In some cases, special, non-permanent sealing cements are required to establish a seal and subsequently remove it by dissolving the cement with common oil-field acids, something that cannot be accomplished with conventional cements. Sorel cement formulations based on caustic calcined MgO grades can be used in such cases. See also Eco Magnesia based Cements.
In drilling operations, drilling or oil-well cements are used to fix the steel casing (a large diameter pipe) to the surrounding geological formation in the recently drilled section of a borehole. Secondary cementing (squeezing or plugging) is used for filling formations, sealing and water shut-off. However, Portland cements suffer volumetric shrinkage upon hardening. The addition of expansion additives provide a net expansion effect which ensures good bonding between cement and casing, with increased shear strength and zone isolation. Hard-burned and dead-burned MgOs are used as expansion additives, especially in high temperature applications (80oC, 120oC or more than 150oC). The MgO must be of low reactivity, exhibiting slow hydration and streamlined expanding properties (expansion not initiating before cement’s setting). In that respect, hard- or dead-burned magnesium oxides achieve the best expansions (2-5%). Addition rates can go up to 10% per cement weight. See also Cement Anti-shrinkage Additive.
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