Extended Reach Drilling (ERD) is a particular challenge for directional drilling, especially in China Bohai Bay. For environment protection reason, oil based mud is forbidden to be used in Bohai Bay, and water based mud is only choice for that time. In the previous campaign, long time drilling has led to serious shale hydration and wellbore collapse in the sloughing formation with low operation efficiency. With the intention to eliminate the wellbore collapse events caused by shale hydration and high Drag and Torque (D T), Synthetic Based Mud (SBM), which is similar to oil-based mud but is relatively non-toxic and has the potential to biodegrade, has been developed and applied for the first time. Based on the wellbore stability model, the adaptability of SBM and sloughing formation is analyzed, and the properties of SBM are optimized; moreover, the Continuous Circulation System is used to reduce equivalent circulating density of drilling fluid, and the micro reamer and cuttings bed breakers have also been employed to reduce equivalent circulating density of drilling fluid. These technologies have been successfully used in more than 10 extended reach wells, true vertical depth of 2000 m and horizontal displacement of 4300 m breaking the records of Bohai Oilfield.

Deep hydrocarbon exploration of shale gas and oil resources meets weak planes (bedding, micro-fractures) with strong anisotropic strength compared to the rock matrix and the time-dependent borehole collapse during the drilling operation. These two factors consist of the internal control factor of frequent wellbore instability and seriously restrict the improvement of drilling quality and efficiency. Based on the fundamental theory of elastic mechanics, this work derives the weak plane strength criterion under polyaxial compression (three-dimensional stress), and the Mogi-Coulomb strength criterion is also introduced to evaluate the failure of the rock matrix. Primarily, one derives a strength expression depending on the dip angle and direction to assess the failure characteristics of the weak plane and reformulates the Mogi-Coulomb criterion to obtain the strength expression of the rock matrix in terms of intermediate and minimum principal stress loadings. These two strength expressions are expected to guide the experiment design and study the strength variation of laminated rock developed weak plane when the stress loading magnitude and direction are applied under polyaxial compression. Besides, to avoid the inversion operation after the Laplace transformation to the isotropic poroelastic solutions, one provides analytical poroelastic solutions for arbitrarily inclined boreholes, including five-time domains related to instantaneous, modified instantaneous, short-time, long-time, and time-independent elastic ones. Thus, the variation of the equivalent density of collapse pressure is analyzed considering the new strength criterion for the weak plane and different time-domain poroelastic solutions drilled through the shale formation. The apparent influence of intermediate principal stress on the rock matrix and weak plane shows that the strength of the rock matrix increases first and then decreases with the increasing intermediate principal stress. The strength of the weak plane does not change with the increasing intermediate principal stress where the applied direction of intermediate principal stress parallels the plane of the weak plane structure. Given intermediate principal stress, the failure angle range of the weak plane gradually increases with the increasing dip direction angle of the weak plane. Given a weak plane occurrence and the intermediate principal stress, the apparent strength of the rock increases with the increase of the minimum principal stress. The equivalent density magnitude of collapse pressure is arranged in descending order when short-time, modified instantaneous, long-time, instantaneous, and elastic solutions are adopted. Severe wellbore instability occurs at a larger angle between the wellbore axis line and the normal line of the weak plane. Studying the influence of three-dimensional stress on the strength of the weak plane and rock matrix is conducive to improving the prediction accuracy of collapse pressure equivalent density. It can help solve the problem of wellbore collapse in troublesome shale formations.

