The Study On Ultra-low Density Slurry Of Coal Bed Methane Well Cementing

China is very rich in coal bed gas resource, but its development and utilization is still in the initial stage.The characteristics of coalbed methane wells are: the shallow hole depth, low bottom hole temperature, poor mechanical stability, long thickening time, poor flow ability, high fluid loss, easily polluted and breakthrough of gas, conventional slurry system cannot meet the requirement of cementing. So this paper has carried on the lab research of ultra-low density cement slurry system.The particle size distributions of raw materials were analyzed by the Particle size laser test machine.Then we can obtain theoretic match of the solid-phase material according to the fractal grading model, combining experiment optimal designed the ultra-low density cement slurry's basic formula.On the basis of the above-mentioned tests, we optimised the dispersant FHJZ-1 (the optimum concentration ranges from l.6% to l.9%) and the fluid loss additive W99(the optimum amount of W99 is about l.6% ), the combination of sodium sulphate and triethanolamine can significantly improve the early-strength of set cement, optimal amount is 1.5% and 0.05% respectively. Besides, after optimizing water-solid ratio under different densities, we can get the final ultra-low density cement slurry's basic formula.The designed ultra-low density cement slurry system that density range is 1.14～1.20g/cm3 possesses good stability and rheological property, lower fluid loss and free fluid which can reach to zero, higher early-strength,big adjustable scope of thickening time, the good ability to prevent channeling. The system can effectively inhibit the coal bed methane wells from leaking and damaging reservoir in cementation, remarkably improve the cementing quality.