Acoustic Characteristics Of Ultra-low Density Cement Stone And Its Influence On CBL Cementing Quality Evaluation Index

With the deepening of oil and gas field exploration and development,the cementing conditions such as low porosity and low permeability,complex pressure system are becoming more and more harsh,for low-pressure easy to leak formations,low rupture pressure strata and low-pressure reservoirs,in order to prevent cementing fluid leakage and effectively protect oil and gas layers,low-density cement slurry is often used for cementing.For some extremely low pressure formation conditions,sometimes the degree of cement slurry can not meet the needs of safe cementing construction,and only ultra-low density cement slurry can be used to solve the problem of equilibrium pressure cementing.Since the existing low-density cementing quality evaluation standards cannot objectively and accurately reflect the real cementation quality of ultra-low density cement slurry,it is of great significance to study the acoustic characteristics of ultra-low density cement,the acoustic propagation characteristics of casing sound field,and the change law of related cementing evaluation indexes.The acoustic characteristics and mechanical parameters of cement stone with a density of1.1～1.3g/cm3 cement slurry under different curing conditions were experimentally tested,and the factors and laws affecting the mechanical and acoustic properties of ultra-low density cement slurry cement stone were analyzed.The downhole sound field model modeling and numerical simulation calculation problems were studied,and the influence of sound source frequency,casing type,microannulus,etc.on casing wave was analyzed.The quantitative calculation of cementation quality index of ultra-low density cement was studied.The results show that density,temperature and curing time will have an impact on the development of sound velocity and compressive strength of ultra-low density cement stone,and temperature has a greater influence on cement stone in the early stage and little influence in the later stage.The higher the density of cement stone,the higher the acoustic impedance and compressive strength,and the linear relationship between acoustic impedance and elastic modulus,and exponential relationship with compressive strength.The optimal logging time is recommended to be after 72 h.The sound field model of casing downhole was established,the frequency of the sound source increased,the amplitude of the first wave of the casing first increased and then decreased,the maximum at about 15 k Hz,the thickness of the casing increased,the amplitude of the first wave of the casing wave decreased slightly,and the amplitude of the first wave of the casing by the interface channel groove increased with the increase of the annular micro-gap.The calculation formula of the relative sound amplitude of the ultra-low density cementing casing was established,and the calculation method and cementing quality evaluation index of the wave amplitude correction of the ultra-low density cement casing were proposed.The density of cement decreases,the upper and lower limits of the relative amplitude increase,and the smaller the density,the greater the relative amplitude that needs to be corrected.The research results provide a basis for formulating the quality evaluation index of ultra-low density cement cementing.