Hydraulic fracture initiation and propagation have been popular topics for research. Hydraulic fracturing research now primarily uses numerical simulation since core size constraints prevent laboratory experimentation. Hydraulic fracturing numerical simulation technology is now developing quickly. ThermalHydro-Mechanical (THM) coupling models have been established for deep, hightemperature, high-pressure formations. Nevertheless, themajority of these models are sequential or multi-software collaborative. The former has a substantial calculation error but a high calculation speed and good convergence. The latter increases computation accuracy, but because of its slow calculation speed and challenging software coupling, it has not been easy to adopt as the conventional method. This work unifies the THM coupling fracture problem into a single model using the finite elementmethod and the cohesive zone method (CZM) method. This model is used to carry out numerical simulations of hydraulic fracturing in high-temperature and high-pressure formations. The impact of important variables on fracture initiation and propagation is further investigated. The simulation demonstrates that fracture internal pressure is a key factor in the development and propagation of hydraulic fractures. Injecting low-temperature fluid into the formation will result in an obvious thermal effect and cause the formation to contract. The fracture extension and formation fracture pressures are lowered by the heat effect. With its fast calculation speed and excellent precision, the model developed in this study offers a new approach to the numerical simulation of hydraulic fracturing and can accurately simulate the hydraulic fracturing problem of high temperature and high-pressure formation.

Abstract Well slots are the most essential resource in offshore oilfield development, and how to efficiently utilize limited well slots in offshore platform instead of building new offshore platform is the key to improve the economic benefits of oil fields. Firstly, well slot expansion technology of old offshore platform including hanging well slots inside and outside the offshore platform considering structure strength of old platform. Secondly, sidetracking new wells from old wells with low production is proposed to develop remaining oil near the old platform and sidetracking point is optimized towards deep to reduce footage. Thirdly, combined with reservoir, multilateral branch wells are drilled to improve oil drainage area and enhance oil recovery, moreover, branch bore open hole anchor and external packer are developed to hang sand control screen pipe and maintain wellbore stability instead of leaving branch hole without any sand control methods. Well slot expansion technology has been used in old platforms, and 35% wells slots have been added to drill new wells to develop remaining oil around old platform. In old well sidetracking, advanced sidetracking tool with high build up rate reaching 120 deg/100 ft is developed and applied in many wells including T shaped well, the sidetracking points have been moved down and the overall footage have been reduced compared with conventional build up tools, and single well construction period have been reduced by 2 days. Multilateral branch wells with 1268 meters long main bore and 2 branch bores have been successfully completed with open hole anchor and external packer to hanger sand control screen pipe, and the production rate is twice as that of the original branch bore without any sand control method. This technology has been applied in Bohai Oilfield and new platforms have been saved every year as re-utilization of well slots, which can also provide reference for other offshore oil fields.

Abstract Deep formation rich in the weak plane (bedding, micro-fractures) has strong strength anisotropy and is the internal control factor of frequent wellbore instability of the long horizontal section of shale gas wells, which seriously restricts the realization of improving drilling quality and efficiency. The conventional single plane or multi planes of weakness criterion only considers the pseudo-triaxial loading state (σ2 = σ3) or the loading direction of the intermediate principal stress σ2 coincides with the failure plane of weakness. Based on the theory of elastic mechanics and the Mogi-Coulomb shear failure criterion, this paper establishes the rock strength criterion considering the influence of the weak plane and three-dimensional stress loading. One analyzes the influence of three-dimensional stress on the strength of rock possessing parallel plane of weakness, the variation of equivalent density of wellbore collapse pressure in different time domains, and the influence of wellbore trajectory on wellbore stability considering different occurrences of the weak plane. The results show the obvious influence of intermediate principal stress on the failure of the rock matrix and the weak plane. The strength of the rock matrix first increases and then decreases with the increasing σ2. At a given small dip azimuth angle, the rock strength is controlled by the rock matrix and the weak plane for smaller σ2, while the rock strength is completely controlled by the rock matrix for larger σ2. The strength of the weak plane does not depend on σ2 where the dip azimuth angle coincides with the failure plane of weakness. At a given σ2, the range of failure angle of the weak plane gradually increases with the increasing dip azimuth angle. At a given minimum principal stress σ3, the apparent strength of the rock matrix increases with the increasing σ2. The larger angle between the borehole axis and the normal direction of the weak plane weakens the wellbore stability related to borehole collapse. Besides, the magnitude of the time-dependent equivalent density of collapse pressure descends in short-time, modified instantaneous, long-time, instantaneous, and elastic ones. Introduction of the influence of three-dimensional stress on the strength of rock possessing parallel plane of weakness and time-dependent stress and pore pressure solutions around a borehole is conducive to improving the prediction accuracy of equivalent density of collapse pressure and can help solve the problem of wellbore collapse in complex shale formations.

