Zhanghua Lian

Experimental Study and Prediction Model of Casing Wear in Oil and Gas Wells

With the development of drilling technology and reinforced exploration and exploitation of unconventional reservoirs, there has been a great increase of complex wells. Meanwhile, however, consequent casing wear is and will continue to be a serious problem that causes enormous economic losses and many safety issues. The purpose of this paper is to find out the mechanism of casing wear and establish casing wear prediction model. Casing wear experiment was carried out to study the effect of contact force, rotation speed, and casing grade on wear depth. Meanwhile, wear coefficients under different working conditions were obtained through the normalizing of data. With the extensive research of downhole drag and torque calculation method, a contact force calculation model was established. Through the combination of crescent-shaped model and wear-efficiency model, the past complicated casing wear prediction models and confusing empirical formulae were greatly simplified. Therefore, the wear volume and depth of the casing string can be accurately predicted. Finally, a prediction software was developed to predict downhole casing wear of oil and gas wells. Comparison with the field data confirmed that the established model and software had enough accuracy to help predict and analyze casing wear at field.

Experimental and numerical study on casing wear in highly deviated drilling for oil and gas

Aimed at studying the casing wear in the highly deviated well drilling, the experimental study on the casing wear was carried out in the first place. According to the test data and the linear wear model based on the energy dissipation proposed by White and Dawson, the tool joint–casing wear coefficient was obtained. The finite element model for casing wear mechanism research was established using ABAQUS. The nodal movement of the contact surface was employed to simulate the evolution of the wear depth, exploiting the Umeshmotion user subroutine. In addition, the time-dependent geometry of the contact surfaces between the tool joint and casing was being updated continuously. Consequently, the contact area and contact pressure were changed continuously during the casing wear process, which gives a more realistic simulation. Based on the shapes of worn casing, the numerical simulation research was carried out to determine the remaining collapse strength. Then the change curve of the maximum casing wear depth with time was obtained. Besides, the relationship between the maximum wear depth and remaining collapse strength was established to predict the maximum wear depth and the remaining strength of the casing after a period of accumulative wear, providing a theoretical basis for the safety assessment of worn casing.

Fracture failure analysis on a drill pipe in directional drilling

This paper addresses fracture failures for drill pipes in a directional drilling construction project for an oil and gas pipeline. The authors analyzed failed drill pipe samples through fracture macro analysis, nondestructive testing for micro-cracks, mechanical properties testing on drill pipe material, microscopic microstructure analysis, the cross-section morphology analysis and the service conditions analysis. This study determined that drill pipe failure is caused by a repeated overload thrust applied to a drill string for the purpose of freeing a stuck drill resulting in a bent drill string and a drill pipe fatigue crack initiated on the outer surface of the bent drill pipe and propagated rapidly. Theoretical calculation and finite element simulation based on the drill string service condition showed a helical buckling deformation before fracture leading to the conclusion that it was the repeated overload thrust that caused the drill pipe fatigue fracture.

Coupled Modeling of Complex Fracture Networks Induced during Hydraulic Fracturing Treatments

Hydraulic fracturing has been extensively utilized to stimulate production in low permeability naturally fractured reservoirs. There have been tremendous efforts to understand how hydraulic fracture develops in the presence of natural fractures to better describe complicated fracture network induced in the reservoirs. Hydraulic fracturing process in homogeneous reservoir is well represented by Linear Elastic Fracture Mechanics (LEFM) as fracture process zone is very small but in heterogeneous formations particularly in presence of microfractures, LEFM may not completely describe the rock failure behavior, hence more sophisticated models like cohesive models should be used. A cohesive zone model (CZM) is developed to couple fluid flow with elastic and plastic deformations of the rock. CZM approach is used to not only model propagation of the hydraulic fracture but also their interactions with natural fractures like crossing natural fractures. Additionally mechanical properties of digenetic cements inside natural fractures is also incorporated into this model. Pressure fluctuations before and after intersection of natural fractures is observed in the field and lab experiments, while this observation was not verified in two dimensional models, the presented model shows pressure fluctuations in non-planar hydraulic fractures due to slippage occurring before and after fracture intersections with the natural fracture.

Application of Titanium Alloy Tubing in HPHT Gas Wells to Reduce Vibration and Buckling

Tubing string vibration and buckling are primary causes of tubing string failure, sustained casing pressure and production casing wear in HPHT gas wells. In order to obtain an improved understanding of tubing string vibration and buckling, finite element analysis is carried out in this paper. Typical tubing string failures in HPHT gas wells are analyzed, and fracture surfaces are tested. The vibration and buckling characteristics of original nickel-base alloy tubing string and modified tubing string with Titanium alloy tubing are calculated and compared. The results indicate that the application of Titanium alloy tubing can effectively reduce tubing string vibration and buckling. Titanium alloy tubing has lower elasticity modulus, so it transforms more energy into deformation energy rather displacement energy. Therefore, it is concluded that Titanium alloy tubing can effectively reduce tubing axial vibration displacement, so as to prevent tubing fatigue and thread looseness. Original tubing string has contacted with the wellbore under sinusoidal buckling. The sinusoidal buckling of modified tubing string is much slighter and tubing doesn’t contact with the wellbore. The work presented in this paper can provide a technological basis for the reduction of tubing string vibration and buckling, as well as application of Titanium alloy tubing.

