Development Of Gravity Heat Pipe Sucker Rod String And Analysis On Its Heat Transfer Characteristics

Due to the viscosity-temperature characteristics of crude oil, the viscosity of oil will increase rapidly as it cools down gradually caused by heat loss during its flow from down-hole to well-head, influencing the normal production of oil well. Therefore, the borehole of oil well needs to be heated. Presently, heating methods of electric, hot fluid circulation and chemical agent addition are applied widely. But there exist many problems in these methods. For electric heating, the energy consumption is tremendous and there are failures of the electric-heated rod string, the electric cable and the control cabinet, etc. The methods of hot fluid circulation or chemical agent addition not only increase the production cost, but also complicate the down-hole operation procedure of production well. In order to solve these problems, the theory of gravity heat pipe was suggested in recent years to be applied in improving the distribution of borehole temperature. But there are many problems should be studied further in order to use gravity heat pipe in oil production. According to the current situation, research work is carried out in-depth on the gravity heat pipe sucker rod string in this dissertation.1. Through analysis, it is appointed out that the following problems in using gravity heat pipe theory in oil production need to be researched profoundly. (1) The liquid pool is too deep and the boiling condition is too harsh; (2) the flow rate of steam is too high, the caliber size is too small, the entrainment limit is small; (3) the steam flow path is too long, the flow resistance is too big.2. The mechanisms and influencing factors of the natural phenomena occurred in the heat pipe are analyzed systematically, which provides the theoretic basis for the design and manufacture of the gravity heat pipe sucker rod string.3. Studies on heat transfer calculation and thermal design on gravity heat pipe sucker rod string are carried out, and the method of heat transfer calculation for gravity pipe made from hollow sucker rod is put forward. The calculation results show that the heat transfer rate of the gravity heat pipe of hollow sucker rod is about 25-30% of the inlet heat enthalpy of crude oil, account for about 80% of the heat loss to the formation. The ratio of the oil temperature at well-head to the bottom-hole temperature is over 70%, consistent with the experimental results, which demonstrates the correctness and validity of the heat transfer calculation method for gravity heat pipe.4. The shell pipe for the gravity heat pipe sucker rod string is developed. The manufacture technical parameters and processing technique of hollow sucker rod specified for gravity heat pipe shell are put forward, and the special sucker rod is manufactured, and the type is FTG . Laboratory experimental results show the tensile strength of this sucker rod is up to 910 MPa, its torque bearing capability reaches 3785 Nm, the rod strength is enhanced greatly. Keep the rod under the pressure of≥20MPa for 5 minutes, there is no leakage, demonstrating the good sealing property. Field test show that the rate of pipe broken and thread tripping of the shell string made up by the special pipes developed in this dissertation reduced obviously; there is no problems of mechanical strength and sealing; the string has a long duration of normal operation. This pipe can be optimally used as the specific shell for the gravity heat pipe.5. Compared with the heat transfer of the typical heat pipe, the different characteristics of the heat transfer of the gravity heat pipe sucker rod string are: the crude oil from formation is both the heat source and the cold source; the division point between the evaporation segment and the condensation segment is not a fixed point; the heat flow density through the pipe wall is different at various well depth. On the basis of the knowledges above, the design idea of the laboratory experimental apparatus is formed; the experimental apparatus of gravity heat pipe sucker rod string is constructed; the experimental methods are put forward; the procedure and technical problems of fluid filled into and gas exhausted from heat pipe are solved.6. Important properties of the water and fluid A prepared in this dissertation, such as physical parameters, boiling condition, flowing resistance of steam and entrainment limit, etc. are compared. The results show that fluid A is better than water. Fluid A can be used as the working fluid of the gravity heat pipe sucker rod string, and the fill amount of working fluid should be 15-20% of the inner volume of the heat pipe.7. Experimental procedure and analysis method for cold/hot water heat exchange on the experimental apparatus of gravity heat pipe sucker rod string is put forward. Through experiments, it is concluded that when the flow rate is controlled at 0.1 kg/s and the heat exchange between the cold and hot water is 7-10 kw, the reduction of water temperature can reach 20-30℃, which can meet the requirements for the heat pipe to operate normally.8. Results of heat transfer experiments show that the gravity heat pipe sucker rod string developed in this dissertation is successful, which exhibits good working effects:(1) For the segment of 0-4 meters at the inlet, abrupt reduction occurs to the temperature of the hot fluid, demonstrating that the boiling heat exchange is very strong at the inlet segment of the heat pipe, great amount of heat of the hot fluid is absorbed and entrained by the heat pipe.(2) When the temperature of the hot fluid reaches the lowest point, the temperature will rise gradually and then decrease gently, which forms a sharp contrast to the test results of cold/hot water circulation system, demonstrating the normal operation of the heat pipe.(3) Thereupon, the temperature inner the heat pipe appears wave-like changes, namely, decreases abruptly at the inlet segment, then rises gradually and transcends the temperature of the hot fluid at the intersect of the two curves, demonstrating that the heat pipe can transfer heat to the outside hot fluid in the tubing.(4) Experimental results show it is right and applicable to chose fluid A as the working fluid for the heat pipe. Working fluid A can overcome a series of drawbacks of the typical working fluid (water), phenomena of flow blocking and too big pressure drop will not occur. And it can be seen from experiments that the rate of temperature between the condensation segment and the evaporation segment is over 0.7, showing that the application of heat pipe in the oil well can obtain a temperature at the well-head being higher than 70% of the bottom-hole temperature.9. The implementation technology of the gravity heat pipe sucker rod string is investigated. According to the field condition of oil production, the heat pipe manufacture technology and the successful laboratory experimental experiences achieved in this dissertation, the procedure to prepare the gravity heat pipe sucker rod string on field is studied preliminarily, and the specific construction process and methods are provided.The research results achieved in this dissertation provide the feasibly way and practical technology for using the heat pipe principle in the oil production, having certain application value and guiding significance.