Yuele Jia Computer Science School, Southwest Petroleum University, Chengdu , Sichuan, China

Transcription

1 doi: / Snubbing Operation Process Simulation Based on String Stress Actuation Yuele Jia Computer Science School, Southwest Petroleum University, Chengdu , Sichuan, China Zhi Chen Engineering Technology Research Institute, PetroChina Southwest Oil & GasField Company, Chengdu , Sichuan, China Yueru Jia Sinopec Zhongyuan Gas Corp.Ltd, Sinopec Corp, Puyang , Henan,China Abstract Snubbing operation is a technical process with high risk in oil and gas production, and the training of snubbing operation is of difficulty. Snubbing operation simulator is a set of electronic devices used for training snubbing technicians, which simulates typical working process of snubbing unit. The current simulation training mainly focuses on operation process, but ignores the influence of downhole troubles on operation security. In order to make simulation process the same as real situation, this paper studies parameter models related to drill string mechanical, and design the process simulation methods driven by the calculation model of neutral point location and drill string axial force. Computer hardware simulation and virtual reality technology is adopted to complete the arbitrary control of snubbing unit, and realize the snubbing process simulation covering accidents triggering and handling. Approved by practice, snubbing operation simulator adopting this mode highly simulates the operation of real devices, and can predict and simulate hazardous situation, which is of great meaning to engineering simulation. Key words: Drill String Mechanical Drive, Snubbing Operation, Subbing Simulator, Virtual Reality 1. INTRODUCTION Snubbing operation is an operation methods completing drilling and workover with snubbing unit(wang Wei, 014). This operation can avoid pollution of killing fluid to reservoir, and protect the production to the maximum. While due to the production condition and complexity of downhole condition, snubbing operation is of great risk. For better production efficiency and economic benefit, and for reducing accident, it is necessary to carry out complete training to snubbing operators. Virtual training system (Zhang, 013) realized by computer simulation technology is now a favorable means in current petroleum industry training (Zhang, 007; Jia, 007). Now the simulation process in this field mainly aims at snubbing unit operation (Chen, 005). It cannot present the effect of downhole string stress situation on snubbing operation process. It is the new idea to add downhole string stress situation analysis and calculation model design in the simulation system, allow trainee to operate arbitrarily, judge triggering condition of accident and add accident treating process in developing snubbing operation simulation.. SIMULATION FRAMEWORK DESIGN Snubbing simulator adopts the working method of combining hardware and software. Hardware devices are the simulation of real site equipment realized by computer communication and computer control technology, providing operation for users. Control device of snubbing unit is realized by hardware simulation embedded with PLC (Zhong, 014), acquiring data and send to software system through communication module. Simulation framework contains parameter simulation, core control and virtual reality display. The relation and function of the three modules is as shown in figure 1: parameter simulation module receives the operation and transfers it into digital signal, and then sends it to core control module; Core control module has the function of non-sequence operation control, accident triggering and automatic assessing. Operation process of user can be judged by data delivered by parameter module, and the operation and phenomenon is sent to virtual reality display module in the form of command. Then virtual reality display module realizes real- time 3D scene rendering and management, and simulating of non-sequence operation, as well as accidents and pressure phenomenon. 351

2 Figure 1. Simulation framework 3. CALCULATION MODEL AFFECTING OPERATION PROCESS Snubbing operation is the process of running and pulling string in the well with pressure. If there is no constraining force, string would be blown out when running or pulling string with pressure. In order to make the snubbing simulation process the same as real site situation, downhole pressure calculation model is added into the control process of snubbing simulator. Through the analysis of string stress situation (Liu, Yao and Yue, 011), the simulation of snubbing operation process is realized. The real time calculation result of the model will affect the operation process of device and the presentation of virtual reality, and assess if the trainee s operation is correct Downhole String Stress Analysis When running/ pulling string in snubbing operation, the vertical stress upon the string should be analyzed to determine the strength of stress. There are mainly five kinds of stress that are applied on the downhole string: external force by devices, friction force of ring seal, string weight, friction force in wellbore, and jacking force of well bore. The string stress condition is as shown in figure : Figure. String stress analysis 35

