Fluid Sampling Monitoring at Yanchang CO2-EOR Demonstration Site, Ordos Basin, China

Abstract

In this paper, we firstly addressed the status and the overall scheme developed for the CO2-EOR Demonstration Project at the Jingbian site, the Yanchang Oilfield, the Ordos Basin, Shaanxi, China, under the funding support of China-Australia Geological Storage of CO2 (CAGS) Program. Then, we are focusing on the fluid sampling monitoring as one of key monitoring indicators of leakage assessment, and the detailed monitoring plan was tailored for the Jingbian CO2-EOR Block in the Yanchang Oilfield with consideration to the existing situation of monitoring technologies for CO2-EOR activity and project planning in China. All the fluid samples were collected from different locations such as surface water ponds, groundwater wells, and monitoring wells around the CO2 injection site. The specific designed multi-layer U-tube based samplers were installed in the monitoring wells at the Jingbian CO2-EOR site. Three 200 m deep shallow monitoring wells were drilled for the samples of underground water and subsurface soil gases. Using 20 Baseline samples collected in 2016 at the Jingbian, and 22 monitoring samples collected in 2017, we found the tracks of some changes in the fluid chemistry and isotope composition. All the regular anions and cations, e.g. Cl-, SO42-, Mg2+, Ca2+, Na+, K+ and HCO3-, were analysed. In addition, total dissolved solids (TDS), pH, conductivity, salinity, oxidation-reduction potential (ORP), organic components, especially petroleum contaminants such as BTEX and PAHs, and isotopic analysis were also traced throughout the sampling analysis. Some of preliminary results can be addressed as follows: 1. The average salinity of formation water is 71.34 g/L, pH is 5.5, and the water belongs to CaCl2 type at the Jingbian site according to the classification of China’s underground water standard. 2. The monitoring indicators for the subsurface water at the Jingbian site indicate good stratification characteristics, and the concentration level of each monitoring indicator decreases with increasing depth. 3. The application potential of the multi-layer U-tube based fidelity sampling technology was demonstrated for the subsurface environmental monitoring. The results of fluid samples provide some significant information for the coming monitoring activities during the implementation of large-scale CO2-EOR or CO2-EWR projects in the Yanchang Oilfield.

Keywords: Yanchang Oilfield; U-tube; CCUS; Environmental monitoring; Subsurface fluid; Ordos Basin