The free water and the solids settling problems exhibited in a cement slurry lead to variation in the density of set cement and channeling. The present study has demonstrated that inclusion of certain biopolymer in the slurry designs can eliminate both the free water and the solids settling problems.
Separation of water/fluid, on standing, from a homogeneous cement slurry is termed "free water". Certain designs, in addition to the free water problem, are incapable to hold the particles uniformly throughout the slurries, thus causing the heavier ones to fall rather rapidly to the bottom of the system. This later problem is commonly known as "settling", or sedimentation, or particle segregation. No matter what one chooses to call this phenomenon, everyone agrees that the free water and the settling problems in cement slurries lead to non-uniform density of the set cement around the pipe, gas migration problems through channels in the cement column, weaker cement at the top (because of the density gradient), lack of hydration of the cement due to water loss to the formation (embrittlement), etc.
Problems, of this nature, are normally handled by adding bentonite, or sodium metasilicate in the cement slurry. These additives are also called 'extenders', meaning that these products can extend or give better yields out of each sack of cement by requiring a tremendous amount of water. However, the effectiveness of these additives decreases remarkably in the presence of strong electrolytes, e.g., NaCl, CaCl2, etc.
The use of polysaccharides in cement is widely known. Hydroxyethylcellulose (HEC), carboxymethylhydroxyethylcellulose (CMHEC) have been extensively used as fluid loss additives in oil field cement. Polysaccharides belong to the carbohydrate family of the general formula CnH2nOn where n is a positive integer equal or greater than four but preferably less than ten.