This paper proposes a method for predicting sustained casing pressure (SCP) induced by the micro-leakage of the premium connection. Firstly, based on the axisymmetric sinusoidal micro-asperity contact simulation of the sealing surface and the laminar flow theory of incompressible viscous fluid, a prediction model for the leakage rate of the premium connection for completion tubing strings was established by considering the coupling effects of leakage rate of the premium connection and SCP. Then, considering the migration and force status of bubbles generated by premium connection leakage, the motion parameters calculation model for the bubble through the annulus protection fluid was developed. Finally, considering the leakage bubble accumulated at the wellhead, a prediction method for SCP induced by premium connection leakage for completion strings has been developed on the basis of recurrence method. The reliability of the model proposed in this paper was verified through case analysis and comparison, and the physical mechanism of SCP caused by the sequence of the premium connection micro- leakage, bubble slip and rise, and wellhead pressure accumulation was revealed. Meanwhile, the influencing laws of the premium connection sealing parameters and completion engineering parameters on the leakage rate of the premium connection coupled with SCP were systematically analyzed, and the measures to reduce the risk of SCP induced by premium connection leakage were proposed. The results show that initial leakage will first occur at the premium connection of tubing strings driven by pressure difference, later the leaked bubbles accelerate within a very short initial time and then rise at a constant speed to the wellhead for accumulation, which induces the SCP and simultaneously reduces the leakage rate at the premium connection leakage point. As the premium connection leakage time increases, the SCP value gradually increases until the leakage stops, and this time the SCP value also stabilizes. The sealing parameters of the premium connection only affect the leakage rate and SCP evolution. The greater the roughness of the sealing surface and the smaller the axial contact length of the sealing surface, the higher the leakage rate of the premium connection, the faster the SCP rises and the shorter its stabilization time; the shallower the well depth at the premium connection leakage point, the higher the tubing inner pressure at the leakage point, and the lower the density of the casing annulus protection fluid, the higher the stable SCP value and the greater the wellhead leakage safety risk. By reducing the roughness of the sealing surface, increasing the axial contact length of the sealing surface, decreasing appropriately the average diameter of the sealing surface, and enhancing the makeup torque control, the leakage rate of the premium connection can be reduced ; while dynamic monitoring of the casing annulus liquid level and timely replenishing it, and increasing appropriately in the density of the casing annulus protection fluid can lower the SCP stable value, thereby reducing the risk of SCP induced by premium connection leakage in tubing strings.

The abnormal swab pressure resulting from packer unsetting poses a great threat to the collapse resistance of production casings in deep high-pressure and high-temperature (HPHT) oil wells. This paper proposes an analytical model to predict the transient swab pressure in the A-annulus after packer unsetting based on a U-type tube and an iterative method. The model can further evaluate the collapse failure risk of the production casing in the whole wellbore. An example study and sensitivity analysis were carried out to reveal the variation characteristics of the transient swab pressure in the A-annulus and the failure risk of the production casing after packer unsetting. Furthermore, some preventative measures are proposed. The largest swab pressure occurs at the initial time of packer unsetting, which will lead to sudden collapse failure of the deeper production casing. A smaller width of the annular clearance between the packer rubber and production casing and a larger initial liquid level depth in the A-annulus can reduce the swab pressure in the A-annulus after packer unsetting and collapse failure risk of the production casing. In the example, when the width of the annular clearance decreased from 2.97 to 2 mm, the maximum swab pressure decreased from 88.71 to 27.4 MPa, a decrease of 69.1%. When the initial liquid level depth in the A-annulus increased from 700 to 900 m, the maximum swab pressure decreased from 122 to 57.05 MPa, a decrease of 53.2%. When the width of annular clearance was 2.97 mm, the collapse resistance safety factors for the production casing were less than 1.1 and may suffer from collapse failure for well depth between 3610 m and 6100 m. When the initial liquid level depth in the A-annulus was 700 m, the production casing will suffer from collapse failure for well depth between 2869 m and 6100 m. When the width of the annular clearance was less than 2.5 mm and the initial liquid level depth in the A-annulus was larger than 900 m, the collapse resistance safety factors for the production casing were all greater than 1.1 and the whole production casing was safe. To lower the collapse failure risk of the production casing because of packer unsetting, a packer rubber with a reasonable larger outer diameter and good deformation recovery ability is recommended, and the initial liquid level depth in the A-annulus should be controlled reasonably. The research results are of great significance for preventing the collapse failure of production casings during packer unsetting.

