Tracer test study on artificial reservoirs in hot dry rock geothermal systems in the Gonghe Basin, Qinghai

This paper is the result of geothermal survey engineering.

Objective Hot dry rock is a clean and vast geothermal resource. China Geological Survey has been continuously implementing China's first hot dry rock geothermal power generation and grid connection project in Gonghe, Qinghai since 2019. Hot dry rock reservoirs are dense and require fracturing to form a permeable interconnected fracture network, creating a reservoir capable of accommodating a certain scale of heat exchange and water conduction. However, research on artificial reservoirs in high−temperature hard rock is relatively limited.

Methods The successful construction of artificial reservoirs and effective inter−well communication are the core of successful hot dry rock development. Tracer tests are an effective means to study the above issues and characterize reservoir hydrogeological conditions. This study focuses on the hot dry rock test site in the Gonghe Basin of Qinghai, selecting sodium fluorescein and sodium bromide as tracers, and conducting tracer tests before and after large−scale fracturing.

Results Before large−scale fracturing, tracer recovery rate and heat exchange volume were relatively low. After the transformation, reservoir fractures became more complex, reservoir connectivity improved, and the tracer recovery rate reached 14.14%. The heat exchange volume of fractures increased to 27 times, and the fractures became relatively homogeneous, effectively reducing the risk of heat breakthrough.

Conclusions Tracer experiments can quantitatively evaluate the effectiveness of hot dry rock artificial reservoir fracturing. The research results have guiding significance for the conduct of hot dry rock tracer tests and the scientific development of high−temperature hard rock thermal storage.