Stress Analysis of Tubing Thread of the Gas Seal Test in Deep Gas Well

Seal test of tubing thread in high pressure gas well is a new technology currently, but there is no uniform standard regarding the accurate sealing test pressure, frequently improper sealing test pressure can easily lead to damage and fatigue failure of tubing threads. In order to solve the above problems, in this paper the axisymmetric finite element mechanical model of the BEAR threads based on the theory of elastic-plastic mechanics was established, and the finite element software used to calculate the results. The results showed that when the test pressure was equal to 100 and 50% of the RIP (Resistance to internal pressure) of tubing, the plastic deformation of the tubing thread occurred, and the plastic deformation area generated in the roots of the last three buckle, when the test pressure was equal to 25% of the RIP of the tubing, the maximum stress of all types of tubing threads was less than the yield strength. Based on this model, the stress of the tubing thread of the high-pressure gas well can be quantitatively and fully evaluated, and the gas seal test technology can be provided with theoretical reference.

P110T Casing Material’s Relation of Creep and Relaxation for the Sealing Surface of Premium Connection in High Temperature Gas Well

Creep or stress relaxation is considered as a mainly factor of reducing the contact pressure on the sealing surface of premium connection, causing the gas leakage in the high temperature gas well. In this paper, the creep tests of P110T casing material were conducted under different temperatures (120, 200, 300 °C). According to the experimental data, the fitting mathematical model of P110T casing material’s steady creep strain rate was obtained under the different temperature and loading by the Least Square method. Then, the relation of creep and relaxation was presented, and an innovation stress relaxation model was put forward from the creep strain rate. Finally, applying with the stress relaxation model, P110T casing material’s stress relaxation varying with time was obtained. The results show that, the stress relaxation was obvious in the initial phase and it decreased with time increasing. The stress decreased significantly and tended to a stable value. And the environmental temperature and initial stress had a significant influence on the stress relaxation. This study can be used for the designing of the premium connection’s structure, or prediction their service life in the high temperature gas well.

Strain-Based Replacement Criterion for Third-Party Damaged Oil and Gas Pipelines

One reason for unpredictable failures in pipeline operation is third-party damage (TPD), which can occur due to actions of any third parties other than pipeline staff. Based on nonlinear dynamics theory, we propose a finite-element model for impact between an excavator bucket and the pipeline surface. The simulation was done for the following situations: gas transportation, bucket tooth striking the pipeline, bucket tooth separating from the pipeline, and unloading the pipeline pressure. We analyzed the pipeline damage mechanism and established the influence of the dent depth on residual strain and residual stress. Comparison of finite-element results with the standard criteria described in the standard ASME B31.8 showed that the latter criteria have a number of disadvantages.

Numerical simulation of fluid flow and experiment on downhole friction reduction tool

In this article, a new friction reduction tool is designed for drilling of horizontal wells, and its performance is investigated using computational fluid dynamics simulation technique, laboratory experiment, and field testing. According to flow field analysis, the spiral diversion channel has obvious disturbance effect on fluid, which can help cutting transport and reduce cutting bed accumulation. Tensile test is carried out and tensile resistance of the friction reduction tool is 90 ton. Shearing test is also conducted to examine the shear resistance capacity of the pins that support rollers. Under the lateral load of 20 MPa, large deformation of the pins is observed, but they are not broken. The function of friction and torque reduction is verified using experimental apparatus for drill string dynamics of horizontal well. Field testing is also completed in a real well. The accumulated operational time of the friction reduction tool is more than 130 h and its fatigue life reaches up to 3×105 cycles. The simulations and experiments indicate the designed tool can effectively reduce friction and torque in horizontal wells. The work presented in this article can provide a technological basis and scientific reference for the development and application of friction reduction tool in horizontal wells.

Casing failure mechanism during volume fracturing: A case study of shale gas well:

A large number of casing failures occur during the volume fracturing operation of shale gas, making normal completion stimulations impossible. To solve this problem, rock mechanical experiments and numerical simulation experiments are carried out in this article. It is found that the macroscopic rock mechanical strength reduces most when the crack angle of fissured rock in Longmaxi Formation is 45°, and it reduces stably when the number of cracks increases to 8. The elasticity modulus ratio, yield strength ratio, and compressive strength ratio are 0.70, 0.71, and 0.68, respectively, based on which this article establishes the finite element model for shale gas well X201. Then, the secondary development realizes the dynamic adjustment of the rock mechanical properties during the fracturing. The correctness of method and model in the article is verified through comparing the simulated calculation of casing deformation and the field multi-arm caliper logging data. The casing failure mechanism is revealed, providing a theoretical basis for the prevention of casing failure caused by shale gas fracturing.