Practical improvements in CaO-swelling cements

Abstract

Calcium oxide-swelling cement expansion behaviour was studied to improve results of annular sealing during well cementing and underground gas storage. Calcium oxide cement compositions expand during hardening. Cement slurry expansion, resistance to flow (to assess pumpability downhole) and thickening time, shear and compressive strengths, and system permeability, which are properties of autoclave-cured hardened cement paste, are discussed in detail. The objective was to avoid calcium oxide sedimentation and reduced swelling tendency that could be anticipated using the calcium oxide-swelling cement. Slurries were initially prepared according to API standards, as quick-look tests to help choose appropriate parameters for further investigations. Expansion and consistency (resistance to flow) tests were conducted in high pressure–high temperature (HPHT) devices at 60, 80, 90 and 120 °C, and 120 MPa. Repeat experiments were conducted by adding sodium silicate solution (water–glass) and the results were compared. As high as 30–40% water–glass solution improves gelation tendency and expansion of the cement slurry from 3.5% to 6.2%. The investigation examines the results obtained, in some test cases, and shows that the influence of about 13% CaO by weight of cement becomes significant when the temperature is between 65 and 80 °C. Calcium oxide-swelling cements composed of 13% calcium oxide, improve tightness of cemented boreholes against gas leakage. Workover and cementing operators could extend the applications of CaO to carbonate formations, where the traditional cementing jobs are difficult because of vugs and secondary porosities.