Study on Synthetic-Based Drilling Fluids for Protecting High-Porosity and High-Permeability Reservoirs

The Wenchang Oilfield’s high-porosity and high-permeability reservoirs are planned to be developed using synthetic-based drilling fluids. However, the induced reservoir damage problems caused by existing synthetic-based drilling fluids in high-porosity and high-permeability reservoirs are still unclear. Currently, through the analysis of reservoir core porosity and permeability characteristics, physical and chemical property analysis, reservoir sensitivity evaluation, and solid-phase and filtrate invasion experiments, the mechanism of reservoir damage is systematically explored, and a synthetic-based drilling fluid specifically for high-porosity and high-permeability reservoirs is developed to reduce reservoir damage. The results show that the average pore radius of this reservoir is 29.4 μm, with well-developed pores and strong permeability; the mineral composition is mainly quartz (with an average content of 55.6%), and the clay mineral content is 21.5%. It has water-sensitive, salt-sensitive, and stress-sensitive damage characteristics. Filter fluid invasion and solid-phase blockage are the core factors causing reservoir damage. Based on its damage mechanism, through the optimization of the plug-forming agent formula and the selection of a sealing agent, a low-harm synthetic-based drilling fluid (hereinafter referred to as KS-9) was developed. Performance evaluation shows that the KS-9 drilling fluid maintains stable rheology after 110 °C/16 h thermal rolling, with an upper temperature limit of 150 °C, and can resist 10% NaCl, 1% CaCl2, and 8% inferior soil pollution; in the core contamination experiment, its static permeability recovery value exceeds 88%, and it has good reservoir protection performance, which can provide technical support for the safe drilling and completion of high-porosity and high-permeability reservoirs in the Wenchang Oilfield.