Effectively analyzing the wellbore stability risk in directional wells plays an important role in the exploration of oil and gas resources in complex deep formations. For the smooth execution of the drilling process, wellbore stability is related to the rock strength characteristics of the formation and the stress state of the rock around the borehole, which in turn is directly affected by the wellbore trajectory inclination and azimuth. The stress state depends on the magnitude of in-situ stresses. However, the uncertainty and non-independence of geomechanical parameters greatly impact the predicting wellbore instability pressure and wellbore stability evaluation. Therefore, this paper effectively combines the Monte Carlo method with the Nataf transformation to sample and simulate the geomechanical parameters and realize the quantitative risk assessment (QRA) of wellbore instability. The parameter sensitivity characteristics of borehole collapse and fracture pressures under different wellbore trajectories and stress states are studied on this basis. The main research shows that the risk assessment results of wellbore instability based on parameter uncertainty indicate that the predicted collapse equivalent density usually increases and the fracture pressure equivalent density decreases where the reliability is greater than 50%, which leads to a significantly narrower safe mud weight window. In addition, the influence of parameter uncertainty on fracture pressure is significantly greater than that on collapse pressure. The correlation coefficient is used to constrain the reservoir geomechanical parameters, which maintains the linear characteristics between the parameters, and then significantly reduces the uncertainty range of wellbore instability pressure. The uncertainty of in-situ stress parameters makes it possible for the formation rock to experience the type of stress state that changes from conventional strike-slip faults to normal faults. However, this possibility of stress state transition obviously affects the selection of wellbore trajectory optimization. The rock mechanics parameters including elastic modulus, Poisson’s ratio, cohesion, internal friction angle, and tensile strength have a weak effect on the collapse pressure and fracture pressure of arbitrarily inclined boreholes. However, the obvious sensitivity of in-situ stresses depends on the change of wellbore trajectory in deviated/horizontal wells. As for the vertical well, the maximum and minimum horizontal in-situ stresses always are the primary sensitivity factors of borehole collapse and fracture pressures, respectively. The methodology shown in this paper provides important guidance for engineering design by calculating the probability of wellbore trajectory optimization.

Abstract The Yingxiongling shale oil in the Qaidam Basin is the preferred choice for Qinghai Oilfield to enter the field of unconventional oil and gas resources, and volume fracturing is the core technology for shale oil exploration and development. Through the analysis of the geological and engineering characteristics of the Yingxiongling shale oil, it is found that there are difficulties in volume fracturing of the Yingxiongling shale oil, such as frequent changes in vertical mechanical properties and limited vertical control range; large stress differences, making it difficult to form complex artificial fractures; difficulty in opening multiple clusters of perforation holes; high water shortage in the plateau, high mineralization of the formation backflow fluid, and difficulty in reusing the backflow fluid. By iteratively modeling three-dimensional geomechanics, a high-resolution three-dimensional geomechanical model is established to guide the optimization of volume fracturing schemes. Through the integration of geological and engineering selection and clustering, three-stage active control technology for cracks, and the dual temporary plugging process of boreholes and cracks, the targeted and complex degree of artificial crack transformation can be improved. Through the study of high salinity fracturing fluid system, it has been found that high salinity backflow fluid can replace slick water and alleviate the demand for fresh water in volume fracturing. The article analyzes the difficulties of shale oil volume fracturing and studies the formation of an effective set of key techniques for volume fracturing, achieving the goal of improving the degree and effectiveness of transformation, and providing support for the exploration and development of the Yingxiongling shale oil in the Qaidam Basin.

