High Temperature Resistance of Conventional Set Sand Cement under Cumulative Working Conditions

In field operations, the set cement with sand in downhole conditions will experience huff-n-puff and steam flooding, followed by transitional stage and fire flooding. To accurately understand the high temperature resistance of the set cement with sand, NMR, XRD and SEM were used to investigate the porosity, permeability, chemical structure and microstructure of the hydration products of the set cement with sand under cumulative working conditions. Laboratory experimental results demonstrated that after curing at room temperature for 14 d, the compressive strength of set cement was 31.8 MPa, which is 90.86% of the compressive strength of the set cement after curing in the same conditions for 28 d. Meanwhile, this set cement had low porosity and low permeability. After fire flooding, the compressive strength of the set cement was reduced by 67.92%, and the porosity and permeability of the set cement were both increased by 53.85% and 77.31% respectively, apparently unable to satisfy the needs of heavy oil production. The reason for the decrease in compressive strength and increase in porosity and permeability of the set cement is that at high temperatures, part of the hydrated calcium silicate with “network” structure or “chain” structure in the set cement is converted into “granular” wollastonite, thereby resulting in increased porosity and permeability and decreased compressive strength of the set cement. Meanwhile, de-hydroxylation of Ca(OH)₂ produces CaO and plenty of big pores and cracks, failing the materials which are beneficial for the set cement to maintain its mechanical performance.