Study On Deformation Law And Damage Mechanism Of Thick-Walled Casing In Salt Rock Creep Formation Of Complex Gas Well

As a good caprock,the salt rock strata often contain rich oil and gas resources.Field caliper logging results of a certain gas field in western China show that,casing deformation and even damage have occurred in some wells.Aiming at the problem of casing deformation and damage occurred in salt rock creep formation of complex gas wells,combined with the actual logging data and casing deformation characteristics,the deformation law and damage mechanism of thick-walled casing in salt rock creep formation are systematically studied through laboratory test,theoretical analysis and numerical simulation in this work.The main work and conclusions are as follows:（1）Based on the triaxial creep tests of salt rock,a new NGKH creep constitutive model is constructed by connecting the improved generalized Kelvin model with Heard model,which can describe the instantaneous strain,attenuation creep stage and steady-state creep stage of salt rock under different stress states.The NGKH creep constitutive model is developed by using UDM interface program of FLAC^3D,which provides the necessary theoretical model for the follow-up research.（2）Based on the random fuzzy statistical method and K-S hypothesis test method,the uncertainty analysis of rock mechanical parameters of salt rock formation is carried out.The optimal probability distribution forms of elastic modulus,Poisson’s ratio,uniaxial compressive strength and cohesion are Weibull distribution,Weibull distribution,lognormal distribution and lognormal distribution respectively,which provides the necessary lithologic parameter data for the follow-up research.（3）Based on Monte Carlo method,the uncertainty analysis method of in-situ stress in deep inclined formation is proposed by considering the factors such as formation dip angle and stratum dip in the horizontal geostress calculation model.The confidence intervals of in-situ stress under different significance levels are given,which can describe the stress state of complex formation more accurately than single value.According to the results of multi factor analysis,the change of formation dip angle has the greatest influence on horizontal in-situ stress.The results of in-situ stress uncertainty analysis provide data support for the subsequent reliability and sensitivity analysis of casing in inclined salt rock creep formation.（4）Based on the multi-objective constrained optimization method,the optimal inversion model of in-situ stress field is established,and the distribution characteristics of plane in-situ stress field in the study area of the gas field are obtained by Levenberg-Marquardt method and numerical calculation.Taking the casing deformation data as the constraint target,the optimal inversion model of non-uniform in-situ stress of single well is established,and the actual non-uniform horizontal in-situ stress of casing deformation formation is analyzed by using the least square method.The results show that the horizontal in-situ stress calculated by the optimized inversion method is in good agreement with the field measured value.The non-uniform horizontal in-situ stress obtained from optimized inversion of casing deformation data can better describe the actual in-situ stress of creep formation after casing deformation to a certain extent.The calculation results provide the necessary non-uniform in-situ stress data support for the subsequent study of casing deformation law and damage mechanism.（5）Through theoretical analysis,the calculation formula of casing collapse strength in creep formation under the action of non-uniform in-situ stress is given.Based on the mechanical model of formation-cement sheath-casing system,a two-dimensional finite element model is established to study the variation of casing external load with creep time under different casing wall thickness and in-situ stress non-uniformity coefficient.On the basis of the above research results,the calculation formula of collapse strength of worn casing in creep formation under the action of non-uniform in-situ stress is given by considering the wear factors.By means of theoretical analysis and numerical calculation,the collapse resistances of worn casing in creep formation under different wear angles,wear depths and in-situ stress non-uniformity coefficient are analyzed.The results show that the collapse strength of worn casing decreases with the increase of non-uniform degree of in-situ stress,and decreases with the increase of wear amount.When the non-uniform external load tends to be stable,with the increase of wear amount,the maximum equivalent stress of the worn casing first decreases and then increases when the wear angle is 0°,and increases approximately linearly when the wear angle is 90°.（6）On the basis of the above research results and considering the geological characteristics of complex gas wells,three-dimensional finite difference models of inclined salt rock formation-cement sheath-casing are established to analyze the distribution characteristics of casing stress and deformation under the condition of complex salt rock creep formation.The results show that the maximum equivalent stress of casing and the maximum deformation of casing increase with the increase of formation dip angle and well deviation angle,and decrease with the enhancement of rock mechanical properties in salt rock formation.When the dip angle and well deviation angle are large,the maximum deformation of casing in salt rock formation increases approximately linearly with the increase of salt rock formation thickness.By setting static boundary conditions at the bottom of the model and applying seismic dynamic load,the deformation law and damage mechanism of casing in complex salt rock creep formation under the joint action of earthquake and formation creep are studied.The results show that the earthquake causes the mutation of the stress state of the formation and casing and the relative slip between the strata,especially at the contact surface of the special geological structure section,which aggravates the interlayer discordance and accelerates the creep rate of salt rock formation.The displacement load and non-uniform external load caused by creep greatly exceed the designed collapse strength of the casing,resulting in casing deformation and damage.The casing deformation trend calculated by the finite difference model of 3D combination is in good agreement with the field measurement results,which can reappear the deformation and damage process of thick-walled casing in salt rock creep formation,and provide a theoretical reference for solving the casing damage problem in deep complex salt rock creep formation.（7）The reliability and sensitivity analysis method of casing in deep complex salt rock formation is proposed by using Monte Carlo method,and the reliability and sensitivity of casing in casing deformation section is analyzed with a project example.The results show that,for the casing deformation section,the elastic modulus of salt rock formation is the most important factor affecting the casing failure.The reliability of casing increases with the increase of elastic modulus of salt rock formation and casing wall thickness,and decreases with the increase of in-situ stress non-uniformity,formation thickness,formation dip angle,well deviation angle and casing outer diameter.In order to ensure the safety and reliability of casing in salt rock creep formation of complex gas wells,special geological structures such as weak strata and faults should be avoided when drilling.When crossing the stratum with large dip angle,the high grade and thick-walled casing should be selected as far as possible under the premise of considering the economic cost.