Study On The Fracking Characteristics Of Granite In CO₂-EGS

The hydraulic fracturing behavior of rock under the influence of CO₂ in the CO₂-Enhanced Geothermal Systems（CO₂-EGS）is a key scientific problem that urgently needs to be solved.According to relevant domestic and foreign literature surveys,it was discovered that fluid injection into the formation not only causes tensile failure,shear failure,but also tensile-shear failure,and also discovers the literature about failure characteristics of rock after CO₂ injection into the formation as a heat exchange medium is relatively poor.Therefore,we take granite outcrop sample from Zhangzhou,Fujian,carried out the thick-wall cylinder method of granite tensile-shear failure test;and we conduct a hydraulic fracturing test under different fluids（CO₂,water）using a self-developed hollow cylinder tester and investigate the fracture characteristics of rock after CO₂ and water infiltration and the mechanism of the effect of rock failure on fracture characteristics.The main research content of this article are:（1）According to the investigation of CO₂-EGS at domestic and abroad,CO₂-EGS is undoubtedly one of the programs that can meet the requirements of new energy development and CO₂ emissions reducing,and has broad prospects for development.In actual projects,the ability to make joints is one of the main factors that determine the efficiency of the project.This article focuses on the effect of short-term CO₂ injection on granite fracture properties.（2）When fluid is injected into the formation,not only simple tension damage will occur,but also tension-shear damage will accompany in the formation,which cannot be ignored.Therefore,this paper combines the tensile-shear strength measured by the thick-walled cylinder method proposed by others and supplements the granite tests.The existence of tension-shear failure is verified by using hollow cylinder tester,which provides guidance for the practical application of CO₂-EGS.（3）A series of physical and chemical reactions occur when CO₂ infiltrate in rocks,which in turn affects the fracking characteristics of rock.At present,most experiments about the CO₂ on the short-term effect of rock are related to sandstone and mudstone.However,the articles related to the granite in this project are very limited.Therefore,this paper will adopt a thick-walled cylinder method similar to the state of engineering stress.Conducting infiltration and fracturing damage test of granite under different fluid and confining pressure conditions to study the effect of CO₂ on the fracking characteristics of granite in a short term.（4）From the engineering and laboratory tests,it was found that most of the fluids influencing the fracking characteristics of rock are related to the physical properties of the fluid itself,among which the viscosity is the most important factor and this article will verify it.Considering the engineering environment and the lack of understanding of the mechanism of fluid fracturing rock,in this paper a new method to explain the mechanism of fluid infiltration and fracturing from the point of view of fluid heat transfer is proposed,and the hydraulic fracture correction model considering thermal stress is given.