CaO- and/or MgO-Swelling Cements: A Key for Providing a Better Annular Sealing?

Abstract The use of expanding cements for improving the sealing efficacy of annulus cementing has been considered for a long time as a possible solution to existing problems. In this paper, the processes of volumetric shrinkage of cements due to hydration as well as the effective mechanisms of CaO and MgO swelling cements are explained. Preliminary results of the current investigations at ITE for testing cement systems of this kind for application in the borehole are presented. The expansion of the cement matrix has been determined with the use of atmospheric and HPHT expansion cells specially developed for this purpose. Further parameters under investigation include the permeability of the system comprising the cement and steel pipe as well as the strength properties. Formation of voluminous Ca(OH)2 and Mg(OH)2 crystals on the surfaces of the cement particles resulted in matrix expansion by more than 10 percent, even under a hydrostatic pressure of 120 MPa and at borehole temperatures up to 175°C. The swelling cements under investigation exhibited especially high shear bond strength. The test results hitherto obtained already indicate that the sealing action of annulus cementing can be improved with CaO swelling cements when employed preferentially in storage wells with BHST up to 70°C, and with MgO swelling cements in deep gas wells with BHST up to 175 °C.