Wellbore instability is one of the most critical challenges during drilling, which may result in complex problems such as stuck pipe, high torque and mud loss, impeding the drilling progress and increasing the cost of drilling operations. Offshore shallow formations are characterized by weak consolidation and low strength, causing serious wellbore collapse frequently. The traditional physics-based wellbore instability models attempt to predict the risk of failure of the wellbore for given drilling fluid densities and provide optimal fluid density for safe drilling. However, these models generally involve quite a few empirical coefficients, determination of which heavily rely on the field experiences of the engineers, and thus the results may vary greatly from person to person. In this study, an artificial neural network (ANN) model has been established for wellbore stability prediction, aiming to reduce the subjectivity while mapping the drilling performance of neighboring wells into the predicted results. Eleven factors that may affect wellbore stability have been identified from the prevailing physics-based model and used as the input of the ANN model, while the wellbore enlargement rate (WER) is used as the output to quantify the wellbore stability performance. The model has been trained using data from 5 wells drilled in the offshore shallow formations of the Bohai Bay Basin in east China and then used to predict the WER of another well in the same region. The results show that the mean absolute percentage error is 6.113%, and at most well depths the predicted wellbore diameter deviate from the measured values not more than 5%. Comparison of the ANN model and some other machine learning models, including random forest model, decision trees model, linear regression model, was conducted, which demonstrated the best performance of the ANN model in terms of the predicted wellbore diameter profile. Finally, the potential application of the ANN model in optimizing mud weight has been illustrated through the predicted wellbore diameter profiles for different mud weights. It is worth noting that the method presented in this paper is of physics and data-driven nature and provides a new research insight for wellbore stability analysis.

Abstract Extended reach wells are drilled to develop the remaining oil in shallow formation just around 600~1400m in China Bohai oilfield. The formation there is terrestrial sediment and heterogeneity is strong, moreover, only water-based drilling fluid can be used as Bohai Bay is an internal bay with weak self-cleaning ability. For these reasons, precise well trajectory control, wellbore cleaning, casing running and cementing are challenges in this region. Facing the problems above, a set of safe and efficient drilling technology for shallow extended reach wells has been developed. Firstly, well profile is optimized referring previous drilling experience, secondly, well trajectory is redesigned using software and directional drilling tools with high buildup rate are selected for precise well trajectory control; thirdly, drilling fluid properties are continuously improved to maintain wellbore cleaning and smooth; Lastly, innovative floating while rotating casing running method and cementing technology are employed to reduce the friction in casing running and avoid leakage during cementing job. The design build rate is about 4 degree/100ft, while the natural inclination reduction rate of formation is serious using classic directional drilling tools that fail to meet this requirement. Thus, uniform wall thickness Positive Drive Motor with 1.5 bend angle and Rotary Steering System of push-the-bit type are selected in surface and second spud drilling to ensure adequate build rate with assistance of software which can track and modify well trajectory while drilling. Water based environmental protection drilling fluid with low viscosity and high shear force are developed to remove cuttings bed and reduce ECD. Meanwhile, the inhibition and plugging of drilling fluid are enhanced to avoid hydration expansion of mudstone. The tripping and back reaming efficiency have been improved by 20%. The drag and torque of casing running in different ways are calculated, innovative floating while rotating casing running method is proposed comprehensively considering the rig capacity and the torsional strength of the casing. Low density cementing slurry system and slurry column structure are optimized to reduce ECD while cementing and cementing quality is excellent. From 2019 to 2021, these technologies have been successfully applied in 7 extended reach wells, with an average drilling efficiency of 97.1%, and the field applications show that the ROP has been improved by 40% while well construction time has been reduced by 28.4% compared with previous operation. Moreover, the horizontal displacement to vertical depth ratio of W1H well is as high as 3.23, breaking the record of Bohai oilfield.

