Uncovering the Influence of Divalent Ions on the Reinjection of Deep Geothermal Sandstone Reservoirs: A Case from Guanzhong Basin in China
Authors: Zhang Wentong , Huang Hai , Shang Xiongtao , Luo Weiyun , Gao Qian , Zhang Jiahui , Li Xinru 
Published: August 2024

ABSTRACT:
Tailwater reinjection is significant for replenishing formation energy ensuring the sustainable utilization of geothermal resources. However, the unclear effects of divalent ions on the tailwater reinjection greatly hinder the geothermal utilization scheme of deep sandstone reservoirs, especially in the Guanzhong Basin of China. In this paper, core flooding (CF) in reservoir conditions, low-temperature nitrogen adsorption (LNG), and X-ray diffraction (XRD) experiments were combined and performed to reveal the impact of divalent ions on the reinjection efficiency of tailwater, mainly composed of Mg2+ and Ca2+. CF results indicate that Ca2+ rather than Mg2+ can promote tailwater reinjection efficiency. LNG results show that Ca2+ can enlarge the pore size while Mg2+ can decrease the pore size of the sandstone reservoir. From the mineralogy level by XRD, the decrease in quartz and dolomite treated by Ca2+ compared with Mg2+. Considering the above results, we can refer to the fact that Ca2+ can decrease the content of quartz and dolomite, further dredging the pore throat of sandstones, with the result that the pore size of sandstone reservoirs treated by Ca2+ increases, resulting in the increased reinjection efficiency. Furthermore, Mg2+ shows a contrary performance compared to Ca2+. This paper sheds light on designing and preparing tailwater reinjection efficiency of deep sandstone reservoirs.

1. INTRODUCTION
With the increasing demand of the greenhouse effect, clean energy is welcome and at the central of spotlight (Barbier, 2002; Tester et al., 2006). Geothermal energy is a renewable clean energy, which arise much interest all over the world, especially in China (Zhu et al., 2015). The geothermal resources widely exist in China. hydrothermal geothermal of the Guanzhong basin is a typical geothermal resource (Xu et al., 2019). The success in the development of the geothermal resources in Guanzhong Basin will benefit the economy, optimize the energy structure, reduce the carbon footprint, and control the environmental pollution. Thus, how to successfully develop the hydrothermal geothermal resources is an urgent and important task.
During the development of hydrothermal geothermal resources in Guanzhong Basin, the reinjection of tailwater into the deep geothermal sandstone reservoirs is important and also a key for effectively exploitation for geothermal resource(Chai et al., 2022; Li et al., 2023; MA et al., 2017; Zhang et al., 2020). Because during the long utilization in the hydrothermal geothermal resource process, long-term exploitation water causes the decrease in the reservoirs pressure, the drop in the water table of underground and destroy in the ground(Guo et al., 2023; J. Xia, H. Tian, et al., 2023). The reinjection of tailwater into the underground promote developed efficiency of geothermal resource of Guanzhong Basin. Thus, focusing on the reinjection of tailwater into the deep geothermal sandstone reservoirs is necessary.