Test And Model Research Of Hydraulic Fracturing And Reservoir Damage Evolution In Enhanced Geothermal System

Energy is the driving force of the society operation and economy development. To ensure the security of national energy structure, countries are actively looking to develop various types of energy. Deep geothermal energy resource possesses many advantages, such as large resource potential, wide distribution and clear. However, the deep hot reservoir must be enhanced to get economically geothermal fluid due to the low porosity and permeability. Enhanced Geothermal System(EGS) is an effective technology for the deep geothermal development from hot dry rock(HDR). Currently, the EGS project in the world is still at the stage of research and development, facing many problems, such as technology, capital, policy and public acceptance, etc. Geology exploration, deep drilling, reservoir stimulation and simulation are the key technologies for the development of EGS. To breakthrough these key technologies can reduce costs, environmental impact and promote the development and commercialization of EGS.Reservoir stimulation and simulation are the main core technologies for EGS. Hydraulic fracutirng is the main method for reservoir stimulation. To make sense of the initial and development of hydraulic fracture is significant for the reservoir stimulation in different geology contions. Besides, the reservoir property would change and damage due to the change of thermal field and mechanical field during the long term water circulation(>20 years), which would have a big impact on the heat production.In this thesis, we carried out the Test and model research for the hydraulic fracturing and reservoir damage evolution in EGS. First, we have carried out the large size hydraulic fracturing test and rock damage test. Based on the test results, the fracture initiation and development procedure and rock damage procedure are both developed. Then, the two procedures are insert to the TOUGHREACT-FLAC3 D in the form of modules. Using this software we established the 3D geological model of Xujiaweizi(XJWZ) target formation and conducted the hydraulic fracturing and hydro-thermal simulation and evaluated the effects of reservoir damage on EGS capacity. Finally, we also conducted the gel-proppant fracturing combined horizontal well technology simulation to assess the hot dry rock geothermal resource potential and proposed the optimal hydraulic stragety and operation stragety. The main research contents are shown as follows:(1) Using the large size(300 mm×300 mm×300 mm) hydraulic fracturing device to conduct the hydraulic fracturing test in homogeneous rock and natural fracture rock respectively. Meanwhile, to discuss the low temperature injection we also conducted the contrast test between room temperature and high temperature fracturing respectively. We analyse the results based on the pressure curve and acoustic emission monitor. It can be concluded that hydraulic fracture extended along the vertical plane to the minimum horizontal principal stress; high temperature fracturing would result in many fractures, which would increase the extension pressure and flow resistance; natural fracture has a big influence on the development of hydraulic fracture; the fracture pressure and extension pressure would decrese with the increase of fracture density.(2) For the initiation and development law of hydraulic fracture, we developed hydraulic fracturing procedure using FISH and FORTRAN language based on the following theories: using elastoplastic theory to derive the fracture initiation pattern and pressure, adopting stress intensity factor as the rule of fracture propagation, using plasticity theory to characterize the hydraulic fracture width, adopting the cubic law to modify fracture permeability. Then, a numerical model was established to compare with the fracturing test. It can be concluded that our procedure have a good performance on simulating the process of hydraulic fracturing.(3) To study the damage development of EGS reservoir due the change of thermal and mechanical field, we conducted triaxial compression test using 25 piesces of indo-chinese epoch granodiorite. The development of elastic modulus and poisson’s ratio would be discussed. Thus, we concluded the elastic modulus and possion’s ratio development law with the change of temperature and confine pressure.(4) We evaluated the development potential of hot dry rock in XJWZ from the aspects of geology, geophysic and geotheric. The target formation is selected and investigated, whose in-situ stress, fracture develop condition, mechanical and thermal parameters are also summarized for the subsequent numerical simulation.(5) We compiled the hydraulic and damage procedure to the TOUGHREACT-FLAC3 D software and conducted the reservoir stimulation and hydro-thermal simulation use this procedure for XJWZ. It is concluded that the hyadraulic fracture length is 230 m, height is 40 m and width is 1.8 cm. For the electricity generation of EGS, the appropriate injection flow rate is just 1 kg/s; the electrical power is 35.0 kw~6.8 kw and reservoir resistance is 0~2.9 MPa/(kg/s)during 20-year operation; fracture space should be greater than 320 m; the electrical power considering the damage is lower than that of unconsidering the damage. Thus, using the lower temperature water would enhance the reservoir conductivity in the reservoir stimulation, while it is not a good choice in the water circulation.(6) In the previous study, the research based on gel-proppant fracturing for electricity generation and heating from EGS have seldom been studied. Due to the different required production water temperature, variouis reservoir conditions, hydraulic stragety and operation stragety, it is very important to choose an optimal stragety to improve the success rate and reduce the cost. Thus, based on the real geology data of XJWZ, we have conducted the following work: ① We coducted the parameters sensitivity analysis of hydraulic fracturing, such as in-situ stress, proppant diameter and sand concentration(sand ratio); ② Based on the fracturing results, we conducted the parameters sensitivity analysis of hydro-thermal simulation for electiricity generation and heating, respectively; ③ Based on the simulation results, the maximum electrical power and the maximum heating power are both obtained. Meanwhile, the corresponding optimal hydraulic stragety and operation stragety in the target formation are obtained in the end.