Study On The Characteristics Of Hot Dry Rock Reservoir And Fracturing Test Model In The Gonghe Basin,Qinghai Province

As a kind of clean and renewable resource,hot dry rock?HDR?has great reserves in the earth's crust.Effective development and utilization of HDR can help to improve China's future energy structure and reduce greenhouse gas emissions.The development of HDR in foreign countries has a history of nearly 50 years,but it is still in the stage of resource exploration and target area selection in China.Enhanced geothermal system?EGS?is the most effective way to exploit dry hot rock.The key technical problems involved are mainly reservoir reconstruction and geothermal exploitation.As one of the most typical unconventional reservoirs,the reservoir reconstruction of HDR is a complex process of temperature-mechanics-fluid?THM?multi field coupling,in which high pressure fluid shears and slides natural fractures or creates new fractures in high temperature and high stress crystalline rock with very low permeability.This process needs to accurately predict the extension and seepage characteristics of the artificial seam network,so the requirements for rock mechanics are very high.The effect of reservoir reconstruction determines the key parameters such as the optimal fluid circulation rate(q_in),reservoir fluid resistance?I_R?and well spacing?L?in the process of geothermal exploitation,thus affecting the productivity of EGS.Therefore,before the construction of EGS engineering site,it is of great significance to carry out the relevant lab test and numerical simulation research for specific site conditions.For the two key technical problems in EGS,this paper is based on the development of HDR in the Gonghe Basin in northwest China,the physical and mechanical properties of dry hot granite,the hydraulic fracturing test of dry hot granite,the model research of reservoir fracturing and geothermal exploitation are carried out.The experimental study of granite is based on the actual reservoir conditions as far as possible,and the relevant parameters are input into the discrete fracture network?DFN?simulator to simulate the characteristics of the complex fracture network formed in the HDR reservoir.Finally,the fracture network formed by fracturing is realized in the multi field coupling software TOUGH2 by the method of"multiple phase interaction continuum"?MINC?,so as to analyze the geothermal productivity of HDR reservoirs with different hydraulic fracture characteristics.First of all,through the special field geological survey,the geothermal characteristics,natural fractures and geostress state of the Gonghe Basin are analyzed from the perspective of geothermal geology to determine the reservoir characteristics of HDR exploitation.Among them,the ground temperature characteristics of the site are determined based on the geothermal geological data,drilling data and logging data.The comprehensive results show that the temperature gradient in the 2500³705m depth of GR1 well is as high as 7.14?/100m,and the formation temperature is between 150²36?.The natural fracture development degree of the target reservoir is based on the drilling core identification,which indicates that the natural fracture may be highly developed.From the aspects of structural analysis,joint fracture statistics,focal mechanism and In-situ stress measurement,it is concluded that the maximum horizontal stress of the field is close to the compressional direction?NNE-NE?of the basin.Secondly,a large number of physical and mechanical tests were carried out on the granites collected from Gonghe area.The micro characteristics,mineral components,thermophysical parameters,porosity and permeability of granite are determined through scanning electron microscopy,X-ray diffraction,thin section identification,thermophysical property test and porosity and permeability test.Then,the Brazilian splitting test was carried out on granite samples in the range of room temperature?about 20??to 170?.The test results show that the tensile strength of granite decreases significantly with the increase of temperature.The relationship between the tensile strength and temperature evolution was obtained by numerical fitting.In addition,the triaxial compression tests of granite at different temperatures?room temperature to 160??and pressures?0 to 40MPa?were carried out.The results show that the brittleness of granite decreases with the increase of temperature,and the plasticity decreases with the increase of confining pressure.The shear failure mechanism is obvious under high temperature and pressure.Based on the triaxial compression test results,the evolution relationship between the mechanical parameters and the formation depth?temperature and pressure?is obtained,which provides a basis for the establishment of reservoir stimulation and geothermal production model.In addition,nine large-scale?300×300×300 mm?natural granite samples collected from Gonghe area were tested under true triaxial fracturing system to study the influence of temperature,injection rate and physical and mechanical characteristics of rock on the fracturing effect,and the apparent geometry and seepage characteristics of self-propping fractures formed by fracturing were analyzed.The results show that:?1?the higher mechanical strength specimens have greater rupture pressure;the higher the porosity,the greater the minimum injection rate capable of fracturing the rock.?2?The higher the injection rate is,the larger the fracture aperture is,and the more easily the fracture effect with multiple fractures will be formed.?3?During high temperature fracturing,a large number of microcracks will be produced in the process of rock heating,which will easily lead to the formation of a complex hydraulic fracture network with multiple fractures,resulting in large fracture aperture and permeability coefficient.The above results indicate that it is feasible to enhance the permeability of HDR by water fracturing.Finally,based on the above experimental results,the fracturing model investigation is carried out in the background of GR1 HDR well in the Gonghe Basin.The fracturing sensitivity analysis is carried out on the factors such as the development degree of natural fracture,the state of in-situ stress,the physical and mechanical properties of rock and the parameters of fracturing operation.The sensitivity results show that the three factors that have a great influence on the fracture area are the consumption of fracturing fluid>natural fracture spacing>the injection rate of fracturing fluid,and the three factors that have a great influence on the fracture aperture are natural fracture spacing>the tensile strength>elasticity modulus.Among them,the natural fracture spacing in the reservoir characteristics is the most critical factor to determine the fracturing results.Therefore,according to the degree of natural fracture development,the HDR reservoir can be divided into four reservoir models:natural fracture development,relatively developed,undeveloped and no natural fracture development.The fracturing simulation study is carried out for four kinds of reservoir models,and the fracture network characteristics are analyzed to determine the suitable fracturing technology for the reservoir with different natural fracture development.The results show that the reservoir with high natural fracture development is suitable for the treatment of shear fracturing,which can obtain a fracture network with an aperture of 2.0⁴.3 mm and permeability of 10^-1210^-13m².For the reservoirs with less natural fractures,it is suggested that gel support fracturing technology be used to obtain the support fractures with average permeability higher than 10^-11m².Finally,hydrothermal simulator TOUGH2 is used to investigate the optimal fracture network of each fracturing model for geothermal exploitation.The simulation results show that the reservoir with higher natural fracture development can build a EGS system with higher geothermal efficiency.In conclusion,it is expected that it is feasible to build a geothermal power generation project in the Gonghe Basin in northwestern China with an operation cycle of 20 years and a stable installed capacity of 0.84MWe³.94MWe.