In recent years, an oil company has intensified its drilling activities in China Bohai Oilfield to meet challenging production targets and has almost doubled its drilled wells. Aiming to minimize environmental impacts of drilling activities on China Bohai Bay which is an internal bay with weak self-cleaning ability, this oil company has always been exploring continuous improvement to minimize the impact of its drilling and production operations on the environment. With the intention of recycling and reutilizing drilling and production waste including cuttings, wasted drilling and production fluids, many efforts are done. In design, well profile is downsized, cement top is restricted and reusable drilling fluids are developed to reduce waste from the root; In the field, environment protection ships (EPS) equipped with solid modified control system, cuttings closed transfer system and cuttings treatment system are used to recycle the produced the waste while drilling platform work as backup when weather is bad; Moreover, offshore waste management online system (OWMOS) has been developed covering the whole life cycle data chain of waste generation, collection, transportation, storage, disposal (utilization). Statistics from OWMOS show that the amount of the waste produced from downsized wells has been reduced by 41~65% compared to similar wells drilled before. Moreover, environmentally friendly drilling fluids with low solid content are developed and liquid phase after solid-liquid separation treatment can be reused. Consequently, the usage amount of drilling fluids has been decreased by 55~75%, while the solid content are packed and transported to onshore for disposal. These wastes can be turned into raw materials of cement stabilized soil and ceramic, which can also provide economy benefit and reduce impact on the environment. Waste recycling and reutilization technology has been successfully applied in more than 40 wells and lay the foundation for the digital transformation acceleration of waste management of offshore drilling and production waste, which can provide more practical reference for waste management of other similar offshore oilfield.

With the thermo-hydro-mechanical coupling process considered, this paper derives a set of analytical porothermoelastic solutions to field variables including the stress, displacement, and pore pressure fields to evaluate the wellbore stability around a vertical borehole drilled through an isotropic porous rock. The thermal effect on the wellbore stability of the low-permeability saturated rock also introduces the thermal osmosis term. The wellbore problem is decomposed into axisymmetric and deviatoric loading cases considering the borehole subjected to a nonhydrostatic stress field. It obtains the time-dependent distributions of field variables by performing the inversion technique for Laplace transforms to the porothermoelastic solutions in the Laplace domain. The results suggest that the thermal osmosis effect should not be neglected on the premise that a lower permeability porous rock is characterized by the substantially large thermal osmotic coefficient and the small thermal diffusivity values. The case that the thermal osmosis effect reduces the undrained loading effect leads to the decrease of the mean shear stress that is determined by the effective maximum and minimum stress around a borehole, since, and accordingly contributes to the wellbore stability to resist the shear failure.

Well performance prediction and uncertainty quantification of fractured shale reservoir are crucial aspects of efficient development and economic management of unconventional oil and gas resources. The uncertainty related to the characterization of fracture topology is highly difficult to be quantified by the conventional model-based history matching procedure in practical applications. Data-space inversion (DSI) is a recently developed inversion-free and rapid forecast approach that directly samples the posterior distribution of quantities of interest using only prior model simulation results and historical data. This paper presents some comparative studies between a recent DSI implementation based on iterative ensemble smoother (DSI-IES), model-based history matching, and conventional decline curve analysis (DCA) for shale gas rate forecast. The DSI-IES method treats the shale gas production rate as target variables, which are directly predicted via conditioning to historical data. Dimensionality reduction is also used to regularize the time-series production data by low-order representation. This approach is tested on two examples with increasing complexity, e.g., a fractured vertical well and a multistage fractured horizontal well in the actual fractured Barnett shale reservoir. The results indicate that compared with the traditional history matching and DCA methods, the DSI-IES obtains high robustness with a high computational efficiency. The application of data-space inversion-free method can effectively tap the potential value directly from historical data, which provides theoretical guidance and technical support for rapid decision-making and risk assessment.

