Dezhi Zeng

Burst Strength of Tubing and Casing Based on Twin Shear Unified Strength Theory

The internal pressure strength of tubing and casing often cannot satisfy the design requirements in high pressure, high temperature and high H2S gas wells. Also, the practical safety coefficient of some wells is lower than the design standard according to the current API 5C3 standard, which brings some perplexity to the design. The ISO 10400: 2007 provides the model which can calculate the burst strength of tubing and casing better than API 5C3 standard, but the calculation accuracy is not desirable because about 50 percent predictive values are remarkably higher than real burst values. So, for the sake of improving strength design of tubing and casing, this paper deduces the plastic limit pressure of tubing and casing under internal pressure by applying the twin shear unified strength theory. According to the research of the influence rule of yield-to-tensile strength ratio and mechanical properties on the burst strength of tubing and casing, the more precise calculation model of tubing-casings burst strength has been established with material hardening and intermediate principal stress. Numerical and experimental comparisons show that the new burst strength model is much closer to the real burst values than that of other models. The research results provide an important reference to optimize the tubing and casing design of deep and ultra-deep wells.

Numerical Analysis of Flow Erosion on Sand Discharge Pipe in Nitrogen Drilling

In nitrogen drilling, entrained sand particles in the gas flow may cause erosive wear on metal surfaces and have a significant effect on the operational life of discharge pipelines, especially for elbows. In this paper, computational fluid dynamics (CFD) simulations based code FLUENT is carried out to investigate the flow erosion on a sand discharge pipe in conjunction with an erosion model. The motion of the continuum phase is captured based on solving the three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations, while the kinematics and trajectory of the sand particles are evaluated by the discrete phase model (DPM). The flow field has been examined in terms of pressure, velocity, and erosion rate profiles along the flow path in the bend of the simulated discharge pipe. Effects of flow parameters such as inlet velocity, sandy volume fraction, and particle diameter and structure parameters such as pipe diameter and bend curvature are analyzed based on a series of numerical simulations. The results show that small pipe diameter or small bend curvature leads to serious erosion, while slow flow, little sandy volume fraction, and small particle diameter can weaken erosion. The results obtained from the present work provide useful guidance to practical operation and discharge pipe design.

Equations to Calculate Collapse Strength for High Collapse Casing

As wells are drilled deeper, the external pressures applied to well casing become greater. Conventional America Petroleum Institute (API) casing strength cannot meet the strength criteria of high pressure, high temperature, and high H2S (HPHTHS) gas wells which are called “3-high” gas wells. When high collapse casing (HCC) is applied in oil fields, it has obviously improved collapse properties in excess of API ratings. HCC shows a very high resistance to tension load, internal pressure, and collapse, as well as being highly resistant to sulfide stress corrosion cracking (SSCC), and it also can be used for deep and sour gas and oil fields. For imperfections of the API 5C3 collapse formula, the joint API/ISO work group ISOTC67 SC5 WG2b have proposed the current API Bulletin 5C3, and a new collapse strength model with manufacturing imperfections, such as ovality, eccentricity, residual stress, etc., improves the casing strength calculation accuracy and increases the benefits for casing strength design, rather than just using API Bulletin 5C3. The study on the new ISO collapse model has found that it is inappropriate to predict the collapse strength of the high collapse casing. As a result, on the basis of my work group results, a new high collapse model for predicting the collapse strength of all HCC has been presented. Numerical and experimental comparisons show that the “new high collapse model” predicts higher accuracy than that of ISO, and this will make great improvements in the casing design of deep and ultradeep wells on the basis of HCC material safety, which was guaranteed.

SIMULATIONS OF FLOW INDUCED CORROSION IN API DRILLPIPE CONNECTOR

Drillpipe failure is an outstanding issue in drilling engineering, often involving great financial losses. In view of the special features of the flow channel in the high failure zone, this article analyzes the drillpipe failure mechanism from the point of view of flow induced corrosion. Based on the Eulerian-Langrangian method and the discrete phase model, a numerical simulation method is used to investigate the flows of the drilling fluid in the drillpipe connector during the operation of three typical drilling methods (mud drilling, air drilling and foam drilling). From the flow field in the drillpipe connector, especially, the velocity and pressure distributions in the threaded nipple and the thickened intermediate belt, one may detect the existence of the flow induced corrosion. Then, some structural optimization measures for the drillpipe connector are proposed, and the optimization effects are compared.

