Effects of Drilling Fluid Encapsulators on Well Cement Slurries

When a drilling fluid is mixed with a cement slurry, the flow of the mixture will become very poor. In a laboratory study, the effects of drilling fluid encapsulators on the thickening time, rheology and the compressive strength of cement slurries were investigated. Using IR, XRD and SEM, the mechanisms with which the cement slurries are contaminated by the drilling fluid encapsulators are studied. Electric conductivity method was also used to evaluate the changes of the electric conductivity of cement slurries before and after the cement slurries are mixed with drilling fluid encapsulators. The study results showed that the mobility of a cement slurry is reduced when an encapsulator is mixed with the cement slurry; a pure cement slurry contaminated with 0.6% encapsulator has its mobility reduced by 52%. Electric conductivity experiment showed that the hydration process of a cement slurry, which is contaminated with a drilling fluid encapsulator, begins accelerated only after 12 hours of hydration. The mixing of a drilling fluid encapsulator into a cement slurry will slow down the hydration process of the cement slurry, leading to a slow development of the compressive strength of the cement slurry. A cement slurry containing 0.2% drilling fluid encapsulator and cured at 90 ℃, has a 3 d strength that 12.8% lower than that of the pure cement slurry. The adsorption groups in the molecules of the encapsulator, such as —OH, —CONH₂, —COO^- etc., can be adsorbed onto the particles of the cement, and the hydrophilic groups of the encapsulator can form hydrogen bond with the Si—O bond in the cement molecules, thereby forming a solvent film on the surfaces of the cement particles, which in turn hinders the contact of water with the cement particles.