Research on acidizing blockage removal and perfusion enhancement technology for sandstone geothermal reservoir recharge wells

Abstract

Geothermal tailwater recharge is an inevitable way to achieve sustainable and efficient development of geothermal resources and water resource recycling. However, as the recharge time of sandstone geothermal recharge wells increases, the recharge rate decreases, severely restricting the development and utilization of geothermal resources. The mechanism of water injection damage and the process of enhancing permeability have not been comprehensively studied, and effective measures to improve the efficiency and permeability of sandstone geothermal reservoirs are lacking. This study takes three reservoir rock samples from the Zhangjiapo Group, Lantian-Bahe Group, and Gaolingqun Group in the Xianyang geothermal field as the research objects. The ``micro+macro'' analysis method was used to study the reservoir characteristics. Water injection damage simulation experiments and scaling trend prediction were conducted. The mechanism of geothermal well damage is clarified from multiple aspects. Four acid systems including mud acid, fluoroboric acid, multi-hydro acid, and solid acid were used to conduct core flooding experiments, revealing the mechanism of permeability enhancement and simulating the acidification stimulation effect indoors. The results show that three layers of the Xianyang geothermal field are composed mainly of sandy mudstone, and fine and medium-fine sandstone, with strong heterogeneity. During the process of tailwater recharge, blockage damage must occur, and environmental conditions such as pressure and temperature changes can easily cause scaling damage. Mud acid causes severe dissolution of the rock core end face, and cannot achieve deep unblocking. The ability of multi-hydro acid stimulation is good, and the permeability increases by 3.18–15.47 times. Multi-hydro acid formed a single acid channel in the core of the Zhangjiapo Group, effectively removing blockages in deep layers. Solid acid can effectively protect the integrity of rock cores. After solid acid stimulation, the core structures of the three layers were intact, and the permeability has increased by 1.66–3.17 times. For loosely cemented reservoirs, solid acids can be used for acid stimulation. This study examined the effectiveness of chemical stimulation with four types of acid in sandstone geothermal reservoirs, demonstrated the feasibility of acidizing sandstone geothermal wells, and provided a scientific reference for improving the permeability and recharge rate of sandstone geothermal reservoirs.