偶极子声波测井在双层套管井中激发的声场特征

多层套管井中的套管-水泥以及水泥-地层在径向上的分布较单层套管井界面多且径向分布范围更深.目前还未有针对多层套管井固井质量评价的有效方法，偶极子阵列声波测井具有工作频率低径向探测深的优势，本文通过实轴积分和三维有限差分计算了偶极子声源在不同胶结状况的双层套管井中激发的声场，旨在探讨偶极子阵列声波在双层套管井中评价外层套管和地层之间胶结状况的可行性.计算结果表明，在两层套管之间充填泥浆时，全波波形中可明显观测到地层弯曲波和套管弯曲波，若外层套管和地层之间充填水泥套管弯曲波的慢度明显高于充填泥浆时的慢度值；在外层套管和地层之间胶结的水泥出现缺失，套管弯曲波的频散曲线会处于完全胶结与完全无胶结之间，其变化量与水泥在外层环空的占比近似成线性关系，几乎不受水泥缺失的径向位置影响.由于偶极子声源的指向性，套管弯曲波的响应特征主要受与声源振动指向的方位扇区胶结状况的影响；在两层套管之间充填水泥时，若外层套管和地层之间充填泥浆，仍可观测到套管弯曲波，模拟波列提取的套管弯曲波的频散曲线与解析解计算的频散曲线吻合，两层套管通过水泥胶结在一起，其弯曲振动的传播速度稍有增加，类似单层套管厚度加厚的响应特征.两种模拟方法得到的结果均表明套管弯曲波对外层套管和地层之间的充填介质敏感，验证了偶极子阵列声波测井在双层套管井固井质量评价中应用的可行性.

The wavefield characteristics of dipole acoustic logging in a double-casing well

Compared with the conventional single-casing well, the multi-casing well has more layered interfaces and a deeper radial distribution of the interfaces. At present, the industry needs a method for evaluating the cement quality of multi-layer casing wells. The dipole array acoustic logging has the advantage of low operation frequency and deep radial depth of penetration. In this paper, the dipole acoustic wavefield is calculated respectively using the real axis wavenumber integration and the three-dimensional finite difference waveform modeling methods. The wavefield excited in the double-casing wells with different cementation conditions was analyzed to explore the feasibility of using dipole array acoustic logging for evaluating the cementation quality between the outer casing and the formation in the double-casing wells. The calculation results show that, when the mud fills the annulus between the inner and outer casing layers, the formation- and casing-flexural waves can be clearly observed in the full-wave data; in addition, the slowness of the casing flexural wave is significantly higher when the outer casing and the formation are filled with cement than filling with the mud; by proportionally decreasing the amount of cement between the outer casing and the formation from fully cemented to no-cement conditions, the casing flexural-wave dispersion curve varies between the two extreme conditions, and its variation is approximately linearly proportional to the cement portion of the annulus, and is insensitive to the radial position of the cement. Due to the directionality of the dipole source, the response characteristics of the casing flexural-wave are mainly affected by the cementation condition in the azimuthal sector in the source vibration direction. In the case that the two layers of casing are well cemented but the outer casing and the formation are filled with the mud, the casing flexural wave can still be observed. The wave dispersion characteristics extracted from the simulated wave data and those calculated from the analytical solution are in good agreement. Cementing the two layers of casing appreciably increases the casing flexural-wave speed, a response characteristic similar to that of a thickened single-layer casing. The results obtained by the two simulation methods indicate that the casing flexural-wave is sensitive to the filling material between the outer casing and the formation, verifying the feasibility of using dipole acoustic logging for cement evaluation in double-casing wells.