Study On The Characteristics And Models Of Hydrothermal Production In Heterogeneous Fractures Of Granite

The optimization and upgrading of energy structure requires improving the utilization rate of clean and renewable energy to ensure that China can achieve a carbon peak in 2030 and carbon neutrality in 2060.In 2021,China’s annual energy consumption reached 5.24 billion tons of standard coal,of which nonfossil energy consumption accounted for 16.6%,an increase of only 0.7% over the previous year.As a result,there is still a long way to go to improve the proportion of clean energy in China’s energy structure,and new nonfossil energy with large reserves and wide distribution is urgently needed to support energy consumption.In view of this,as a clean energy widely stored in rock and soil,the large-scale exploitation of geothermal energy is expected to provide solutions for China and the world’s emission reduction targets.At present,the development of high-temperature geothermal resources in a rock formation is mainly provided by natural or artificial fractures,so the flow and heat transfer process in fractures is one of the critical factors affecting reservoir productivity.Field observations and laboratory measurements have widely confirmed that the fractures have the inherent geophysical properties of anisotropy of aperture and random contact obstacles.At the same time,the traditional parallel plate model can not describe the channelized flow induced by heterogeneous characteristics and its influence on the heat transfer process.Therefore,it is necessary to further clarify the influence of fracture heterogeneity on the flow and heat transfer process and heat extraction performance,so as to improve the evaluation accuracy of reservoir productivity.Based on the above problems,firstly,the pressure-sensitive paper is used to measure the contact obstacles characteristics of granite.Based on the measured results,granite fracture samples with different contact rates were prepared.Then,the fracture seepage test is carried out to analyze the influence of random contact obstacles on the fracture permeability.The test results show that the water pressure gradient in the fracture is discrete due to the difference in the distribution of the contact obstacles,and the permeability property of the fracture has the opposite evolution trend before and after the critical contact rate.According to the dominant role of confining pressure and contact rate,the influence of contact rate on fracture permeability can be divided into two stages: the stage dominated by void closure deformation and the stage dominated by contact obstacles.Before the critical contact rate,the pressure gradient,equivalent hydraulic aperture,and equivalent permeability of fractures decrease,increase and increase with the increase of contact rate,respectively.After the critical contact rate,the pressure gradient,equivalent hydraulic aperture,and equivalent permeability of the fracture show the opposite evolution characteristics with the increase of the contact rate.Then,the heat transfer experiments of fractures were carried out by using the regular contact obstacles model,and a new theoretical model of convection heat transfer coefficient considering the effective heat transfer area was established based on Newton’s cooling law.The heat transfer test results show that the flow difference of cracks with different contact rates is the main factor affecting the heat transfer of fractures.With the increase of contact rate,the flow through the fracture increases first and then decreases,resulting in the outlet temperature of the fracture decreases first and then increases,while the heat generating power and cumulative heat generation first increase and then decrease.The traditional theoretical model of convection heat transfer coefficient underestimates the actual convection heat transfer coefficient of fracture,and the degree of underestimate increases with the increase of contact rate.The new theoretical model of convective heat transfer coefficient shows that there is a positive correlation between the convective heat transfer coefficient and the contact rate after the critical contact rate,which reveals the mechanism of the contact obstacles in the heat transfer process of the fracture.In order to further analyze the evolution law of channelized flow caused by heterogeneous characteristics and its influence on hydrothermal production,the Hydrothermal coupling model of single fracture and fracture network reservoir unit was established.Then,the degree of channelized flow in fractures is quantified by effective flow parameters,and the scale effect of effective flow and its correlation with fracture intensity P32 are discussed.The simulation results show that the average effective flow intensity of single fracture is 35%～55%,while the average effective flow intensity of fracture network system is 20%～38%.With the increase of correlation length,the dominant flow channel changes from braided to banded,and the number of flow channels decreases.With the increase in correlation length,the dominant flow channel changes from braid to strip,and the number of flow channels decreases.The flow channel is straight and smooth under the condition that the water pressure gradient is parallel to the main range direction,while the flow channel is curved under the condition that the water pressure gradient is perpendicular to the main range direction.With the increase of correlation length ratio,the tortuosity of streamline increases and the effective flow intensity decreases.In addition,the effective flow intensity of a single fracture has dimensional stability,and the effective flow intensity of fracture network has power-law and exponential relationships with fracture intensity P32 and rock equivalent permeability,respectively.In the fracture network,due to the uncertainty of the aperture distribution,the dominant flow path in the system changes between the fracture subchannels.The hydrothermal coupling results also show that the production temperature of fractured reservoir units is dependent on the correlation length,and the variation coefficient of production temperature increases with the increase of correlation length and fracture intensity P32.The traditional parallel plate model underestimates the production temperature of the heterogeneous aperture model,and the greater the correlation length,the higher the degree of underestimate.For the fracture network with small correlation length,the cumulative heat production of homogeneous model is slightly lower than that of heterogeneous model.However,with the increase of correlation length,the homogeneous model began to overestimate the cumulative heat production of heterogeneous model.There is a strong logarithmic correlation between the effective flow intensity and the cumulative heat production.The logarithmic model has different levels according to the size of fracture intensity P32.The research results show that the flow channelization induced by heterogeneity plays a decisive role in the productivity of geothermal reservoir.Finally,taking a geothermal field in Jilin City as the geothermal geological background,an enhanced CBHE production model combined with natural fractures is proposed,and the long-term impact of the aperture change of the natural fracture system on the thermal performance of the field geothermal field is studied,and the sensitivity of the system productivity to the well section length,circulation flow and heating cycle mode is discussed.The results show that the production temperature curve can be divided into three stages.The constant aperture model underestimates the heat production performance of the variable aperture model,and the underestimated degree is positively correlated with the aperture variation coefficient.Increasing the length of the well section leads to a significant decrease in pump energy consumption,at the same time,it also reduces the thermal power of the reservoir and finally reduces the cumulative net thermal energy.Although the increase of circulating flow leads to the reduction of production temperature,the heat generating power and accumulated heat energy are significantly increased,and finally,a higher accumulated net heat energy is obtained.The effect of the heating cycle on the heat production performance of enhanced CBHE is mainly determined by the cumulative injection flow.Extending the heating time reduces the production temperature and heat production power,which leads to a significant increase in cumulative heat production.In addition,the thermal performance and actual life cycle of the system are related to the minimum heating temperature,so reservoir design and injection production management should be carried out according to the heating conditions.