Making use of data generated from drilling process are challenging in oil industry, an intelligent decision system for drilling based on big data has been established to improve efficiency and reduce drilling cost of more than 400 wells are being drilled in Bohai oilfield every year. The data acquisition standard of offshore drilling and completion is formed, and the data are transformed and stored in database system. Based on the analysis of regional geological, seismic, logging and drilling data, various risks of the design well are evaluated to achieve the best drilling and completion design scheme. Meanwhile, real time intelligent prediction models with machine learning methods, such as Artificial neural network, Convolutional neural network, K nearest neighbor, etc, are established to predict and avoid pipe stuck, lost circulation, gas kick events. Moreover, a continuous improvement system is established to improve drilling efficiency through drilling parameters optimization. An intelligent decision system for drilling with the core of "pre drilling rehearsal-risk assessment-parameter optimization" has been formed. This technology has been successfully applied in 50 wells, the average well construction time has been reduced by 4.5 days compared with previously drilled wells. Bohai oilfield is accelerating digital transformation pace of drilling and completion, and the filed application show that digitization and big data technology can greatly help increase efficiency and reduce cost, which can also be a reference to other oilfield.

With the in-depth development of Bohai Oilfield, China National Offshore Oil Corporation (CNOOC), the water cut of some old wells become too high to produce while nearby remaining recoverable reserves are still considerable. In order to maximize recovery and reduce well construction cost, herringbone multilateral horizontal well drilling and completion technology is employed to increase drainage area of single well and make full use of well slot and old wellbore. Considering the current development of oilfield and the geological characteristics of reservoir, the technical difficulties of herringbone multilateral horizontal well drilling and completion technology including high build-up rate, easy blockage of drilling tools in the sidetracking in open hole, easy collapse and instability of sandwich wall and high requirements for drilling fluid performance are analyzed and solved. This technology has been successfully applied in three wells with total 6 branches and the production of three wells is twice higher than that of conventional horizontal wells with no water cut, which fully verified the reliability of the branch well tools and the feasibility of the technology. Herringbone multilateral horizontal well drilling and completion technology provides a new idea for the treatment of low production and low efficiency wells in a sustainable way and will be widely promoted and applied in Bohai oilfield, which can also provide reference for other high water cut oilfields.

ABSTRACT Wellbore instability is one of the most critical challenges during drilling, often manifested as wellbore collapse, shrinkage, falling rocks, and formation fracturing, which may result in complex problems such as pipe sticking, high torque, mud loss, thus impeding the drilling progress and increasing the cost of the drilling operation. Conventional wellbore stability prediction relies on some deterministic physical models, involving some empirical coefficients which are difficult to determine and often dependent on field experience. In addition, some complex factors, such as natural fractures, cannot be explicitly and quantitatively characterized in existing wellbore stability prediction models. Artificial intelligence technique has shown unique advantages in nonlinear issues. The artificial intelligence technique is used to predict wellbore stability in this study, including artificial neural networks (ANNs) and support vector machine (SVM). The logging data and drilling data were collected from the field. According to the correlation analysis between influencing factors and wellbore enlargement rate, 16 parameters were extracted, such as mud density, formation density porosity, acoustic interval transit time, weight on bit as the input data of the models, and wellbore enlargement rate as output. Both SVM and ANNs models have exceptional performance in predicting wellbore stability. When the kernel of the SVM model is Linear, predictions perform optimally. In the ANNs model prediction results, the result performs optimally when the total number of neurons is 1024 in the hidden layer. Overall, ANNs model performs better than SVM model with a coefficient of determination (R2) of 0.991, therefore it is recommended to apply ANNs to predict wellbore stability. The present analysis supplies knowledge that can be used to predict wellbore stability problems before drilling, optimize drilling parameters, and reduce drilling accidents and costs. INTRODUCTION Wellbore instability refer to a series of responses resulting from mechanical, chemical and other effects of the rock around the wellbore. Wellbore instability is one of the most critical challenges during drilling, often manifested as wellbore collapse, shrinkage, falling rocks, and formation fracturing, which may result in complex problems such as pipe sticking, high torque, mud loss, thus impeding the drilling progress and increasing the cost of the drilling operation.