The deep shale formation exhibits anisotropic and fractured properties. Previous models of shale wellbore stability have primarily focused on fractured or mechanical anisotropies of shale. Furthermore, thermal effects are inevitably considered when drilling deep shale formations. Nevertheless, the instability mechanism of a wellbore under the combined effects of anisotropy, fractures, and thermal-hydro-mechanical coupling is unclear. Thus, based on the assumption of generalized plane strain, anisotropic porothermoelastic theory, and dual-porosity medium theory, this study established a thermal-hydro-mechanical coupled dual-porosity medium model for inclined wellbore considering complete material anisotropy. The finite element formulation was employed to solve this model. Parametric analysis was performed to investigate the effect of dual-porosity medium properties and material anisotropy parameters on effective stress, fracture pore pressure(pII), and matrix pore pressure(pI). Through model comparison, the effective stress, pore pressure, and failure zone were observed to be completely different from those of the traditional elastic isotropic dual-porosity medium model and elastic anisotropic single-porosity medium model when subjected to the combined action of influence dual-porosity medium and anisotropy. With the elastic anisotropy index increases, the elastic anisotropic pI is smaller than the elastic isotropic pI. The effective stiffness of the rock increases with the elastic anisotropy index, which leads to the generation of 'negative' thermal stress, reduces the effective radial stress and hoop stress. When the well inclination exceeds 60 degrees, the evolution of the induced pI in elastic anisotropy is significantly different from that in elastic isotropy in the X direction, but pII in is not sensitive to the change of well inclination. When a horizontal well is drilled parallel to the bedding direction, the risk of wellbore shear failure will be reduced for a higher ratio of anisotropy in elasticity, solid thermal expansion, and permeability.

The gas-liquid two-phase flow during the production of ultra-deep gas wells continuously impacts the production string, inducing nonlinear flow-induced vibrations, which severely compromise the integrity and safety of production string in ultra-deep gas wells. Considering the nonlinear characteristics of the production string's material, geometry, and boundary conditions, and taking into account the equivalent nodal loads under axial load, as well as the impacts of gas-liquid twophase impact loads and nonlinear contact collision forces, a nonlinear axial-lateral coupled vibration model for the production string of ultra-deep gas wells is established. By solving the vibration model of the production string using the Newmark-beta method, the effects of different production and production string parameters on the nonlinear vibrations are investigated. Through example analysis of ultra-deep directional wells, the results show that the lateral vibration response at the lower position of production string is more pronounced, leading to repeated instantaneous contact and collisions between production string and casing. The axial vibration response at the middle and upper positions of production string is more severe. The effects of gas and water production rates on the axial vibration displacement are relatively small, while the influences of bottomhole pressure, production string outer diameter, and wall thickness are significantly more pronounced. Medium gas production rate (50-75 x 104 m3 & sdot;d-1), increasing production string wall thickness, and reducing water production rate, bottomhole pressure, and production string outer diameter contribute to the reduction of both radial and axial vibration responses of the production string. This study provides a solid theoretical foundation and support for the rational optimization of the structural parameters and production system of production strings in ultra-deep gas wells.

Formation mobility is a key parameter for characterizing reservoir permeability and predicting productivity. Formation mobility can be obtained in real-time by formation testing while drilling (FTWD), which is vital for improving the efficiency of oil and gas development and reducing production costs. In terms of FTWD, adopting an appropriate formation mobility inversion method is the key to accurately obtaining the mobility of the formation. The objective of this research is to evaluate the applicability and accuracy of formation mobility inversion methods in various coupled formations. Therefore, the mobility inversion methods for FTWD were reviewed in detail. Then, the process of finite element simulation of FTWD in various coupled formations, including uncoupled, hydro-mechanical coupled, thermal-hydro coupled, and thermal-hydro-mechanical coupled, was introduced. Based on the pressure response of FTWD generated by numerical simulation, four case studies of formation mobility inversion were performed using three different inversion methods, namely pressure drawdown (PDD), area integration (AI), and formation rate analysis (FRA). Finally, an evaluation was performed to assess the applicability of the inversion methods to different coupling formations, and the suggested method for each formation was provided. The results showed that the interpreted formation mobility based on the pressure response of FTWD is the apparent mobility near the wellbore, which can be influenced by the multi-physical coupling effect. The PDD method has a good applicability to high-mobility formations, a general applicability to low-mobility formations, and a poor applicability to tight formations. Both the AI and FRA methods can be used to invert formation mobility under different coupling conditions, and the accuracy of the FRA method is generally superior to that of the AI method. If the formation is in a coupled thermal-hydro-mechanical state, it is recommended to use the AI method, PDD method, and FRA method to invert the mobility for high-mobility, low-mobility, and tight formations, respectively. The results of this paper are beneficial for the accurate interpretation of formation mobility, which can promote the development of oil and gas resources.

