Study On The Relationship Between The Transition State Evolution Of Cement Slurry And The Pressure Transmission Capacity Of The Slurry Column

In cementing operation,because the hydrostatic pressure of cement slurry is reduced,some unexpected accidents are often happened,which cause the cementing quality be reduced or blowout.Cement slurry is time-varying and its will change from liquids to solid state in the hardening stage,changing the properties and pressure transfer capability of cement slurry.Thus,in this study XRD,TG/DTG,TAM,ESEM,EDS,surface area measurement and pore size analyzer and some designed equipment and methods are used to investigate the evolution of properties and structure of cement slurry,and explore the mechanism of hydrostatic pressure drop of cement slurry in hardening stage.The results represented in this paper show that the hydrostatic pressure of cement slurry with different system is reduced and the hydrostatic pressure is affected by settlement stability and hydration reaction of cement slurry.The hydrostatic pressure of cement slurry is reduced during liquid-solid transition.In this stage,the hydration reaction is accelerated and a lot of hydration products are formed such as Ca(OH)2,C-S-H.The specific surface area of cement grains is increased rapidly and much free water change into capillary water and gel-water.Combining the ultrasonic technique and ESEM,the microstructure of cement slurry in liquid-solid stage is analyzed and it can be obtained that cement slurry is particles accumulate and the surface of cement grains only has a layer of hydration shell.When cement slurry is in liquid-solid stage,accelerated hydration of cement slurry make the cement grains be dissolved into pore solution.When the pore solution is supersaturated,some hydration products begin nucleating on the surface of cement grains,lead to the surface of cement grains become rough(the specific surface area is increased).Moreover,because the dissolution of cement grains and the formation of hydration products on the surface of cement grains are simultaneous to make the hydration products form a large number of micropores.Meanwhile,the supersaturated pore solution can also nucleate and grow in the pore solution and the pores are filled with the hydration products.The transmission of hydrostatic pressure of cement slurry follows Pascal's law,only liquid solution can effectively transfer hydrostatic pressure.However,when the settlement stability of cement slurry is poor,grains are difficult to suspend in pore solution,and the gravity of cement grains is transferred difficultly.Finally,the density of transmission medium is decreased and the hydrostatic pressure of cement slurry is reduced.With the increase of hydration time,these micropores in cement slurry are formed to make the free water change into capillary water or gel water,and the transmission medium is reduced.Owing to some pores are filled with hydration products,the height of transmission medium is decreased.Finally the hydrostatic pressure of cement slurry is reduced,even is lost.