Coiled tubing (CT) is used as a velocity string to transport high-velocity gas in drainage gas recovery technology. Sand particles flowing at high speed can cause serious erosion of the pipe wall. Long-term erosion wear leads to the degradation of the string strength and can even cause local perforation. In order to study the erosion wear problem of CT, a gas–solid erosion experimental device was established for a full-size pipe with different radii of curvature. A 3D laser confocal technique was used to examine and characterize the microscopic erosion morphology of the inner wall of the CT. The CFD erosion model was selected based on the erosion test data of the inner wall of the CT, and the erosion results of the Finnie model show minimal error and good agreement compared with other models. The average value of the error of the maximum erosion rate at different radii of curvature is 8.3%. The effect of the radius of curvature, gas velocity and solid particle size on the maximum erosion rate of the inner wall of the CT was analyzed based on the Finnie model. The results reveal that erosion wear occurs on the inner wall of the CT’s outer bend. As the radius of curvature is reduced, the maximum erosion rate and area increase, and the position of the maximum erosion rate gradually shifts toward the inlet. The maximum erosion rate is positively correlated with the gas flow rate. However, as the particle size increases, the maximum erosion rate shows a trend of first increasing, then decreasing and finally stabilizing, with a critical particle size of 200 μm. This study can provide theoretical guidance and methods for improving the service life of CT. The erosion rate of the tubing in old wells can be reduced by controlling production and employing appropriate sand control methods, while the erosion rate of tubing in new wells can be reduced by adjusting the wellbore trajectory.

The high production period of oil Wells in tight reservoirs represented by Yingxi area in Qaidam Basin is less than 60 days after fracturing, and the initial decline rate is more than 70

ABSTRACT Steam huff and puff is the main production mode of heavy oil, but the casing damage problems during the cyclic thermal injection process are very serious. The casing damage of thermal recovery wells is an important factor affecting oilfield production and economic benefits. The conventional casing strength calculation method does not consider the effect of corrosion and thermal degradation in terms of long-term casing strength designing. The establishment of a long-term casing strength prediction method to check and optimize casing strength is an important guarantee for the safe production of heavy oil thermal recovery wells. In this paper, the strength degradation characteristic of two kinds of casing steels (N80 and TP100H) are tested under the cyclic thermal effect using a High Temperature Tensile Testing Machine. The heat temperature is from 30˚C to 350˚C, and heating cycles are 1, 3, 5 respectively. The results show that the N80 casing strength decreases by 18.2% in the first thermal cycle, and the average damage of each subsequent thermal cycles are about 0.88%. However, the TP100H decreases by 13.7% in the first cycle and subsequently with 0.31% degradation averagely. The long-term corrosion rates of N80 and TP100H were measured in HTHP autoclave, which are 0.43m/a and 0.3m/a respectively. Combining the obtained experimental results with the standard casing strength calculation formulas, a newly prediction method of casing long-term strength for cyclic thermal injection well is presented. This method has been successfully used in Lvda 21-2 block of Bohai oil field CNOOC, which provides a technical basis for casing strength design in cyclic thermal injection wells. INTRODUCTION Casing damage in oil fields has now become a pressing problem worldwide, and many fields have encountered casing damage during development. The factors affecting casing damage are extremely complex. Steam injection is the most widely used and effective method in the exploitation of heavy oil reservoir. However, in the process of steam huff and puff, the high temperature steam injected in circulation makes the casing undergo great alternating stress, and the residual stress increases constantly, reducing the casing strength. In addition, after steam injection, thermal cracking of heavy oil produces acidic gas and the casing is in a corrosive environment. This leads to the casing strength decay and reduces the casing service life. Therefore, casing damage is a difficult problem in the development of heavy oil reservoir.