3 Jacking force calculation model: OD F=pa (1) In the formula: F Jacking force, KN Pa Wellhead pressure, Kpa OD Outside diameter of tubing, mm To ensure the snubbing operation carried out normally, the down force needed in the process depends on the following three factors: (1)String between slips is stable and effective, that is, down force is smaller than the critical buckling load of string. ()Axial friction force of string is smaller than down force, to ensure normal lifting and lowering of string. (3)Stress generated by snubbing string should meet the strength condition. That is, with the lifting and lowering of string, the stress of string should meet the strength condition in different well depth. Based on the above analysis, beside the conventional strength check and pressure control calculation in snubbing process, special stress analysis and calculation is also needed, including the calculation of neutral point depth and critical buckling load. 3.. Determining Neutral Point Lubinski believes that according to string stress condition, string in the well should be considered as two parts. The weight of the upper part in drilling fluid is equal to the weight of elevator or hook load. The weight of the lower part in drilling fluid is equal to weight on bit. The point to divide the two parts is the neutral point. The neutral point location can be determined by zero axial stress cross-section area. When F=0, the corresponding well depth is neutral point. (1)Above the neutral point: F>0, tubing is pulled down. ()Below the neutral point: F<0, tubing is lifted upward. When operated above the neutral point, raise/ lower string by applying external force with snubbing unit; when operated below the neutral point, raise/ lower string as the same operation by conventional drill rig or workover rig. Without considering friction force, neutral point calculation model can be deduced based on jacking force calculation model 1: OD Pa H () 4 g steel fluid In the formula: H Neutral point, m Pa Wellhead pressure, Mpa OD Outside diameter of tubing, mm ρ Density, kg/m 3 According to this model to calculate: outside diameter of the tubing is 60.3mm; inside diameter is 50.6mm, unit mass is 6.99kg/m. The neutral point depth in gas well and water well changes with well head pressure. The changing trend is as shown in table 1: Table 1. The values of neutral point depth on different Wellhead pressure Neutral point depth (m) Wellhead pressure (Mpa) Water well Gas well From the above table, the relation between neutral point and well head pressure can be shown as in figure 3: 353

4 Figure 3. Neutral point calculation curve To determine the neutral point change in different control pressure and the axial force of string can simulate the process of running and pulling tubing in snubbing operation Calculation of Critical Buckling Load When the maximum downward force applied in snubbing operation is calculated, the critical buckling load(grant, 008) should be determined so that the devices work normally. When SR SRc, buckling load is equal to main axis buckling load. When SR < SRc, buckling load is equal to part buckling load. SR:Slender ratio SRc:Critical slender ratio Determine the maximum unbraced length according to critical buckling load, the calculation model is as follows: 1) Calculate main axis buckling load with Euler model: E I Feb (3) In the formula: E Young modulus, Mpa I Inertia moment, mm OD ID ID Inside diameter of tubing, mm Feb Jacking force, N ) Calculate part buckling load with Johnson model: In the formula: Sy Yieled stress of string, Mpa Flb Jacking force, N L I (4) 64 L 1 Rg Flb Sy As SRc OD ID As Cross sectional area of tubing, mm SRc Critical slender ratio As (6) 4 (5) 354

5 Rg Radius of intertia, mm L Unbraced length, mm E SRc Sy Rg I As SR L (9) Rg (7) (8) When SR SRc, buckling load = Feb, When SR < SRc, buckling load = Flb. According to the calculation models, the relation between string stress and unbraced length is as shown in figure 4. To ensure the security and efficiency of the operation, the maximum of cylinder stroke should be less than 70% of the unbraced length limit(qing, 01). Figure 4. Relation between buckling load and unbraced length 4. CORE CONTROL MODULE DRIVEN BY PRESSURE MODEL According to the above analysis of string stress driven process, the changing trend of corresponding parameters and process intervention is designed in the core control model. In the process of operating snubbing simulator, operation signal is fed back to core control module after collected by hardware. Core control module realizes the process control and operation assess according to calculation results and hardware signal feedback. The system judges the technological process according to neutral point calculation model, judges whether the string will be buckled by operation according to unbraced length calculation model, and judges if the string will be blown out according to string stress calculation model. Thereafter, the system judges comprehensively whether the operation process complies with the requirement, and whether the operation aim can be achieved. The working principle of core control module is as shown in figure 5 Take gas well snubbing operation as an example, pressure in tubing is blocked by tubing plug, pressure in annulus is blocked by snubbing device in this operation, and string is lifted and lowered with hydraulic cylinder and slips system. Collecting snubbing device parameters, when wellhead pressure is less than the rated value, lift/ lower string with annular type operation; when wellhead pressure is in the controllable range and is greater than the rated value, lift/ lower string with annular and ram type operation. After the operation type is determined, calculated neutral point location according to wellhead pressure and tubing size. If string length is greater than neutral point depth, run and pull tubing with only load-bearing slips of snubbing unit. If string length is less than neutral point depth, run and pull tubing with pressuring slips and load-bearing slips alternatively. Core control program monitors the lifting height at real time, and compares with unbraced length, to make sure if it complies with operation requirement. 355

