Casing Cementing
In addition to wellhead insulation, the best available procedure of reducing casing failure due to external corrosion is the placement of a uniform cement sheath opposite all corrosive formations, e. g.. chlorine — and sulphur-rich formation waters. Diffusional supply of chlorine and sulphate ions to the interface of the casing can be inhibited by reducing porosity and permeability of the cement sheath. Most API oilwell cements contain tricalcium alumina, which forms complex salts of calcium chloroaluminate upon contact with chlorine ions, and calcium sulphoalu — mina hydrates upon contact with sulphate ions. Both of these reaction products lead to the formation of porous and permeable set cement. Upon long exposure (2-5 years) to these environments, the cement matrix begins to deteriorate and ultimately collapses leaving the casing without any protection (Rahman. 1988).
Full-length cementing of surface casing and production casing is recommended for deep wells. Pozzolan blended ASTM type I cement (API Class В or C). which is resistant to chlorine and sulphate attack and at the same time develops strong cement matrix, should be used. Additives such as fuel ash. blast furnace slag or silica flour is added to the cement to improve its properties (porosity, permeability and strength).