Safety Analysis Of Completion Strings For Steam-Injected Thermal Recovery Wells

The extraction methods of heavy oil reservoirs include Steam stimulation,steam flooding and burning oil layers,among which steam stimulation is the most widely used method to improve the recovery ratio.However,the high temperature steam in steam injection well can cause the casing to creep,produce the heating stress,and the performance of the casing material will also decrease.Therefore the porosity and permeability of formation change as the injection-production time rises and variation of in-situ stress.The risk of casing failure will increase and the actual life will also continue to decrease.In the process of thermal recovery of heavy oil reservoirs,sand production can also cause the failure of completion sieve tube and the instability of gas injection tubing string.The research on temperature field,stress field,safety protection and safety evaluation of wellbore tubing string in thermal production is of great theoretical significance and engineering application value.In this paper,a comparative systematic study is carried out for the safety protection of tubing string in heavy oil thermal recovery well,and the main conclusions are as follows:(1)The high temperature mechanical properties of the materials are tested indoors,and the relationship between the material properties and the temperature are studied,and the stress-strain curves of the materials under high temperature are considered.With the increase of temperature,the elastic modulus,yield strength and strength limit of the material decreased,while the material elongation rate and the reduction rate of material section were increased.The higher the temperature,the less stress required under the same strain.The welding performance and welding quality of casing wellhead of HSTG80 SH and TP90 H casing Gas seal thermal recovery wells are evaluated and compared.The joints between the HSTG80 SH orTP90H casing and 35 CrMo are good solderability.(2)The theoretical models of the formation heat conduction and the wellbore heat conduction are established.The effects of thermal conductivity of different insulation pipes on temperature of the wellbore system and casing are studied;the temperature at different well depths as well as the temperature in the joint of the heat-insulated pipe is also studied;the force of the thermal recovery casing stirng under complicated constraints is analyzed and studied.Results show that the temperature of the casing,the cement sheath and the adjacent formation are directly proportional to the thermal conductivity of the insulated pipe.The temperature of the joint of the insulated pipe is obviously higher than that of other parts.The temperature is the control factors to the Von Mises stress of casing.The pre-stress measures should be taken to help reducing the Von Mises stress of casing in the Steam-Injected thermal recovery well.(3)The theoretical analysis of stress is established for the completed screens of the thermal recovery well.A three-dimensional finite element model is established as well which is used in the analyis of the stress distribution and collapsing strength of horizontal well completion screen under uniform load.The influence of screen parameters on casing stability during thermal recovery is also studied.Results show that When the hole density(within a certain range)or the phase angle is constant,the larger the hole diameter,the lower the collapsing strength of sieve tube.When the phase angle is fixed,the greater the hole density,the lower the collapsing strength.When the aperture is fixed,the small phase angle(less than or equal to 60°)and high hole dense can be used for casing completion.Increasing the hole density and wall thickness,and reducing the hole redius appropriately can be used to improve the overall stability of perforating sleeve when the hole area is fixed.In order to comprehensively consider the balance between safety and economy,it is suggested that the design criteria of small phase,high hole density and large hole redius be used in the design of perforation casing.(4)The formula between porosity as well as permeability and formation pressure during reservoir pressure drop are derived,and the casing stress and deformation are studied while reservoir pressure dropping.The influences of different bottomhole pressures and reservoir pressures on the stress distribution of casing,cement sheath and perimeter of perforation casing are analyzed by the three-dimensional ANSYS software.The influence of sand production on casing stress and deformation are studied,as well as further considering the influence of casing with initial bending.The prediction method of sand production in reservoir is analyzed.The casing stress while considering and not considering the creeps itself are compared.Results show that the Von Mises stress of the casing and cement sheath is larger when the bottom hole flow pressure and reservoir pressure is smaller.The stress of the perforated casing or cement sheath is higher than that of the unperforated segment,and the Von Mises stress is the largest in the casing perforation section.(5)The Von Mises stress value of the casing and cement sheath is obviously increased due to sand production.With the increase of sand production,the Von Mises stress and axial stress of the casing increased sharply,and the stress variation in the casing perforation is the most significant.The smaller the elastic modulus of formation,the greater the Von Mises stress in the casing perforation.The influence of internal pressure on the axial stress is not obvious.(6)In thermal recovery wells,the design of casing based on strain should fully consider the influence of casing creep.The total strain and Von Mises stress of casing increase with the increase of temperature and time and there will be a mutation in the building up section.When the permeability of formation and the drainage rate of bottom hole are the lowest value at the same time,the Von Mises stress of the wellbore is the largest and the tubing string is in the most unsafe state.Failure will first occur in the casing inner surface of the perforation section.(7)Based on the 3D numerical simulation method,the casing evaluation model of the casing sorking process of thermal recovery well is studied.A safety evaluation method of casing safety during the whole cycle of steam injection is proposed based on the casing residual stress corresponding to different steam injection cycles with the corresponding casing failure criterion.(8)The software “Software of Oil and Gas Well Casing String Design Based on Strained is developed(Version 1)“ is deleloped based on experiment results and wellbore 3D stress analysis method.The main modules in thermal recovery well calculation include: the strain design module of thermal recovery well,ANSYS software calling module and near wellbore in-situ stress calculation module.The mechanical calculation and safety evaluation of the tubing string in the multiple thermal recovery wells are carried out in the engineering field.