低密度水泥固井质量评价方法的改进

为了防止低压易漏地层发生固井液漏失并有效保护油气层，固井过程中经常采用低密度水泥浆体系。由于低密度水泥浆体系中密度减轻剂的使用，其水泥环的声阻抗值较之常规密度水泥环的声阻抗存在差异，若采用常规密度水泥的固井质量评价标准来评价低密度水泥浆体系的固井质量，则其真实可靠性存在问题。为此，进行了3种常用低密度水泥浆体系的水泥石抗压强度与声阻抗实验，分别测定了水泥石养护时间24 h和48 h的抗压强度和声阻抗值。结果表明，不同类型不同密度的低密度水泥石24 h和48 h条件下的抗压强度与声阻抗具有良好的线性关系，由此得出了考虑第一界面胶结良好时，基于抗压强度的低密度水泥浆体系套管波声幅值计算公式，进而提出了基于低密度水泥石抗压强度的低密度固井质量评价的改进方法，为全面改进低密度水泥浆体系固井质量评价标准奠定了基础。

Improvement on the cementing quality assessment method for light-weight cement sheaths

Light weight liquid cement systems are frequently adopted in the well cementing process in order to prevent low pressure, mud loss prone formations from developing cementing fluid loss and to protect oil/gas bearing formations effectively. The lightening admixture used in a light weight liquid cement system contributes to the acoustic impedance difference between its cement sheath and normal weight cement sheath. Therefore, if the cementing quality assessment method for a normal weight sheath is adopted in a light weight liquid cement system, the accuracy and reliability of the assessment results will be in doubt. In this paper, compressive strength & acoustic impedance tests were conducted on the cement stone of three common types of light weight liquid cement systems to determine the compressive strength and acoustic impedance after 24 hours and 48 hours of curing, respectively. As shown from the test results, the compressive strength and acoustic impedance of the light weight cement stone of different types and weight had a good linear relationship under 24 hour and 48 hour curing conditions. Therefore, an equation was developed for calculating the compressive strength based acoustic amplitude of casing waves of a light weight liquid cement system in times of favorable cementation of the first interface. After that, based on the compressive strength of light weight cement stone, an improvement method used for light weight well cementing quality assessment was thus suggested. This lays a foundation for improving the cementing quality standard of a light weight liquid cement system.