Research On Medium-Deep Geothermal Heating Potential And Model Assessment In Songyuan,Jilin

Energy issues have been a key factor affecting the stability and development of human society for centuries.The world has set its sights on the development of new renewable energy sources in response to the current environmental degradation and depletion of fossil energy.As a new type of clean energy,geothermal resources have become the most promising renewable energy source after hydropower and biomass energy due to their wide distribution and unlimited space restrictions.China has abundant geothermal resources,but the development and utilization of these resources is still in the development stage.Geothermal heating is an important way to effectively use geothermal resources.It can alleviate the pressure of winter energy supply and effectively reduce environmental burden.At present,the utilization of geothermal resources for heating in China involves extracting shallow and low-grade hydrothermal geothermal resources.However,in actual engineering,the spatial distribution of hydrothermal geothermal resources is limited by structural topography and cannot be implemented in most areas while shallow geothermal layers for heating are often accompanied by issues in heating quantity and efficiency.Especially in the northeastern region of China,winter is extremely cold,and the commonly used geothermal heating mode cannot be effectively promoted.In response to the above problems,in order to promote the rational promotion of geothermal resource heating in the northeast region,this paper relies on the New Energy Program of Jilin Province?NO.SXGJSF2017-5?,the National Natural Science Foundation Project“Mechanism and modeling study on the low temperature induced reservoir fracturing of enhanced geothermal systems”?NO.41602243?and the“13th Five-Year Plan”national key R&D plan?2018YFB1501803-02?.Explore new methods for geothermal heating in Jilin.This study first evaluates the potential of medium-deep geothermal resources in the study area by collecting published data combined with regional structure,stratigraphic conditions,and geothermal characteristics.The heat sources in the study area are mainly Yanshanian and Hualixi granites.The basement granite is a good hot–dry type of thermal reservoir that is covered with nearly 2000 m of Cretaceous lower caprocks.The various fracture systems form a good heat transfer channel,resulting in good geothermal generation conditions for the study zone.The geothermal heat flow is in the approximate range of 73–79 mW/m².After calculation and evaluation,the hydrothermal geothermal resource in the medium-deep layers of the Songyuan area is6.5×10²⁰ J,which is equivalent to 2.22×10¹⁰ t standard coal.The amount of hot–dry geothermal resources is 7.38×10²¹ J,which is equivalent to 2.52×10¹¹ t of standard coal.The hydrothermal and hot–dry geothermal resources have abundant reserves and have good prospects for development under the current policy situation.In the laboratory experiment,the reservoir parameters were obtained via the laboratory thermal property test and high-temperature mechanical experiments by using a granite sample from the study area.The characteristics of the medium-deep reservoirs in the study area were then discussed.The pressure failure characteristics of the reservoir granite and the seepage effect after fracturing were analyzed via the indoor clear water fracturing test.The granite density in the study area ranged from2.763 g/cm³ to 2.926 g/cm³.The porosity ranged from 1.39%to 2.78%.The permeability ranged from 0.255 md to 0.327 md.The average velocity of the longitudinal and transverse waves were 5.132 and 3.104 km/s,respectively.The specific heat capacity ranged from 0.704 kJ/?kg·K?to 0.748 kJ/?kg·K?.The axial and radial thermal conductivity ranges of the rock were 2.455–2.932 and 2.491–2.887w/m·K,respectively.The variation of mechanical parameters of the rock,such as uniaxial tensile strength,triaxial compressive strength,cohesion,internal friction angle,fracture toughness,elastic modulus,and Poisson's ratio,at different temperatures were analyzed.The variation law of rock fracture pressure affected by confining pressure and flow velocity was studied.The results indicated that the fracture permeability of the sample after fracture in the laboratory was between 5.2×10^-141.1×0^-138)².For the field test,the medium-deep heat transfer field test was performed for two months in the research area of Songyuan Chuangxin-1 well.The experimental results show that the geothermal gradient in the study area is divided into the following four steps in the longitudinal direction:0-103 m at 1?/hm,104-451 m at 7.19?/hm,452-901 m at 3.79?/hm,and 902-2044 m at 5.4?/hm.The bottom temperature and average geothermal gradient are 107.3?and 5.07?,respectively.The fluid inlet and outlet temperatures during the initial operation of the system were 36.7?and58.2?,respectively;these values gradually decreased to 29.1?and 43.0?,respectively,after two months of operation.In the aspect of model research,a medium-deep coaxial casing heating exchanger model was established for the field test in the study area.The on-site heat transfer test process was simulated and verified,and the heat extraction capability under different working conditions was analyzed and studied.The simulation results showed that after two months of system operation,the corresponding heat losses of the outlet fluid temperature under the thermal loads of 100,200,300,and 400 W/m were 0.42°C,0.52°C,0.69°C,and 0.73°C,respectively.The 300 W/m working condition was compared with that of the field test,and the result showed a good fit.The decrease in temperature of the formation at the bottom of the coaxial casing was related to the distance between the casing and the rock?soil?.At a distance of 6 m,the formation temperature was almost unaffected by the fluid.Subsequently,the Enhanced Geothermal Systems?EGS?heating model of the medium-deep geothermal resources in the study area was established.A total of 16fracturing models under different construction conditions were established on the basis of the stratum characteristics,and the fracturing simulation analysis was performed.The simulation results showed a fracture length between 179.2 and 290.0m;crack height between 121.6 and 137.7 m;maximum opening of 14.8 mm;and maximum fracture conductivity and permeability of 1038.1 mD?m and 70.14 D,respectively.The effects of proppant concentration,construction displacement,and fracturing fluid volume on the fracturing results were analyzed,and scheme 12 was selected as the optimal fracturing scheme in the study area.On the basis of the fracturing results,the three-dimensional hydrothermal coupling simulation of EGS was conducted to analyze the sensitivity of the design parameters affecting the heat transfer effect and to evaluate the effects of water injection rate,well spacing,and injection temperature for the heat transfer efficiency.Finally,an optimal design scheme of the geothermal heating model of the Chuangxin-1 well in the study area was proposed,and the medium-deep geothermal heating potential was comprehensively analyzed.The environmental and economic factors were also evaluated.The optimization scheme was set with injection flow rate,well spacing,and water injection temperature of 8 kg/s,400 m,and 30°C,respectively.The simulation system was assumed to run for 20 years,with the average heat extraction of the EGS heating system being 1.99 MW.The flow impedance reached 2.43 MPa/?kg/s?,and the average energy efficiency reached up to 11.43 and remained above 9.9.During the 20-year operation,the cumulative production capacity of the EGS heat exchange heating system was 1.25×10¹⁵ J.If the net production capacity were used for heating,then the average annual heating demand can be guaranteed at 39,000 m².The investment cost was expected to be recovered in 10–15years.During the operation period,the accumulated coal consumption was 6.08×10⁴t,the CO₂ emission was reduced by 1.08×10⁵ t,the SO₂ emission was reduced by 8×10⁵ kg,and the NO_x emission was reduced by 1.46×10⁶ kg.