Hongjun Zhu

Calculation analysis of sustained casing pressure in gas wells

Sustained casing pressure (SCP) in gas wells brings a serious threat to worker safety and environmental protection. According to geological conditions, wellbore structure and cement data of gas wells in the Sichuan-Chongqing region, China, the position, time, environmental condition and the value of SCP have been analyzed. On this basis, the shape of the pressure bleed-down plot and pressure buildup plot were diagnosed and the mechanism of SCP has been clarified. Based on generalized annular Darcy percolation theory and gas-liquid two-phase fluid dynamics theory, a coupled mathematical model of gas migration in a cemented annulus with a mud column above the cement has been developed. The volume of gas migrated in the annulus and the value of SCP changing with time in a gas well in Sichuan have been calculated by this model. Calculation results coincided well with the actual field data, which provide some reference for the following security evaluation and solution measures of SCP.

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.

A New Model for Predicting Dynamic Surge Pressure in Gas and Drilling Mud Two-Phase Flow during Tripping Operations

Investigation of surge pressure is of great significance to the circulation loss problem caused by unsteady operations in management pressure drilling (MPD) operations. With full consideration of the important factors such as wave velocity, gas influx rate, pressure, temperature, and well depth, a new surge pressure model has been proposed based on the mass conservation equations and the momentum conservation equations during MPD operations. The finite-difference method, the Newton-Raphson iterative method, and the fourth-order explicit Runge-Kutta method (R-K4) are adopted to solve the model. Calculation results indicate that the surge pressure has different values with respect to different drill pipe tripping speeds and well parameters. In general, the surge pressure tends to increase with the increases of drill pipe operating speed and with the decrease of gas influx rate and wellbore diameter. When the gas influx occurs, the surge pressure is weakened obviously. The surge pressure can cause a significant lag time if the gas influx occurs at bottomhole, and it is mainly affected by pressure wave velocity. The maximum surge pressure may occur before drill pipe reaches bottomhole, and the surge pressure is mainly affected by drill pipe operating speed and gas influx rate.

Experimental Validation on a New Valve Core of the Throttle Valve in Managed Pressure Drilling

This paper is concerned with experimental investigation of the linear opening-pressure drop (L−ΔP) relationship of new valve core in managed pressure drilling (MPD). Based on the linear L−ΔP relationship in theoretical results, the actual L−ΔP relationships are obtained by testing the flow rate, displacement of the valve core, and pressures at the import and export. Under all three working conditions, the theoretical L−ΔP relationship almost agrees with the experimental result, and the accuracy of the theoretical design is validated. In addition, the flow coefficient (C v ) is also tested in the experimental process to judge the throttling characteristics of the new valve core and the proper working conditions.

A Repeated Impact Method and Instrument to Evaluate the Impact Fatigue Property of Drillpipe

It is well known that drillpipe failures are a pendent problem in drilling engineering. Most of drillpipe failures are low amplitude-repeated impact fatigue failures. The traditional method is using Charpy impact test to describe the fracture property of drillpipe, but it cannot veritably characterize the impact fatigue property of drillpipe under low amplitude-repeated impact. Based on the Charpy impact and other methods, a repeated impact method and instrument have been proposed to simulate the low amplitude-repeated impact of downhole conditions for drillpipe. Then, a series of tests have been performed using this instrument. Test results demonstrate the drillpipe upset transition area nonhomogeneity is more severe than drillpipe body, which is the key factor that leads to washout and fracture frequently of it. As the one time impact energy increases, the repeated impact times decrease exponentially, therefore, the rotational speed has a great effect on the fatigue life of drillpipe, and it is vital to select a suitable rotational speed for drilling jobs. In addition, based on SEM fractographs we found that the fracture surface of repeated impact is similar to the fatigue fracture, and there are many low cycle fatigue characteristic features on fracture surface that reveal very good agreement with the features of drillpipe fatigue failures in the field.

Numerical Investigation of Temperature Distribution in an Eroded Bend Pipe and Prediction of Erosion Reduced Thickness

Accurate prediction of erosion thickness is essential for pipe engineering. The objective of the present paper is to study the temperature distribution in an eroded bend pipe and find a new method to predict the erosion reduced thickness. Computational fluid dynamic (CFD) simulations with FLUENT software are carried out to investigate the temperature field. And effects of oil inlet rate, oil inlet temperature, and erosion reduced thickness are examined. The presence of erosion pit brings about the obvious fluctuation of temperature drop along the extrados of bend. And the minimum temperature drop presents at the most severe erosion point. Small inlet temperature or large inlet velocity can lead to small temperature drop, while shallow erosion pit causes great temperature drop. The dimensionless minimum temperature drop is analyzed and the fitting formula is obtained. Using the formula we can calculate the erosion reduced thickness, which is only needed to monitor the outer surface temperature of bend pipe. This new method can provide useful guidance for pipeline monitoring and replacement.

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.