Study on the Density Enhancement of a Solids-Free, Potassium-Based Phosphate Kill Fluid by Cesium Formate and Its Molecular Dynamics Mechanism

In the development of high-temperature and high-pressure oil and gas reservoirs, high-density, solid-free, well-killing fluids are essential for well control operations. Traditional weighting agents suffer from insufficient density and high corrosivity. Potassium-based phosphates are widely employed owing to their low cost and low corrosivity, but their density is capped at around 1.8 g/cm³. Although cesium formate brine can achieve a density up to 2.3 g/cm³, its high cost makes it impractical for sole use. In this work, we formulated a high-density, solid-free, potassium-based phosphate killing fluid by mixing potassium-based phosphates with cesium formate and incorporating 2 wt % corrosion inhibitor R-51. The density and fundamental performances of the as-prepared fluid were experimentally evaluated. Meanwhile, molecular dynamics (MD) simulations were performed to clarify the intrinsic mechanism behind the density enhancement by cesium formate. The results show that the maximum density of the optimized fluid reaches 1.913 g/cm³ at room temperature, which breaks the density limitation of conventional potassium-based phosphate killing fluids. The fluid possesses good thermal resistance up to 180 °C and stable rheological behaviors. It achieves a rolling recovery rate higher than 91% and a corrosion rate of 0.0184 mm/a, with the core permeability recovery rate over 85%. NMR T₂ characterization confirms that the prepared fluid imposes no substantial damage to the dominant seepage pathways of rock cores. MD simulations showed that the addition of cesium formate disrupted the original hydrogen-bond network of bulk water and reduced the average number of water-water hydrogen bonds per water molecule. In addition, the coordination number of water molecules around phosphate increased, and more water molecules surrounded the phosphate groups through hydrogen bonding with the oxygen atoms in phosphate, forming a compact hydration shell. These features suggest that cesium formate promotes more efficient atomic packing throughout the solution, thereby increasing the solution density.