Formation testing while drilling (FTWD) is useful for acquiring dynamic formation parameters in real time. However, the FTWD is inevitably affected by the thermal-hydro-mechanical (T-H-M) coupling effect as the current drilling depth and the formation temperature continue to increase. In order to clarify the mechanisms of the effect of T-H-M coupling on the pressure response of FTWD, a novel T-H-M coupling mathematical model of the pressure response of FTWD was proposed. The parametric studies of the pressure response of FTWD under T-H-M coupling condition were simulated using the finite element method (FEM). The influence of different borehole wall conditions and coupling conditions on the pressure response of FTWD were compared, and the effect of T-H-M coupling on the interpretation of formation parameters was also discussed. The results indicated that the stress perturbation of drilling has little effect on the FTWD, but the change in formation pressure and temperature around the wellbore will significantly affect the pressure response of FTWD. The T-H-M process can create the “coupling skin” effect in the formation, which further leads to the greater pressure drop in the pressure drawdown stage and the lower growth rate in the pressure recovery stage. Formation permeability and temperature differential have a large effect on the pressure response of FTWD, but in situ stress and probe orientation have almost no effect. When the inversion of the original formation parameters is performed using the formation rate analysis (FRA) method, the near-wellbore supercharging can lead to an overestimation of the original formation pressure and the T-H-M coupling can lead to an underestimation of the original formation mobility. The model established in this paper is more in line with the actual test conditions, and the results can provide theoretical support for parameter optimization and data interpretation of FTWD.

This paper proposed a semi-theoretical model to quantitatively predict leakage rate of tubing and casing premium connections. The geometric parameters of the sealing surface profile approximated by a sinusoidal micro-convex surface were first obtained based on the random normal distribution sampling method. With the actual area prediction formula for elastic–plastic contact of an axisymmetric sinusoidal micro-convex body based on the equivalent simulation principle, the circumferential leakage width and radial average leakage height of the micro-leakage channel between sealing surfaces were then acquired with the surface roughness and geometric mean contact pressure. At last, the actual micro-leakage rate of the premium connection was derived by considering the non-uniform contact pressure distribution between sealing surfaces. An example was investigated to validate the model and reveal the sealing and leakage characteristics, and anti-leakage measures were proposed. The results show that average contact pressure, circumferential leakage width, and radial average leakage height between sealing surfaces were non-uniformly distributed. The leakage rate of a premium connection decreases exponentially with an increase in radial interference between sealing surfaces. In order to reduce leakage rate, it is beneficial to increase radial interference and lower sealing surface roughness.

Drilling is a key step in the exploitation of deep oil and gas resources. In order to clarify the stress state of the rocks and the mechanism of rock breakage in deep-well drilling, a thermal-hydro-mechanical coupling model for deep-well drilling was established, and the effects of drilling on the temperature, pressure, and stress in the formation were studied. Furthermore, the effects of the formation parameters and wellbore parameters on the bottomhole stress were analyzed. The results revealed that after the formation was drilled, the temperatures in different horizontal in situ stress directions were not significantly different, but the difference in the pore pressure between the maximum and minimum horizontal stress directions was large. The average effective stress at the bottom of the hole was the smallest, and in some areas, it was tensile stress. For deep-well drilling, as the formation pressure increased, the in situ stress increased, and the permeability decreased, leading to greater average effective stress of the bottomhole rock. As a result, it was harder to break the rock, and the drilling efficiency decreased. Reducing the wellbore pressure and wellbore temperature is conducive to forming tensile stress near the borehole axis in the bottomhole, causing tensile damage. The average effective stress of the formation near the shoulder of the drill bit was compressive stress, and it is advisable to take advantage of the rock shear failure characteristics to improve the drilling efficiency in this area. The results of this study can help us to understand the stress state of the bottomhole rocks and the mechanism of rock breakage and can provide a reference for the optimization of drilling tools and drilling parameters in deep-well drilling.

