实际测井数据分析结果符合较好.

Numerical study on the characteristics of multipole acoustic logging while drilling in cracked porous medium

Cracks and pores universally exist in earth formation rocks, significantly affecting LWD acoustic waves. Using a cracked porous medium elastic wave theory, we derive LWD acoustic wave field for a fluid-filled borehole containing a drill collar and study the velocity dispersion, attenuation, and sensitivity of multipole wave modes (e.g. Stoneley, flexural and quadrupole waves) as a function of crack density and gas saturation. Increasing crack concentration and gas saturation can produce significant velocity change and attenuation. Relatively speaking, the velocity is more sensitive to crack density, while the attenuation is more sensitive to gas saturation. The dispersive LWD dipole and quadrupole waves characterized by a "Airy Phase" are strongly attenuated by gas saturation. This phenomenon can be used to detect the presence of gas in tight formations. The modeling and analysis results of this study are in agreement with results from field LWD data analyses.