Drilling Fluid Technology for Plugging and Strengthening Formations with Narrow Mud Weight Windows in Deep Water Drilling

Formations drilled in deep water drilling are generally poorly compacted and have narrow safe mud weight windows, and mud losses in drilling these formations hence occur frequently. To solve these problems, a flexible particle plugging agent was developed with vinyl monomers, a macromolecular crosslinking agent and a silicate mineral with layered structure. Laboratory experimental results show that this flexible particle plugging agent has good toughness, and functions normally at temperatures as high as 160 ℃. In 10% salt water, the plugging agent has stable characteristics and can effectively plug permeable cores, fractures and sand-bed, significantly increasing their pressure bearing capacity. A high temperature drilling fluid for deep water drilling was treated with this plugging agent, and its properties before and after aging at 160 ℃ were stable. This drilling fluid has proper viscosity and gel strengths, and the ratio of the yield points at 4 ℃ and 25 ℃ is less than 1.35, indicating that the drilling fluid has constant rheology at low temperatures. Cores plugged with this drilling fluid have permeability that is near zero, and the pressure bearing capacity of the cores is 11 MPa. This drilling fluid has strong resistance to the contamination by clays and salt, and is of low damage to the permeability of reservoir formations; permeability recovery of plugged cores is at least 90%. The application of this drilling fluid in the Lingshui block in South China Sea effectively increased the pressure bearing capacity by 6-11 MPa, ensuring the safe drilling operation in the complex hole sections.