Development of a Solids-Free High-Density Oil-Based Drill-In Fluid with Enhanced Interfacial Stability and Reservoir Protection

To mitigate weighting-agent settlement, reservoir pore-throat plugging, and water-sensitivity damage during deep high-temperature drilling, a solids-free high-density oil-based drill-in fluid was developed using synthetic base oil as the continuous phase and potassium formate brine as the dispersed phase. Within the formulation range investigated, the optimized system employed 75 wt% potassium formate solution and a GMS/SDS compound emulsifier at a mass ratio of 9.7:0.3, corresponding to an HLB value of 4.5. Two representative formulations were evaluated: System I, with an oil/water ratio of 20:80, 5% emulsifier, and a density of 1.35 g/cm3; and System II, with an oil/water ratio of 15:85, 10% emulsifier, and a density of 1.46 g/cm3. Both systems maintained favorable electrical stability and filtration performance after thermal treatment at 150 °C, with sedimentation factors of 0.505–0.507 after 72 h of static aging. Molecular dynamics simulations suggested that the co-adsorption of GMS and SDS at the oil/brine interface, involving hydrogen bonding and ionic interactions, may contribute to interfacial-film stabilization, consistent with the experimental observations. Formation-damage tests using three sandstone cores covering low-, medium-, and high-permeability levels yielded permeability recoveries of 92.8–96.2% under the tested conditions. The formulation also exhibited an anti-swelling rate close to 100%, tolerance to the investigated contaminants, and a P110 corrosion rate of 0.042 mm/a. These results demonstrate the feasibility of the proposed solids-free high-density oil-based formulation within the investigated composition range and laboratory conditions, while its long-term performance and applicability under more severe HPHT and field conditions require further validation.