滑移界面表征的套管井声场的理论计算

经典弹性波理论往往在套管与水泥环之间增加一层流体环来模拟套管与水泥环之间胶结差时的测井响应, 但流体环的存在, 使得套管与水泥环之间缺失了剪切耦合, 即使几个微米的厚度, 套管波幅度也会明显增大, 接近甚至超过自由套管时的30%.因此经典弹性波理论常常无法解释现场测量数据中套管波幅度的渐变现象.实际套管井中胶结界面粗糙或不规则起伏使套管与水泥环界面两侧介质有所接触, 往往存在剪切耦合, 用剪切刚度和法向刚度表达的滑移界面理论应用到套管与水泥环或水泥环与地层界面的胶结响应, 可模拟界面从完全胶结到完全不胶结的过渡状态.通过将滑移界面理论模拟的波结构与在套管与水泥环之间加一层软固体层时的经典弹性波理论模拟的波结构对比, 验证了滑移界面理论在模拟水泥胶结时的适用性.滑移界面理论与经典弹性波理论结合分析了水泥不同胶结状态下井孔内外声场的辐射特征, 随着界面水泥胶结质量的下降井内接收的套管波以及套管波泄漏到地层中的纵横波幅度逐渐增强, 透过井壁辐射到地层中的纵波和横波逐渐降低.将滑移界面理论应用到套管井水泥胶结状况的模拟, 弥补了经典弹性波理论在模拟水泥胶结测井响应时的局限性, 也为定量刻画水泥环界面的耦合刚度提供了可能.

Theoretical calculation of casing well sound field represented by slip interface

The classical elastic wave theory can be used to simulate the casedhole acoustic logging response by adding a fluid annulus between casing and cement sheath when the cement bonding is poor. The classical simulation results show that the casing wave amplitude is about 30% of the free-pipe amplitude even with 0.01 mm fluid annulus. The limitation in the traditional modeling of poor cement bond is that it requires an ideal interface shape such that the materials on both sides are completely separated. In field conditions, however, the interface gap (or micro-annulus) is often irregularly shaped with roughness or asperities, which, in the passing of casing waves, behaves like a partially coupled/decoupled interface. This is the scenario to apply the slip-interface modeling and use the shear coupling stiffness to characterize the degree of coupling. The slip-interface modeling is focused on the transition from the well-bonded to the un-bonded conditions. By comparing the wave structure simulated by slip interface theory with that simulated by classical elastic wave theory when a soft solid layer is added between casing and cement sheath, the applicability of slip interface theory in simulating cement cementation is verified. The combination of slip interface theory and classical elastic wave theory can simulate the response characteristics of casing wave amplitude from well cemented to free casing in cased wells. Combined with the radiation of compressional and shear waves in the formation, the radiation characteristics of acoustic field inside and outside the borehole with different cementation conditions are analyzed. With the decrease of cement cementation quality, the amplitude of casing wave and leaky waves into the formation gradually increase, the compressional wave and shear wave radiated into the formation through the borehole wall gradually decreases. The application of slip interface theory to simulate cementation in cased wells compensates the limitations of classical elastic wave theory and provides the possibility to quantitatively characterize the coupling stiffness at the interfaces.