6 Initial state Test wellhead pressure Pa Input string parameter Determine Block pressure in the well Pa>Rated value Annular and ram type control Pa Rated value Annular type control Calculate Model real-time calculation String stress calculation Determine neutral point Determine unbraced length Control Carrying out process Judge neutral point depth Operation process below neutral point Operation process at neutral point Operation process above neutral point Judge unbraced length Lifting height>unbraced length Accident trigger: String buckling Lifting height Unbraced length Normal operation Judge string stress condition String stress unbalence Accident trigger: String blown out String stress balence Normal operation Figure 5. Working principle of core control module 5. VIRTUAL REALITY DISPLAY MODULE BASED ON CORE CONTROL Virtual reality display module is developed by 3D animation engine. With the command transmission by network communication with core control module, functions such as scene initialization, technical animation control, collision handling and real-time animation rendering, etc. are realized. Just as shown in figure 6: virtual reality display module responds the control command at real time, and realizes the simulation of site scene, model initial state, snubbing operation process, devices working process, accident phenomenon and handling methods in form of 3D animation synchronized with hardware operation. Figure 6. Virtual reality display module 356

7 The simulation of snubbing operation process mainly involves five control parts: hydraulic dynamics system (Peng, 008), BOP system, bleed off/ balance system, slips system and hoisting system (Liu, 013). Under the command of core control system the five control parts cooperate with each other according to hardware operation, completing the real-time presentation of virtual devices operation process. The effective picture of running virtual devices is as shown in figure 7: 6. CONCLUSIONS Figure 7. Virtual devices working process This paper has stated the methods of mathematical calculation models of snubbing computer simulation system (Ding, Xu and Jia, 015), formed a set of snubbing simulation plan based on string stress driven control, and completed the verification of snubbing simulation plan. Snubbing operation simulation system developed by this mode can complete the safe operation training for relevant technicians, and can provide scientific, accurate, high-efficient use mode. This study is initiative in the field of snubbing computer simulation. The successful development can improve the standard of snubbing operation, which is good for the promotion of snubbing technology, with the advantages of less accident, cost reduction and more economical benefits. REFERENCES Wang Wei (014) The Technical Status and Application Analysis of Snubbing Operation Device, China Petroleum Machinery, 4(10), pp Zhang Jing (013) Application of Automata Theory in Drilling Simulator, International Journal of Applied Mathematics and Statistics, 51(3), pp Zhang Jing (007) The Design of Drilling Simulator Graphic System Based on Object Oriented, Journal of Southwest Petroleum University, 9(), pp Jia Yuele (007) Drilling Simulator Graphics Program Development Based on OpenGL, Journal of Southwest Petroleum University, 9(), pp Chen Weiqian (005) Foreign Snubbing Operation Machine, China Petroleum Machinery, 33(1), pp Zhong Gongxiang (014) The Hydraulic Control System of Snubbing Unit Based on PLC, Chinese Hydraulics & Pneumatics, 31(), pp Liu Jubao, Yao Jinjian, Yue Qianbei (011) The mechanical analysis of drillstring in snubbing, Journal of Harbin Institute of Technology, 43(1), pp Grant J. Duncan (008) Avoid downhole Explosions, Buckled Pipe and Parted Tubing During Snubbing Operations, Proc. Conf. on. SPE s Annual Technical Conference and Exhibition, pp

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