During large-scale multistage hydraulic fracturing of shale gas horizontal wells, disturbances to the non-uniform in-situ stress and the wellbore thermal stress can lead to integrity issues such as casing damage and cement sheath failure. Based on the principles of elastic mechanics and thermal stress theory, considering the coupling effects of non-uniform in-situ stress and thermal stress to establish a coupling stress model for the casing-cement sheath-formation system, and the influence of wellbore temperature, non-uniform in-situ stress and casing eccentricity on the coupling stress in shale gas horizontal wells is analyzed. When considering the coupling of non-uniform in-situ stress and thermal stress, the peak values of the system’s radial, circumferential, axial, and Von Mises stresses increase by 12.87

The drilling fluid invasion is influenced by both wellbore environments and formation properties. To reveal the characteristics of drilling fluid invasion in the coupled wellbore-formation system, a novel mathematical model for drilling fluid invasion was established by integrating the annular drilling fluid flow, the dynamic formation of mud cake, and the multiphase seepage within the formation. A hybrid finite-difference/finite-element numerical scheme was implemented to solve the model. The characteristics of drilling fluid invasion in high-, medium-, and low-permeability formations were analyzed. The differences in drilling fluid invasion depth among these different permeability formations were also discussed. The results indicated that during the initial stage of drilling fluid invasion, only a transition zone is present. As the duration of invasion increases, a flushed zone begins to form near the wellbore, and it is more likely to form in high-permeability formations. The formation of mud cake inhibits the continuous invasion of drilling fluid, but it is hard to form in low-permeability formations. Irrespective of mud cake formation, the propagation rate of the transition zone front is significantly higher than that of the flushed zone front, and the former is approximately 14.42 times faster than the latter. The model established in this study enables the forward simulation of drilling fluid invasion in a coupled wellbore-formation system, and it is of significance for reservoir evaluation.

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 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.

In this paper, based on the elastic-plastic finite element theory, the finite element model of expansion process for Solid Expandable Tubular is established, the axial length shrinkage, expansion force, residual stress and collapse strength after expansion are studied. According to the finite element simulation results and practical experience, the optimum velocity of expansion cone is from 6 m/min to 9 m/min, which provides a theoretical basis and reference for the design of Solid Expandable Tubular.

A more comprehensive,systematic experimentical ground tests of the expansion process of expandable casing were performed. In the tests, 6 5 1/2 '' N80 casings were selected, of which five had been carried out expansion tests, another one casing was used to test the collapse strength, not been expanded. In the course of the tests,some important parameters of the expandable casing were measured before and after expansion,including the inner and outer diameter, wall thickness, ovality, wall thickness of inequality and the axial deformation, etc..These data provide a reliable basis for the mechanism research of the N80 casing expansion. After expansion tests, these six casing pipes had been performed the collapsing tests in Xi'an Tubular Goods Research Institute of CNPC.The test results show that after expansiong the collaps strength of expandable casing get decreased compared with unexpanded casing which provides a practical basis and reference for the design of expandable casing.

With the development of the world economic integration, technical standardizatin plays a very important role in the interantional competition, and the standardization of oil industry has important strategic significance for the oil companies and the technology itself.Solid Expandable Tubular technology which is known as a technological revolutin in 21st century is also necessary to be standardized. Solid Expandable Tubular technology include expansion casing and expansion cone,Enventure company which is the leader of Solid Expandable Tubular technology has developed some specfications of expansion casing,but the standards related expansion cone have not been seen.In order to achieve the standardization of the expansion cone,based on the study in domestic and foreign scholars,the effect of structure parameters on the expansion casing after expansion is analyzed,the structure parameters of expansion cone are optimized by using of computer simulation,and the specfication of expansion cone matching with the expansion casing are initially developed. The results provide theoretical grounds for the development of the specfication of expansion cone.

Internal corrosion of oil tubular goods is becoming increasingly serious in the gas & oil exploitation. Lined steel pipe is a new anti-corrosion material in preventing gas & oil material internal corrosion. The mechanical property after forming is one of the key factors affecting the service characteristics of lined steel pipe. A parametric finite element analysis (FEA) numerical model is established in this paper to simulate the plastic forming process of lined steel pipe. Furthermore, FEA numerical simulation has been made for the residual stress-strain of carbon steel tubing Φ70×3.5 mm and stainless steel tubing Φ63×1.16 mm. Studies show that the inner pipe produces residual compressive stress and the outer pipe produces residual tensile stress after forming. The bonding force between inner and outer pipes increases linear to the forming force. The deformation law of outer pipe in the loading & unloading process is analyzed by resistance-strain test method. Experimental results point out that the plastic deformation of lined steel pipe is nonlinear in the loading process, but rebounds linearly in the unloading process. FEA numerical simulating results are close to the experimental data, of which the accuracy can meet the engineering requirements. Studies in the paper can provide theoretical reference for the manufacturing and application of lined steel pipe.