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PurposeTo maintain the well integrity, the strength and sealing ability of premium connection should be in the safe scope. ISO 13679 is widely used for evaluating the ability of tubing and casing connection all over the world. FE is adopted to simulate the ISO 13679 tests.Design/methodology/approachBecause of the disadvantage of experiment such as long period, high cost and high requirement on the facility, considering the convenience and universality of finite element method, as well as the contacting nonlinearity and material nonlinearity, three-dimensional finite element model of a certain type of premium connection is established with the consideration of helix angle. The loads exerted on the premium connection are the loads in series B test and thermal cycle test of ISO 13679. The distributions of Von Mises stress and contact pressure in various cases were studied.FindingsThe results showed that the bending load has a great influence on the distribution of Von Mises stress and contact pressure for premium connection. The Von Mises stress and contact pressures on the sealing surface are smaller on the tension side and greater on the compression side. With increasing axial compression load, the contact pressures on the tension side are too small, which may lead to sealing failure. The influence of temperature on the performance of premium connection cannot be ignored when choosing or designing premium connections. Both the Von Mises stress and contact pressure decrease slightly during a period of thermal cycle. Although the performance of the premium connection is good in a period of thermal cycle, its performance in a long period should be evaluated. Finite element simulation can effectively simulate the ISO 13679 test procedure and obtain the stress and contact pressure distribution. It can be used as a reference for evaluating the performance of premium connections.Originality/valueConsidering the convenience and universality of finite element method, as well as the contacting nonlinearity and material nonlinearity, three-dimensional finite element model of a certain type of premium connection is established with the consideration of helix angle.

In horizontal wells or directional wells, a casing bends to be in accordance with the wellbore curvature of the wells. The collapse strength of a curved casing correspondingly decreases, which may cause well failure if neglected. Herein, a curved casing is taken as a bending beam under uniform load according to material mechanics. The differential equation of the deflection is established. The relationships among axial stress, hoop stress, radial stress of casing, and wellbore dogleg is obtained based on Lame theory. The equation of collapse strength of the curved casing is deduced using distortion-energy theory. To verify the theoretical result, a finite-element model of the casing is established with ANSYS software. The collapse strengths of casing in four cases are analyzed using theoretical and finite-element method. Results show that the theoretical results well match the numerical ones. The collapse strength of the casing decreases nonlinearly with increased curvature. For the casing with outer diameter (OD) of 139.7 mm and thickness of 10.54 mm, collapse strength decreases from 104.74 MPa to 99.30 MPa with increased wellbore curvature from 2°/30 m to 12°/30 m. The relative reduction in collapse strength increases nonlinearly with increased wellbore curvature. OD has a greater influence on the relative reduction than thickness. The maximum value of relative reduction is 6.099% when OD is 139.7 mm but only 4.039% when OD is 114.3 mm. These results provide a reference for casing selection in production or other cases.

Due to the perforated holes on casings after perforating, the strength of perforated casings will be decreased and the well integrity will be influenced during sequential well services. To get the remaining strength of perforated casings, experimental and theoretical study were completed. The samples of perforated casings were obtained from conventional oil field casings perforated in simulation well according to oilfield perforating practice, and the stress concentration coefficients around perforated holes were measured and confirmed by experiments and finite element analysis. Taking the perforated holes as semi-elliptical surface cracks on casings, the formula for calculating stress intensity factors and revised coefficients of perforated holes were derived. Then, both strength safety and stress intensity factors were applied to evaluate the safety of perforated casings. So, the safety analysis method for perforated casings is improved and more considerate. Considering the similarity of compressed rod and pipe, the formula for calculating the remaining collapse strength of thin walled casings were developed, taking account the stress concentration of perforated holes. The experimental and theoretical study shows that: the stress concentration coefficients around perforated holes were about 1.3 and the remaining strength will be decreased more when both stress concentration and stress intensity factors are took into consideration. Taking the mostly used perforating parameter as an example, the collapse strength of a perforated casing with 16 perforations/m and 90°helix may be decreased 12%. Based on the results of the experimental and theoretical study of this paper, the working parameters were decided and guidelines were developed to ensure the safety of perforated casing and well integrity.

To maintain the well integrity, the strength and sealing ability of premium connection should be in the safe scope. ISO 13679 is widely used for evaluating the ability of tubing and casing connection all over the world. Because of the disadvantage of experiment such as long period, high cost and high inquire on the facility, considering the convenience and universality of finite element method, as well as the contacting nonlinearity and material nonlinearity, three dimensional finite element model of a certain type premium connection is established with the consideration of helix angle. The loads exerted on the premium connection is the loads in series B test and thermal cycle test of ISO 13679. The distributions of Von Mises stress and contact pressure in various cases were studied. The results showed that the bending load has a great influence on the distribution of Von Mises stress and contact pressure for premium connection. The Von Mises stress and contact pressures on the sealing surface are smaller on the tension side and greater on the compression side. With increasing axial compression load, the contact pressures on the tension side are too small, which may arise sealing failure. The influence of temperature on the performance of premium connection can not be ignored when choosing or designing premium connections. Both the Von Mises stress and contact pressure decrease slightly during a period of thermal cycle. Although the performance of the premium connection is good in a period of thermal cycle, its performance in a long period should be evaluated. Finite element simulation can effectively simulate the ISO 13679 test procedure and obtain the stress and contact pressure distribution. It can be used as a reference for evaluating the performance of premium connections.

To obtain the dynamic response of perforating string under gun-shock loads, a dynamical model was established based on the vibration mechanics theory of cantilever beam, and the differential equations of longitudinal vibration were deduced. The dynamic response was obtained according to the measured data of impact loads. The results show that vibration response of displacement, velocity, and acceleration were changed with a cycle of 15ms. The Von Mises stress near the packer is the largest owing to stress concentration. The analytic solution of natural frequency is close to the finite element analysis value, and the maximum relative error is below 5%.The velocity of stress waves propagating along the string approximates 5000m/s, which is close to the theoretical analysis value and is about 3.6 times that of sound velocity in water.

Considering the influence of key parameters such as bore angle, azimuth and sounding of wellbore, based on the spatial five-time PH curve with continuous unit tangent and directional curvature, no need for iteration, using Bessel polynomial interpolation method, optimization the spatial model and the curve description can accurately deduct the spatial parameters of the fifth point according to the determined four points of the space, and then establish the wellbore trajectory analytical equation to generate the well trajectory curve. Taking a directional well as an example, the wellbore trajectories of the wells from 403.9 meters to 976 meters were fitted by PH, cubic spline and linear interpolation method respectively, and compared with the real measuring points. The results show that the wells fitted by PH method, the differences of the bevel angle and the measured point, the azimuth angle and the measured point are 0.910° and 5.7°. The differences of the cubic spline and the measured point, the straight line method and the measured point are 4.240°, 5.86° and 6.54°, 9.09°. For the PH method, the trajectory inclination and azimuth differences are the smallest, which are 3.51% and 3.17%.