Study On Fracture Of Casing-Cement Sheath Cement Interface In Hot Dry Rock Geothermal Wells Under Thermal-Fluid-Solid Coupling

Hot Dry Rock geothermal resources is a kind of clean energy,and it is also one of the key measures to optimize the national energy strategy.The development of geothermal resources of deep dry and hot rocks requires fracturing to form a fluid circulation system,and the change of wellbore temperature and pressure caused by fracturing fluid will cause the fracture of casing-cement sheath cementation interface of Hot Dry Rock geothermal wells,which involves fluid percolation,material deformation,heat transfer and so on.This paper focuses on the fracture of casing-cement ring cement interface in Hot Dry Rock geothermal wells under the action of variable temperature and high stress field.A hot pore elastic model of casing-cementation surface-cement sheath-formation combination is established,which considers the effects of defect distribution,pore pressure,heat conduction and thermal permeability,and the model solving algorithm is established based on Laplace transformation.The influence of temperature and pressure parameters on stress is analyzed.The main contents and results are as follows:（1）The theoretical model of casing-cement surface-cement sheath-formation combination under in-situ stress is established,and the boundary stress conditions are decomposed into uniform stress boundary conditions and non-uniform stress boundary conditions.The theoretical solutions of stress field under two kinds of boundary conditions are derived.Considering the mechanical properties of the cemented surface and the influence of its internal pore structure,with the help of the finite element analysis method,the sum of dimensionless stress influence coefficient is introduced to reflect the influence degree of pore number and pore structure on the stress field.The maximum stress influence coefficient isK₁（28）1.0265,K₂（28）1.1524.A theoretical model of casing-cement surface-cement sheath-formation combination considering the effect of pores under in-situ stress is established,and the accuracy of the model is verified.The results show that the number of pores on the casing-cement sheath cementation surface and the shape of the pore structure will aggravate the non-uniformity of the stress field.When the pores exist,the Mises equivalent stress increases by 12.50%and the radial stress increases by 9.30%.The maximum increase of Mises equivalent stress caused by changing the thickness,elastic modulus and Poisson’s ratio of casing-cement sheath bonding interface is 0.06%,2.57%and 3.15%,respectively.（2）Considering the influence of cementation surface,cement sheath,formation pore,pore water pressure caused by water pressure in casing,thermal load caused by temperature difference between injected water in casing and underground formation,and thermal permeability effect（T-O）,a thermal pore elastic model of casing-cementation surface-cement sheath-formation combination in hot dry rock geothermal wells is established,which overcomes the shortcomings caused by the assumption of cementation surface,cement sheath and formation continuity.Then the general solution of casing-cement sheath cementation interface is obtained by using state equation,geometric equation,equilibrium equation and Laplace transform in Laplace domain.Through Laplace inverse transformation and definite solution condition,the expressions of stress field,temperature field and seepage field in time domain are obtained.Combined with boundary conditions and continuity conditions,the undetermined constant coefficient matrix is established.Through Matlab software programming,the coefficients of each interface of hot dry rock geothermal well casing-cementation surface-cement sheath-formation combination in time domain under the action of thermal-fluid-solid coupling are obtained,and the thermal-fluid-solid coupling model is verified.（3）Based on the theory of near-field dynamics,the numerical model of porous cementation surface is established,and the basic mechanical parameters of casing-cement sheath cementation interface are obtained.Based on the engineering background of GR2 well in Qiabuqia area of Gonghe Basin,Qinghai Province,the temperature curve,radial stress curve and radial displacement curve of wellbore system developed with time are obtained.The paper further analyzes the influence of thermal-fluid-solid coupling parameters on the radial stress of casing-cement surface-cement sheath-formation combination in hot dry rock geothermal wells.The results show that when the elastic modulus,Poisson’s ratio and Tmuro coefficient of the cement surface are close to the corresponding properties of the cement sheath,and the lower formation temperature and smaller porosity of the cement surface are more conducive to the integrity of the casing-cement surface-cement sheath-formation system.