The present investigations on sealability evaluation for tubing and casing premium connections depend on the FEM with testing. This paper proposed a theoretical model to evaluate the sealability of a sphere-type premium connection based on make-up torque, which combines Hertz contact pressure and the von Mises yield criterion for calculating elastic–plastic contact pressure distribution on sealing interface and adopts the gas sealing criterion obtained from Murtagian’s experimental results for deducing gas sealing capacity. With the proposed model, the effects of additional make-up torque from the sealing interface on the sealing contact pressure distribution and key sealability parameters, including contact width, yield width, average contact pressure and gas sealing capacity, were analyzed and compared. The results show that additional make-up torque from the sealing interface closely influenced sealability parameters’ variation and gas sealing capacity. The gas sealing index based on the sealing contact energy theory should be recommended for sealability evaluation other than average contact pressure on the sealing interface. For improving gas sealability, make-up torque should be controlled accurately for ensuring enough average contact pressure and contact width but a proper yield width, and a lager sphere radius should be selected for reducing the risk of yield sticking.

Considering additional make-up torque from sealing interface, the elastic-plastic contact stress analytical model for sphere to cone premium connection is established on the basis of Hertz contact stress and Mises yield criterion. The effects of additional make-up torque from sealing interface on the contact stress distribution are analyzed. The results show that with the additional make-up torque from sealing interface increases, sealing interface contact state gradually shifts from completely elastic contact to elastic-plastic contact, the contact stress distribution by gradually opening downward parabola to parabolic cup, and the contact width, yield width and average contact stress all increase. In order to improve the gas sealing performance of premium connection, the sufficient average contact stress and contact width should be ensured and the yield width also reasonably controlled. In the example, the additional make-up torque from sealing interface should be controlled between 731.7 N·m and 1826.4 N·m. The results are of great significance for guiding the make-up torque operation and improving the gas sealing performance for premium connection.

The present research studies about sealability of premium connection almost focus on qualitative description by sealing contact stress distribution, which is not convenient for sealing capacity evaluation and sealing parameter design. This paper proposes a quantitative model to calculate directly gas sealing capacity of cone to cone premium connection, different from the usual finite element method with testing, which combines the elastic thick wall cylinder interference fit theory for calculating sealing stress with the gas sealing criterion obtained from Murtagian’s experimental results. With the proposed model, the effects of pipe wall thickness, seal cone taper, radial sealing interference, and axial sealing length on the gas sealing capacity have been investigated. Furthermore, the gas sealing capacity envelope curve based on radial sealing interference and axial sealing length is also calculated and a new sealing parameter design method is proposed for cone to cone premium connection. The results show that the internal upset pipe is good for sealability, and increasing both radial sealing interference and axial sealing length can significantly enhance gas sealability while seal cone taper has an unobvious effect on it. To meet sealing capacity, the designed sealing parameter combination ( δ d , L d ) should be located in the upper right region of the gas sealing capacity envelope curve.

Premium connections, which usually own modified buttress threads, special mental-to-metal seals and torque shoulder, have been increasingly adopted in casing and tubing strings to improve their joint strength and sealing performance in high pressure-high temperature (HPHT) gas wells. Because of complicated geometric structure, it is very difficult to establish an analytical joint strength model for premium connections while typical finite element analysis (FEA) does not also reveal the general failure law. In fact, the coupling joint strength of premium connection will decrease because of the make-up torque resulted from shoulder and the torque shoulder never allows to separate under limit axial working loads. Considering the make-up torque and prevention of shoulder separation, this paper proposes methods to calculate the joint strength of premium connections with API buttress thread teeth based on elastic mechanics. An example is also analyzed by modifying a premium connection from an API 177.8 mm P110 grade buttress thread casing. The results show that the joint strength of the premium connection is largely influenced by the ratio value zeta of the torque resulted from the shoulder to the total torque and that larger or smaller ratio values zeta both decrease the joint strength. To enhance the joint strength and tensile connection efficiency in practice, not only should the total make-up torque be controlled but the ratio values zeta should also be appropriately controlled. The proposed methods more comprehensively reflect the failure patterns of premium connections and their great significance for joint strength calculations, makeup torque control and connection parameter design.