Fatigue Strength Prediction of Drilling Materials Based on the Maximum Non-metallic Inclusion Size

Abstract In this paper, the statistics of the size distribution of non-metallic inclusions in five drilling materials were performed. Based on the maximum non-metallic inclusion size, the fatigue strength of the drilling material was predicted. The sizes of non-metallic inclusions in drilling materials were observed to follow the inclusion size distribution rule. Then the maximum inclusion size in the fatigue specimens was deduced. According to the prediction equation of the maximum inclusion size and fatigue strength proposed by Murakami, fatigue strength of drilling materials was obtained. Moreover, fatigue strength was also measured through rotating bending tests. The predicted fatigue strength was significantly lower than the measured one. Therefore, according to the comparison results, the coefficients in the prediction equation were revised. The revised equation allowed the satisfactory prediction results of fatigue strength of drilling materials at the fatigue life of 107 rotations and could be used in the fast prediction of fatigue strength of drilling materials.

Corrosion Behaviors of Q345R Steel at the Initial Stage in an Oxygen-Containing Aqueous Environment: Experiment and Modeling

The ingress of oxygen into pressure vessels used in oil & gas production and transportation could easily result in serious corrosion. In this work, the corrosion behaviors of Q345R steel at the initial stage in 1 wt.% NaCl solution were investigated using electrochemical techniques. The effects of oxygen concentration, temperature and pH on corrosion behaviors were discussed. Simultaneously, a numerical model based on the mixed potential theory was proposed. The results show that the proposed model accords well with the experimental data in the pH range from 9.0 to 5.0. In this pH range, the oxygen reduction reaction, H+ reduction, water reduction, and iron oxidation can be quantitatively analyzed using this model. However, this model shows a disagreement with the experimental data at lower pH. This can be attributed to the fact that actual area of reaction on the electrode is much smaller than the preset area due to the block effect resulted from hydrogen bubbles adsorbed on the electrode surface.

Effect of Plastic Deformation and H2S on Dynamic Fracture Toughness of High Strength Casing Steel

The effects of plastic deformation and H2S on fracture toughness of high strength casing steel (C110 steel) were investigated. The studied casing specimens are as follows: original casing, plastic deformation (PD) casing and PD casing after being immersed in NACE A solution saturated with H2S (PD+H2S). Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness (JId) was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows: the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly.

Theoretical and experimental study of the thermal strength of anticorrosive lined steel pipes

Bimetallic lined steel pipe (LSP) is a new anti-corrosion technology. It is widely used to transport oil, gas, water and corrosive liquid chemicals. At present, the hydroforming pressure for LSP has been investigated theoretically and experimentally by most researchers. However, there are a few reports on the thermal strength of bimetallic LSP. Actually, the bimetallic LSP will be subjected to remarkable thermal load in the process of three layer polyethylene (3PE) external coating. Reverse yielding failure may occur on the inner pipe of the bimetallic LSP when it suffers from remarkable thermal load and residual contact pressure simultaneously. The aim of this paper is to study the thermal load and strength of the bimetallic LSP. A mechanical model, which can estimate the thermal strength of the bimetallic LSP, was established based on the elastic theory and the manufacture of the bimetallic LSP. Based on the model, the correlation between the thermal strength of the bimetallic LSP and residual contact pressure and wall thickness of the inner pipe was obtained. Reverse yielding experiments were performed on the LSP (NT80SS-316L) under different thermal loads. Experiment results are consistent with calculated results from the theoretical model. The experimental and simulation results may provide powerful guidance for the bimetallic LSP production and use.

Numerical and Experimental Study on Working Mechanics of Pear-Shaped Casing Swage

Pear-shaped casing swage (PCS) repair technology is highly efficient in repairing deformed casing and the value of repairing force is a very important parameter for designing and optimizing the casing swage and structure parameters. A new three-dimensional simulation analysis of casing swage in the well and the finite element analysis (FEA) model of 7′′ API deformed casing and PCS are established based on the elastic-plastic mechanics, the finite element theory, and application of numerical simulation analysis for the actual process of repairing deformed casing. According to the model, the repairing force required to repair the deformed casing is obtained; furthermore, the correlation between the repairing force and confining pressure is obtained. Meanwhile, the repairing test of deformed casing was performed by using PCS in the lab. Experimental results are consistent with simulation results. It indicated that the mechanical model can provide theoretical guidance for design and optimization of the structure of tool and reshaping technological parameters.