Study On Genesis Mechanism And Exploitation Potential Of Zhacanggou Geothermal Field In Guide County,Qinghai Province

The development of human society is accompanied by changes in energy use and the rapid growth of energy use.The traditional fossil fuel-based energy structure has brought tremendous pressure on the environment.Therefore,in order to solve the sustainable use of energy,it is urgent to adjust the energy structure and promote the development and use of renewable energy.As an important renewable energy source,geothermal energy has attracted more and more attention because of its clean,centralized and stable advantages.China has abundant geothermal resources and huge dry-hot rock reserves.The dry-hot rock reserves in the southern part of the Qinghai-Tibet Plateau account for 20.5%of China's total.EGS?Enhanced Geothermal Systems?is an important method to develop deep dry hot rock geothermal resources.In order to solve the two core issues of reservoir fracturing and thermal energy recovery in EGS,planning and designing EGS projects that better meet the actual site requirements,we must first explore the reasons for the formation of geothermal sites,evaluate the geothermal resources of geothermal fields,and reasonably predict the site heat.Production capacity,and then designing a reasonable fracturing and heat production plan based on site data,can provide theoretical guidance to actual engineering.Aiming at the above problems,this article takes zhacanggou geothermal field in Guide County,Qinghai Province as the research object.First,according to the structural conditions,stratigraphic characteristics and geothermal geological conditions of the study area,the regional geophysical characteristics,hydrogeochemical characteristics,and drilling data are analyzed,a conceptual model of the Zacangou geothermal field is proposed and the geothermal resources are calculated and evaluated.The base of the Zacangou geothermal field is mainly Indosinian granodiorite,which is a good dry-hot rock thermal reservoir.The main strata are the Middle Triassic and Neogene.The lithology of the Middle Triassic strata is hard sandy feldspar sandstone,feldspar quartz sandstone,and sandstone pebble that are conducive to good thermal storage.The lithology of Neogene stratum is sandstone,mudstone and thin layer calcareous sandstone,which has the function of heat insulation.The main faults in the area include two groups of thermo-optical faults and Zhacang faults.The thermo-optic fault strikes NW and NNW reverse faults and belongs to compressive and torsional faults;the Zhacang fault strikes NE normal fault and belongs to torsional faults.Zhacang fracture is a thermally conductive water channel,which is conducive to the upward movement of deep heat sources and hot water.According to the geothermal geological conditions and the laws and characteristics of the occurrence of underground hot water,the geothermal system above 3km in the zhacanggou geothermal field is a fractured deep-circulation hydrothermal geothermal system,and the below 3km is a strongly structural active zone dry hot rock geothermal system.The layered geothermal flux values below3000m are 89.0mW/m² and 134.3mW/m²,which are much higher than the Chinese average and global average heat flux values.The geothermal reserves of the stratum above 4.7km are 1.01×10¹⁹J,which is equivalent to 345 million tons of standard coal heat,the hydrothermal area?0³km underground?belongs to a medium-scale medium-temperature geothermal field,and the dry-hot rock area?3⁴.7km underground?belongs to a large-scale high-temperature geothermal field.In order to explore the physical and mechanical properties of underground thermal reservoirs,rock samples from wells ZR1 and ZR2 and field outcrops of geothermal fields were collected and tested for mineral composition,chemical composition,thermophysical properties and mechanical properties.The main lithologies of thermal reservoirs are granodiorite,biotite diorite,and quartz diorite.Their main mineral components are quartz,potash feldspar,plagioclase,and biotite.The content of Si in the rock accounts for 66%,followed by Al,Mg,K,Ca,Fe,Na.The average density of zhacanggou thermal reservoir is 2709kg/m3,the average thermal conductivity is 3.23W/?m·K?,and the specific heat capacity is 0.741kJ/?kg·K?.There are fewer pores and fractures in the rock,and most of the pores exist alone.Artificial fractures are required to establish a well-connected fracture network in the underground thermal reservoir.Based on the site logging data and temperature measurement curve,the fracture zone is mainly concentrated in four layers of 1110m¹130m,1430m¹490m,3700m³740m and 4210⁴320m.According to the reservoir temperature and fracture development degree,4210⁴320m underground is selected as thermal reservoirs.By comparing the fracturing schemes of 3300³400m underground in zhacanggou geothermal field and 3600³700m in Gonghe Qiabuqia site.The fracturing results of the 4210⁴320m underground thermal storage fracturing scheme in the zhacanggou Geothermal Field include a fracture half length of 500 m,an average fracture height of 110 m,a fracture opening of 5 mm,and a conductivity of 100 mD·m.The average fracture permeability rate is 20 D.Based on this fracturing result,a numerical model of hydrothermal coupling was established for heat transfer simulation.During the operation of the system,the production temperature of Zacangou geothermal reservoir decreased from 192.89°C to 169.49°C,and the economic life was about 27 years.The total power generation It is 1211.8GW·h,which is roughly equivalent to the annual power generation of 10 Yangbajing geothermal power stations.In order to explore the optimal production scheme,first,a sensitivity analysis of the ground temperature gradient,thermal conductivity,porosity,injection-production depth,fracture permeability,well spacing,water injection temperature and water injection speed is performed to obtain the factors that have a greater impact on productivity.The neural network algorithm further analyzes the effect of single factor and factor interaction on the EGS system.The analysis results show that the injection speed and well spacing have a great impact on the production temperature,followed by fracture permeability,and the injection temperature has the smallest effect;the injection temperature and fracture permeability have a greater effect on the flow resistance,and the injection speed and well The effect of spacing is the smallest;the impact of injection speed and well spacing on electrical power is greater,followed by the injection temperature and the fracture permeability is the smallest;the injection speed has the largest impact on power generation efficiency,followed by fracture permeability,the injection temperature and the well spacing The impact is small.Comprehensive analysis shows that the well spacing is 600m,the fracture permeability is 2×10^-10m²,the injection temperature is 55°C,and the injection rate is60kg/s is the best production plan.In 30 years,the cumulative power generation is1739.6GW·h,which is roughly equivalent to 15 times the annual power generation of Yangbajing Geothermal Power Station,and The cumulative heat production is3.44×10¹⁶J,which is equivalent to 1.17×10⁶ tons of standard coal.Compared with the basic model,the heat generation is increased by about 45%.Within 30 years of system work,the total cost of the proposed EGS power plant in this work is expected to be$56.99 million,with cumulative power generation and carbon trading income of$111.98 million,and total revenue of$54.99 million,reducing greenhouse gas emissions from 0.588 to 2.032 Mt.