The use of polymer gel for water control and oil addition is a common technical method in oilfield development. The polymer and hydrated bentonite react under the action of an organic crosslinking agent to form a composite gel. The particle-size change and microstructure of the composite gel were analyzed via shear thinning, thixotropic, viscoelastic, and start-up stress rheology experiments. The experimental results show that the polyacrylamide/bentonite organic crosslinked composite gel was a gel system with bentonite as the core aggregate structure, and the large particle-size distribution was mostly increased with increasing crosslinker content. The composite gel presented shear thinning characteristics, the content of bentonite or crosslinking agent was increased, and the shear resistance was stronger at a high shear rate. The composite gel exhibited positive thixotropic properties, and the thixotropy increased with increasing bentonite content. The composite gel had good viscoelastic characteristics, the elastic characteristics of the composite gel showed more significantly with bentonite increases, and the viscosity of the composite gel showed its characteristics more significantly with the crosslinking agent increased. After loading at a rate on the composite gel, the shear stress increased significantly with time and reached its maximum value, and then the shear stress decreased and gradually stabilized.

The significant decreased wellbore temperature and increased casing pressure during fracturing fluid injection present a big challenge for the mechanical integrity of cement sheath in fracturing wells. Based on the theories of elastic mechanics, thermodynamics, and a multi-layer composed thick-wall cylinder, this paper proposed a new mechanical model of cement sheath for fracturing wells, coupling pressure, and thermal loads, which consider the failure modes of de-bonding, radial cracking, disking, and shear failure. The radial nonuniform temperature change and the continuous radial stress and radial displacement at two interfaces have been considered. With the proposed model, the radial distributions of failure stress and the corresponding safety factor for cement sheath during fracturing fluid injection have been analyzed and compared under four failure modes. Results show that the decreased wellbore temperature will produce significant tri-axial tensile stress and induce cement failure of de-bonding, radial cracking, and disking. The increased casing pressure will significantly lower the risk of de-bonding but also aggravate radial cracking and shear failure. For integrity protection of cement sheath, increasing the injected fluid temperature, maintaining higher circulation pumping pressures, and adopting cement sheath with a low elasticity modulus have been suggested for fracturing wells.

Geothermal energy development has increasingly been studied in recently decades because of its renewable and sustainable features. It can be divided into two categories: traditional geothermal (hydrothermal) systems and enhanced geothermal systems (EGS) based on the type of exploitation. The hot dry rock (HDR) in the EGS incorporates about 80% of all thermal energy, and its value is about 100–1000 times that of fossil energy. It is pivotal for geothermal wells to improve the flow conductivity of the HDR mass, enhance the communication area of natural fractures, and constitute the fracture network between injection and production wells by hydraulic treatments. While the wellbore temperature significantly decreases because of fracturing, fluid injection will induce additional thermal stresses in the cement sheath, which will aggravate its failure. Considering the radial nonuniform temperature change, this paper proposes a new thermal stress model for a casing-cement sheath-formation combined system for geothermal wells during fracturing based on elastic mechanics and thermodynamics theory. This model is solved by the Gaussian main elimination method. Based on the analytical model, the thermal stresses of cement sheath have been analyzed. The effects of the main influencing parameters on thermal stresses have also been investigated. Results show that the radial and axial tensile thermal stresses are both obviously larger than tangential tensile thermal stress. The maximum radial and axial thermal stresses always occur at the casing interface while the location of the maximum tangential thermal stress varies. Generally, thermal stresses are more likely to induce radial and axial micro cracks in the cement sheath, and the cement sheath will fail more easily at the casing interface in fracturing geothermal wells. For integrity protection of the cement sheath, a proper decrease of casing wall thickness, casing linear thermal expansion coefficient, cement sheath elasticity modulus, and an increase of the fracturing fluid temperature has been suggested.

A theoretical model for calculating the hermetic seal pressure of special screw threads subjected to an applied torque from a spherical-conical sealing surface is established.The influence of the spherical radius,cone and taper,and applied torque of the sealing face are studied.According to the results,the hermetic seal pressure of the screw threads declines according to a power law with increasing spherical radius;the hermetic seal pressure declines gradually with an increase in the cone and taper;and the hermetic seal pressure increases parabolically with increase in the make-up torque added to the sealing surface.Generally,the spherical radius and make-up torque added to the sealing surface have a significant impact on the hermetic seal performance of the special screw threads.It is suggested to optimize the spherical radius while properly controlling the applied torque such that the stress distribution of the sealed contacts can simultaneously satisfy the hermetic seal performance and stop the yielding of the sealing surface.

