ZOU Pengfei, XU Xueqiu, CHENG Yuanzhi, DU Jianguo, WANG Kuanbiao, XIE Jianlei, MAO Guanhui, GUAN Yu
Driven by the integrated high-quality development of the Yangtze River Delta region and the national strategy of Peak Carbon Dioxide Emissions and Carbon Neutral, generally shortened to Dual Carbon, there is a strong demand for clean energy. The Yangtze River Delta is rich in mid-deep geothermal resources. It is of great significance to study the exploring technology and method of geothermal resources in mid-deep layers in the Yangtze River Delta. Moreover, its innovation and application which will support the low-carbon development of the Yangtze River Delta will promote the exploitation and utilization of geothermal resources in mid-deep layers in this region. Based on the exploration practice of geothermal resources in the Yangtze River Delta region, this paper analyzes and studies the applicable conditions and application effects of the main technical methods of geothermal exploration in different geological structure conditions, geothermal systems and heat storage types, and forms a method system of geothermal resources exploration in the middle and deep depths of the Yangtze River Delta region. The selection of medium and deep geothermal exploration technology and method should be carried out on the basis of fully analyzing the geological background of the exploration area. It is necessary to select appropriate technology and effective combination of methods according to different geothermal geological conditions and different exploration stages to ensure good exploration effect. Gravity and magnetic exploration are suitable for detecting the information of basement, fault structure and magmatic rock distribution in sedimentary basins. We utilize controlled source audio-frequency magnetotellurics (CSAMT) and wide-field electromagnetic method(WFEM) for detecting the location of hidden thermal reservoir and thermal control structure with high accuracy, which is the most effective method to determine the location of geothermal well. The ability of microtremor survey method(MSM) to distinguish geothermal reservoir and cap rock is better than other geophysical exploration methods. Temperature measurements in shallow drillholes and radon gas survey are the most effective means to explore convective geothermal resources in shallow cover area. Different combinations of techniques and methods should be adopted in exploring geothermal resources in different geological structural units. It is advisable to use the combination of CSAMT, WFEM, MSM, gravity and magnetic method to explore deep-buried conduction-type geothermal resources in tectonic uplift area. The combination of CSAMT, temperature measurement and radon measurement should be adopted to explore convective geothermal resources. A combination of CSAMT, large pole distance electric sounding and MSM is used to explore conduction-type pore geothermal resources in sedimentary basins. The combination of CSAMT, MSM, high precision gravity and shallow hole temperature measurement is used to explore the composite geothermal resources in the uplift and depression fault zone. In the area with strong hydrothermal activity, the underground heat storage structure and geothermal anomaly range are determined by analyzing the enrichment law of some special elements in soil and groundwater. The cationic chemical temperature scale and SiO2 temperature scale in geothermal fluid are used to estimate geothermal reservoir temperature. In general, hydrogen and oxygen stable isotopes are used to study the genesis of geothermal system. Different drilling, well formation and well washing methods should be adopted according to different geothermal reservoir types. Pore type geothermal wells with soft lithology of thermal reservoir generally adopt positive circulation mud drilling, spray well washing, polyphosphate well washing and other methods, the casing is inserted into the whole hole, and the water intake interval is formed by winding filter water pipe. For fissure geothermal wells with high lithologic hardness of hot reservoir, positive circulation mud drilling is generally adopted, which is suitable for injection well washing, polyphosphate well washing, liquid carbon dioxide well washing, compressed air well washing and other washing methods. Open hole well formation or filter pipe well formation is adopted for water intake interval. For the karst geothermal well whose reservoir lithology is carbonate rock, reverse circulation drilling and positive circulation drilling techniques can be used, and the water intake section can be formed by open hole or filter pipe. Acidizing fracturing is the most effective way to increase the water yield of karst - fissure geothermal well.
