Improvement of magnesium oxysulfide cement used in geological environment of oil and gas wells

Magnesium oxysulfide cement (MOS) has excellent acid solubility properties and can be used for severe loss plugging in oil and gas wells. However, the solidification time of MOS cement is fast, making it difficult to be used in the geological environment of oil and gas wells. The MOS suitable for construction operation in oil and gas wells under a temperature of 60 degrees C was studied by optimizing the raw material ratio and modification. The influence of MgO/MgSO4 and H2O/MgSO4 mole ratios on the compressive strength of MOS was studied by a single-factor experiment. The response surface methodology (RSM) was used to optimize the raw material ratio. The sodium hexametaphosphate (SHMP) was applied to adjust the thickening time of MOS, and its effect on MOS was studied. The microstructure and phase composition of different MOS were analyzed. The results show that under hydrothermal conditions at 60 degrees C, the optimal mole ratio of MgO: MgSO4: H2O of MOS is 11.39:1:19.26, and the 24-hour compressive strength of MOS is 12.89 MPa. The thickening time of MOS was extended by 100 min, and the compressive strength increased by 25.26% in 72 h with 6 wt% SHMP at 60 degrees C. The SHMP-modified MOS has high compressive strength and a longer thickening time, and its strength develops steadily with time. The short rod-shaped 517 phase connected in MOS is beneficial for improving the strength. SHMP can regulate the stability of MOS, promote the formation of the 517 phase, and improve the strength of MOS. The research results support the application of magnesium oxysulfide cement in oil and gas wells.