Research on A New Material to Prevent the Strength Decline of Set Cement Under Ultra-High Temperature

In wells with bottomhole temperatures of 200 ℃ or higher, the strength of the set cement with sand will inevitably decrease with time, causing the zonal isolation by the cement sheaths to fail. Wells with this kind of problem will not be able to produce for the designed long time. Researches have been done to investigate the mechanisms of strength retrogression of sanded set cement, showing that the root cause of strength retrogression is the increase in the porosity of the set cement resulted from the generation of parallel loose xonotlite from the hydration of that cement at ultra-high temperatures. To deal with this problem, a new material against strength retrogression of set cement at ultra-high temperatures, DRB-4S, has been developed with optimized aluminum mineral having nanometer crystal layer spacing and nanotube materials. At 240 ℃, a set cement containing 10%–20% DRB-4S has a strength of 50 MPa and does not decrease. Studies on the mechanisms of DRB-4S against strength retrogression of set cement with SEM, XRD and pore structure analysis showed that the use of the nanometer crystal layer spacing aluminum mineral in DRB-4S helps the production of chalcomorphite, a hydration product that is stable at elevated temperatures, in the set cement. The nanotube used in DRB-4S inhibits the conversion of tobermorite to xonotlite and reduces the porosity of the set cement through bridging and filling. Cement slurries treated with DRB-4S have filter loss of less than 50 mL, easy-to-adjust thickening time, good stability and operational performance, all these are of great technical support to the long-term production of high-temperature oil and gas wells.