Study and evaluation of drilling fluid system for tight sandstone reservoir protection

According to the characteristics of a fractured tight sandstone reservoir, this study analyzes the reservoir lithology, physical properties, and hole roar characteristics of the block and designs a set of oil-based drilling fluid systems with good reservoir protection performance according to the requirements of drilling and completion work in the block. The experimental results indicate that the average porosity of the reservoir in this block is 3.62~4.33%, the permeability is 0.0001~1.3040×10-3μm2, the connectivity of pore throat is poor, the clay mineral content is moderate, primarily illite, and some of them are interbedded between Iran and Mongolia. The reservoir fractures are developed, and their widths are mostly less than 100 m. After mastering the reservoir characteristics, the key treatment agents of oil-based drilling fluid were screened and proportioned indoors. A set of white oil-based drilling fluid systems with a density of 1.20~2.00 g/cm3was prepared. Its specific composition was 5 # white oil + 20% CaCl2brine + 3~4% emulsifier EMP-4 + 0.5~1% auxiliary emulsifier FRHJ-3 + 1~2% wetting agent WER-3 + 2~3% fluid loss additive (FL-ZR: FL-ZHY = 1:1) + 2~3% organic soil 5 # + 2% Calcium oxide (CaO) + barite (oil-water ratio: 90: 10~80: 20). Its basic performance was tested to verify the adaptability of the system to the reservoir in YS block. The results exhibit that the system has good emulsification stability, with a demulsification voltage of approximately 1000 V. The anti-pollution ability, thermal stability, inhibition, and suspension stability have positive effects and can meet the needs of drilling and completion work in the YS block, providing a technical basis for using oil-based drilling fluid in the YS block's fractured tight sandstone reservoir.