Preparation and Application of a Block Removing Fluid for Eliminating Reservoir Contamination by High Density Oil-Based Drilling Fluids

High density oil-based drilling fluids have been widely used in the well drilling operation due to their high stability and inhibitive capacity. However, barite filter cakes formed after filtration and migration of the high density oil-based drilling fluids cause great harm to the reservoir, resulting in reduced oil and gas production. The contamination caused by the barite filter cakes is difficult to remove by conventional methods such as acid dissolution and alkali dissolution. In this study, a stable and efficient block removing fluid was formulated with the chelating agent DTPA as the main component, potassium carbonate as the conversion agent, oxalic acid as the catalyst, ammonium persulfate as the oxidant, AEC as the surfactant and ethylene glycol butyl ether as the organic solvent. This block removing fluid, through penetration, chelation, solubilization and washing on the mud cakes, destroys the internal structure of the mud cakes formed by oil-based drilling fluids, reduces the inter-substance adhesive force, and enhances the removal of the contamination by high density oil-based drilling fluids. The composition of the solid-phase chelating liquid for high-density drilling fluids is:20%DTPA+3%oxalic acid+6%K₂CO₃+0.06%ammonium persulfate+KOH, and the final composition of the oil-phase cleaning liquid is: 12% ethylene glycol butyl ether+0.5%AEC. Evaluation of the performance and oil cleaning ability of this block removing fluid shows that under different temperatures (120℃-180℃), the composite block removing fluid can effectively degrade the filter cakes produced by high-density drilling fluids. Single-stage treatment at 120℃ for 4 hours can reach a filter cake dissolution rate of 66.7%, and multi-stage treatment at 120℃ for 4 hours has a filter cake dissolution rate of 72.22%. Moreover, the filter cake dissolution rate increases with temperature. After the multi-stage treatment at 180℃ for 8 hours, the dissolution rate is 89.24%, and the highest corrosion rate is 1.1537 g/(m²·h). It has excellent block removal performance and temperature resistance. In the same experimental conditions, the dissolution efficiency of the composite block removing method for the mud cakes produced by oil-based drilling fluid is about 20% higher than that of the conventional chelating block removing method. Field tests have proved that this system can effectively solve the contamination and block by high-density drilling fluids, and the production capacity recovery rate can be as high as about 90%.