Preparation and properties of magnesium oxysulfate cement and its application as lost circulation materials

Abstract

Loss of drilling fluids in large porous and fractured zones inevitably up-regulates the overall cost of drilling. As a type of acid-soluble cement, magnesium oxysulfate (MOS) cement is arousing huge attention for the less hygroscopic nature and less damaging to steel casings compared with magnesium oxychloride (MOC) cement. The present study developed MOS cement as a fast setting, high strength and acid-soluble lost circulation material to reduce the problem of losses. As suggested in this study, a higher strength of MOS cement at 70 °C could be achieved by elevating MgO/MgSO4·7H2O molar ratio or down-regulating H2O/MgSO4·7H2O molar ratio. Boric acid and borax could act as effective retarders. Plugging slurry based on MOS cement could effectively block the simulated porous loss zones exhibiting a diameter from 1.24 mm to 1.55 mm, as well as the fractured loss zones with a width from 2 mm to 5 mm and bearing a pressure difference up to 8 MPa. Permeability recovery test demonstrated that it facilitated future oil and gas production. The successful field application in the Junggar Basin, Xinjiang, China verified the significant plugging effect of MOS cement for severe loss problems.