Abstract: Obtaining underground fluid samples with high quality is important for CO₂ leakage monitoring in CO₂ geological utilization and storage (CCUS). The gas-operated underground fluid sampler has the advantages of high sampling accuracy, low formation disturbance, and in-situ monitoring through integrating downhole monitoring technologies, and has been widely applied in several CCUS applications. However, during practical application processes, sampling volume of the sampler is unstable and greatly affected by groundwater level. In order to solve these problems, a visualized experimental device to simulate subsurface fluid sampling in a wellbore was developed. A set of fluid sampling experiments at different liquid heights were carried out to explore sampling process and sampling mechanism of the gas-operated sampler. The results show that there is no disturbance to the liquid height during the sampling process, indicating the gas-operated sampling technology is characterized by passive sampling and low interference to external environment. The sampling volume depends on the size of the storage container and the liquid height in the wellbore. The sampling volume can be calculated accurately by the liquid height and the pipeline size. The measured sampling volumes under different liquid heights are basically consistent with the theoretical values. The error between the measured volume and calculated one decreases with the increasing liquid heights. The maximum error is less than 6.6%. Both the sampling time and average sampling rate mainly depend on the injection pressure and present a power function relationship with the injection pressure with a correlation coefficient of greater than 99%. The above experimental results can help to understand the sampling mechanism of gas-driven sampler and are of great significance to feedback and guide sampling performance in field by optimizing the structure. In addition, it can improve the ability of warning environmental risks in CCUS and contaminated sites.

Key words: underground fluid sampling, gas-operated, sampling volume, sampling time, sampling rate