Geophysical and geological analysis of hot spring drilling

Geophysical and geological analysis of hot spring drilling

Geothermal resources are renewable energy resources, but at the same time, they are limited resources, and their replenishment process is slow. With the continuous expansion of the geothermal market and the increasingly significant development benefits, a large amount of geothermal water has been extracted, and the actual amount of water extracted has greatly exceeded the maximum production capacity available for geothermal fields. This has caused the water level to decline year by year, and some places have experienced resource depletion, seriously affecting sustainable development. Therefore, the sustainable development of geothermal resources is necessary, which not only promotes the economic development of the country but also greatly improves people's living standards. Hot dry rock (HDR), also known as enhanced geothermal system (EGS) or engineering geothermal system, generally refers to high-temperature rock masses with temperatures greater than 200 ℃, burial depths of several kilometers, and no or only a small amount of underground fluids (liquids) inside. The composition of this rock mass varies greatly, with the majority being intermediate acidic intrusive rocks since the Mesozoic era, but it can also be metamorphic rocks from the Mesozoic and Cenozoic eras, or even thick blocky sedimentary rocks.

Dry hot rock is mainly used to extract heat from its interior, so its important industrial indicator is the temperature inside the rock mass. The principle of developing hot dry rock resources is to drill a well (injection well) from the surface into the hot dry rock, seal the wellbore, and inject low temperature water into the well under high pressure, generating very high pressure. In the case of dense and crack free rock mass, high-pressure water will cause many cracks in the rock mass along the direction roughly perpendicular to the minimum geostress. If there are already a small number of natural joints in the rock mass, these high-pressure water will cause them to expand into larger cracks. Of course, the direction of these cracks is influenced by the geostress system. With the continuous injection of low-temperature water, cracks continue to increase, expand, and interconnect, ultimately forming a planar artificial hot dry rock thermal storage structure. Drill several wells at a reasonable distance from the injection well and connect them to the artificial thermal storage structure. These wells are used to recover high-temperature water and steam, and are called production wells. The injected water moves along the cracks and exchanges heat with the surrounding rocks, producing high-temperature and high-pressure water or water vapor mixtures with temperatures up to 200-300 ℃. Extract high-temperature steam from production wells that connect artificial thermal storage structures for geothermal power generation and comprehensive utilization. After using the warm water, it is injected back into the dry hot rock through the injection well to achieve the purpose of recycling. Geothermal systems can be divided into two types based on their genesis analysis: medium low temperature conduction geothermal systems; Medium low temperature convective geothermal system; High temperature convective geothermal system; There are four types of geothermal systems, including high-temperature conductive geothermal systems. In nature, a single type is rare and often complex. This is a type of geothermal resource mainly buried in large and medium-sized sedimentary basins (such as Bohai Bay, Songliao, northern Jiangsu, Sichuan, Ordos and other basins), with enormous energy potential. Chen Moxiang (1988) determined a medium low temperature conductive geothermal system based on geothermal research in the Bohai Bay Basin. It is estimated that the recoverable resources of 10 major sedimentary basins in China can reach the level of 18.54 × 108t standard coal. At present, the main development and utilization of geothermal resources in large and medium-sized cities such as Beijing, Tianjin, Xi'an, and rural areas are of this type.

Based on our detailed research on the geothermal field in Yuxiong County in recent years, we have noticed that this type of conductive geothermal system may also have local convection, and therefore there may also be conductive convective geothermal systems.

Xiongxian County is located in the southwest of the geothermal system in Niutuo Town, in the northern part of the Bohai Bay Basin. The entire 524km2 area contains geothermal resources. Geothermal development mainly involves the pore thermal storage of Neogene sandstone and the karst fissure thermal storage of bedrock. Among them, the Wumishan Formation thermal storage of the Jixian Formation has a wide distribution range, large thickness, developed karst fissures, and good permeability, making it the most important thermal storage in the entire geothermal field.

Geophysical and geological analysis of hot spring drillingHeat source and water source

The Xiongxian geothermal system is located in the Bohai Bay Basin, and the heat source consists of two parts: the heat generated by radioactive elements in the crust and the heat from the upper mantle. Due to its location in the core of a rift basin and thin crust, the proportion of mantle heat sources is relatively high, making it a typical mid to low temperature conductive geothermal system in the eastern hot basins of China. Geothermal water is mainly supplied by atmospheric precipitation, but due to its distance from the supply area, the supply speed is relatively slow.

Thermal reservoir

The shallow thermal reservoir of the Xiongxian geothermal system is the Neogene sandstone thermal reservoir, and the deep thermal reservoir is the Jixian Wumishan Formation dolomite thermal reservoir. The Neogene sandstone thermal reservoir and the Jixian Wumishan Formation thermal reservoir are separated by the tight mudstone of the Neogene lower part and the Paleogene, forming two independent thermal reservoirs with hydraulic differences. The Jixian series thermal reservoirs are buried at depths of 950-1050m and are the main thermal reservoirs for geothermal development and utilization in Xiongxian County.

Water channel

The fractures and secondary fissures in the bedrock of the Xiongxian geothermal system constitute the main conduit for geothermal water. The Niudong fault plane contacts the Paleogene, causing obstruction of bedrock water flow. The upper part of the fault is composed of multiple faults, which are combined into one at a deeper location. Its mechanical properties are tensile, and it serves as a conduit for water flow, allowing deep lateral runoff to surge up along these channels after being blocked on the fault slope, replenishing the top crack zone of the protrusion, giving it higher temperature and more abundant water.