Abstract

Severe loss of drilling fluids that occurs in Xinjiang, China, increases the Non-Production Time (NPT) and the overall cost of drilling. Magnesium oxysulfate (MOS) cement as lost circulation material has the advantages of short curing time, acid-soluble property and low cost, showing great potentials to solve severe loss. In this paper, the loss treatment of adding MOS cement directly into the drilling fluids was purposed and practiced. The effects of the amount of MgO, MgSO₄·7H₂O and borax on the compressive strength and curing time of the drilling fluids at 50 °C and 70 °C were studied. The plugging performance of the curable plugging slurry in minimizing fractured loss was evaluated. The results indicate that with the increase of MgO and MgSO₄·7H₂O, the compressive strength increases while the curing time would be shortened. The addition of borax retarder could extend the curing time while it would also decrease the compressive strength. The curable plugging slurry could effectively reduce the loss rate of fractured loss bearing a pressure difference above 8.0 MPa. The successful field applications in Xinjiang, China were introduced. This paper demonstrates that using MOS cement could provide a convenient, fast and efficient way to solve severe loss challenges.