Study On Energy Flow Characteristics Of Solar-gas Combined Heating System For Settling Tank Of Oil Field

Vertical settling tank is a commonly used oil-water separation equipment for oilfield treatment of oil production wastewater.However,the high viscosity and poor flowability of the separated crude oil affect the oil recovery operation.Usually,a gas heating boiler is used as a heat source to improve the fluidity of the oil layer in the settling tank.However,this method has prominent problems such as high energy consumption and high carbon emission intensity.With the increasing water content of extracted crude oil year by year,the operating cost of settling tanks continues to increase,and the development and utilization of clean and efficient heating technology is urgent.In recent years,the development of solar thermal utilization technology has gradually matured.However,solar energy cannot meet the needs of large-scale continuous energy supply due to factors such as weather and day and night.Solar heating systems still need to be assisted by existing gas heating methods.Therefore,a solar-gas combined heating technology for settling tank is proposed.However,for the combined heating technology,the issues related to flow and heat transfer,energy matching characteristics,and efficiency performance of the system are not yet clear,which restricts the development and application of such new heating methods.For the individual settling tank,the limited heat exchange area of its heat tracing coil leads to low heating efficiency.Therefore,fins are added to the outside of the bare coil to transform it into a finned coil.However,considering the influence of the coil structure differentiation on local crude oil flow,which in turn affects the overall convective heat transfer effect of the original oil layer,further clarification is needed to strengthen the heat exchange mechanism.Insulation laying is an important way to reduce passive heat dissipation,and its structural parameters still need to be further optimized.Using numerical simulation and system simulation methods to solve the above problems,a simulation model of the solar-gas combined heating system for the settling tank is constructed from the overall perspective of the system,and the energy flow transport characteristics and comprehensive energy-saving benefits of the system are studied.Starting from the individual equipment and based on the actual operation of the settling tank in the oil field,a heating flow heat transfer model for the settling tank is established.The enhanced heat transfer characteristics of the heat tracing coil after adding fins and the optimization benefits of the insulation structure are analyzed.The main research content and results are as follows:1.The finite volume method is used to establish a two-dimensional flow heat transfer model of the crude oil and water in the settling tank.The influence of the structure and surface temperature of the heating coil on the heating effect of the crude oil is investigated from the temperature field,the average temperature,and the viscosity of the crude oil and the thickness of the condensate layer.The results show that,without changing the structure of the hot tracing coil,increasing the surface temperature of the hot tracing coil can effectively increase the average reservoir temperature,reduce the condensate layer thickness and condensate rate,and thus improve the crude oil flow effect under the oil receiving condition.However,from the perspective of heating benefits and possible adverse effects of high temperature surface on crude oil,it is not feasible to maintain temperature and reduce viscosity only by raising the surface temperature of the tracing coil to 180℃.2.Extending the heat exchange area of the bare coil is one of the effective ways to realize the heating gain of the tracing coil.The results show that the average crude oil temperature of2-,4-,and 6-fin coils increased by 1.23℃（3.1%）,2.66℃（6.7%）,and 5.84℃（14.6%）,respectively,compared with that of finless coils.The 4-fin,surface heating temperature of 75℃,and 6-fin,surface heating temperature of 65℃ can meet the heating and temperature maintenance requirements of the settling tank.Furthermore,the optimal coil structure is selected from the economic point of view.The results show that during the design life（15-20years）,the operation and maintenance cost of the 6-fin coil is CNY 295,700-408,500 lower than that of the 4-fin coil（10.7%-11.2%）.Based on the perspective of economic benefit maximization,it is found that the average annual cost is the lowest when the thickness of insulation layer is 11 cm,which is CNY 41,500.3.The dynamic simulation model of the solar-gas combined heating system of the settling tank is established.The influence of the oil collection period of the settling tank and the control strategy of the temperature difference of the opening of the phase change heat storage tank on the system performance is analyzed from the aspects of the heat collection of the solar collector,the heat supply of the gas boiler and the heat storage of the phase change heat storage tank,and the optimal combination scheme of the control of the oil collection period and the temperature difference of the phase change heat storage tank is recommended.The results show that when the oil collection period is scheme E（4:00-8:00）and the temperature difference of phase change accumulator is controlled as 6/0,the annual cumulative heat collection of solar collectors is the highest,the annual total heat supply of gas boiler is the lowest,and the heat storage and release efficiency of the phase change heat storage tank is the highest.During the service life（15 years）,the annual energy saving,total energy saving cost,dynamic payback period and CO₂ emission reduction of the solar-gas combined heating system for settling tank are 727,988.69 MJ,CNY397,007,9.14 years and 753,069.8 kg,respectively.