Objective To establish a CVS-11 pseudovirus particles ( pp)-based assay for detec-tion of neutralizing antibody against rabies virus. Methods An improved rapid fluorescence focus inhibition test ( RFFIT) for detection of neutralizing antibody against rabies virus ( RVNA) was established based on the CVS-11 pseudovirus expressing a luciferase reporter gene. Forty-six human serum samples were analyzed with the improved RFFIT and the results were compared with those by using standard RFFIT. Moreover, the improved RFFIT was used to detect the titers of RVNA in 91 serum samples collected from pet dogs and pet-breeders in Beijing. Results The coincidence rate of the improved RFFIT and the standard RFFIT was 100% regarding to the analysis of 46 human serum samples and 5 negative reference serum samples. Moreo-ver, the RVNA titers of all serum samples obtained with CVS-11 pseudovirus-based assay showed a signifi-cant high correlation with those obtained with standard RFFIT (n=46, r=0. 94, P<0. 000 1). All of the 91 serum samples collected from pet dogs and pet-breeders in Beijing were positive for RVNA as indicated by the improved RFFIT with a mean titer of 33. 01 IU/ml. Conclusion We established an improved RFFIT based on the CVS-11 pp expressing luciferase reporter gene, which might be used as a reliable alternative RFFIT for measuring RVNA titer. Analysis of the 91 serum samples collected in Beijing with the improved RFFIT showed that all samples were positive for RVNA.

Dectin-1 signalling in dendritic cells (DCs) has an important role in triggering protective antifungal Th17 responses. However, whether dectin-1 directs DCs to prime antitumour Th9 cells remains unclear. Here, we show that DCs activated by dectin-1 agonists potently promote naive CD4 + T cells to differentiate into Th9 cells. Abrogation of dectin-1 in DCs completely abolishes their Th9-polarizing capability in response to dectin-1 agonist curdlan. Notably, dectin-1 stimulation of DCs upregulates TNFSF15 and OX40L, which are essential for dectin-1-activated DC-induced Th9 cell priming. Mechanistically, dectin-1 activates Syk, Raf1 and NF-κB signalling pathways, resulting in increased p50 and RelB nuclear translocation and TNFSF15 and OX40L expression. Furthermore, immunization of tumour-bearing mice with dectin-1-activated DCs induces potent antitumour response that depends on Th9 cells and IL-9 induced by dectin-1-activated DCs in vivo . Our results identify dectin-1-activated DCs as a powerful inducer of Th9 cells and antitumour immunity and may have important clinical implications.

The wellhead casing pressure (WHCP) resulted from gas production, downhole operations, casing and oil tubing or packing leakage poses great challenges to the mechanical integrity of cement sheath in HTHP gas wells. This paper proposed an analytical model of cement sheath on the basis of the multi-layer thick wall cylinder theory and Mohr–Coulomb failure criterion. The pressure and temperature loads were both considered in the model and the calculated stresses are all in good agreement with those obtained by FEA method. Meanwhile, the influences of the WHCP on cement sheath stress and integrity have been investigated with the proposed model, also a safety factor diagram of cement sheath at the weakest casing interface has been drawn by considering WHCP ranged from 10 to 70MPa and wellbore temperature change ranged from −60 to 60°C. Results show that the greater the WHCP, the lower the safety factor and the smaller the influence of wellbore temperature change on it. If the WHCP is less than 40MPa, wellbore temperature effect cannot be ignored. For a specific gas well, the safety factor diagram can be used to determine the reasonable maximum allowable WHCP, which is of great guiding significance for protecting the cement sheath integrity and improving the WHCP management level in HTHP gas well.

This paper describes a new downhole multistage choke technology for lowering the wellbore pressure and temperature profile of a high-pressure and high-temperature (HPHT) gas well. With the nodal system analysis of a gas well, the wellbore pressure and temperature distribution model of a downhole multistage throttle was established when coupling the throttling pressure and temperature dropping model. An example of a HPHT gas well was analyzed and also the choke hole diameters, numbers and positions of the downhole chokes were optimized according to the presets of the wellhead pressure, temperature and acceptable sustained casing pressure (SCP) in the A annulus. The results show that when adopting two-stage downhole throttling, the wellhead tubing pressure, average wellbore temperature, and the wellhead casing pressure in the A annulus have been decreased by 64.4%, 37.8%, and 43.6%, respectively, compared to the case with the surface choke technology. Therefore, using multi-stage throttling technology in HPHT gas wells can reduce wellhead pressure and wellbore temperature significantly, which can effectively lower the risk of SCP, ensure the integrity and security of wellbore and also greatly reduce the platform space for fixed offshore gas wells.