Abstract
The results of tests carried out to develop a durable corrosion-inhibiting cement blend for use in the annulus between casing and surrounding formation are described. The blend was tested in the laboratory under corrosive conditions of typical Saudi Arabian fractured vugular formations saturated with highly corrosive water. The high chloride and sulfate ion contents present in the formation water have resulted in loss of strength and sloughing of the casing cement, thus leading to increased corrosion and pitting on the outer casing surface and consequent leaks. API class cements and their derivative blends are usually specified for improved sulfate resistance. Such cements, however, do not afford adequate protection of the casing against corrosive environments. The new cement blend presented in this work is a pozzolan blended portland cement which is composed of ultrafine particles having a C3A (tricalcium aluminate) content of 8–12%. The ultrafine particles hydrate to form a cement with a markedly reduced permeability, thus creating a barrier which decreases the amount of corrosive formation water reaching the casing. The high C3A content, in turn, provides enough aluminate to lock the corrosive elements into an insoluble compound, preventing them from reaching the casing and thus inhibiting corrosion. Furthermore, the ultrafine particle size of the cement, with a higher C2S (dicalcium silicate) to C3S (tricalcium silicate) ratio, and the presence of pozzolan afford a stronger cement of low permeability which is more durable and less susceptible to sulfate attack.
Published Version
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