Research On Motion Control And Energy-saving Control Of Sucker Rod Pumping Systems Using Direct-driven Electro-hydraulic Technology

The sucker rod pumping system is the predominant artificial lifting method and the main power consumption source in the oil field. The operational efficiency of sucker rod pumping system is greatly concerned, which is generally of a low level with the usage of traditional beam pumping unit. Constrained by the crank rocker mechanism for nonlinear motion transmission, the defects such as unbalanced drive power, low load rate of electrical motor, low energy efficiency,difficult to adjust operational parameters, overlarge bulk and huge mass are found in the beam pumping unit and its improved type. Many non-beam pumping units have been invented to overcome the preceding shortcomings of the beam pumping unit. In this thesis, aimed at improving the operational efficiency of the sucker rod pumping system, the direct-driven electro-hydraulic technology is applied to develop a new type hydraulic pumping unit that features in linear motion transmission and balanced power consumption. The design of hydraulic pumping unit, the motion control of the polished rod as well as the energy-saving control of the sucker rod pumping system are studied, which contribution to the energy saving and efficiency improvement of the oil production. The thesis provides some references for energy saving in sucker rod pumping systems and the application of direct-driven electro-hydraulic system, which have great value in industrial practice and academic research.With the polished rod load analysis, the method to reduce installed power and improve electrical motor load rate of the pumping unit is studied from two aspects of balanced drive speed and balanced drive force,and then a direct-driven electro-hydraulic pumping unit is proposed and designed. To achieve the polished rod motion control of this new type hydraulic pumping unit in condition of only noised displacement measurement and unknown disturbance, the method to estimate unknown state and disturbance is investigated, and the sliding mode position tracking controller with disturbance compensation and adaptive switching gain is studied. Considering the sampled control character of industrial digital controllers, the discrete-time sliding mode controller is also studied.On the basis of the study on high efficiency pumping unit and its motion control, the energy-saving operation of sucker rod pumping system is further studied. The dynamic model of the downhole system consisting of the sucker rod string and the downhole pump is presented. The application and effect of different buoyancy methods on rod string dynamics are analyzed. The viscous damping coefficient of wave equation is deduced based on equivalent friction energy loss.The downhole system is simulated with different oil well gas ratio and different polished rod motion, and then the pump condition and the polished rod load are compared under different simulation settings. The well production, downhole energy consumption and efficiency of downhole system are simulated under different stroke frequencies further considering the inflow performance of oil well. Finally, an energy-saving operation strategy based on downhole pump fillage control is proposed, and the method to calculate the pump fillage from surface polished rod load and displacement is presented.The thesis is outlined as follows:In chapter 1, the background and significance of the studies in the dissertation are discussed.Then the working principle of the sucker rod pumping system is introduced and the research works on the efficiency improvement of the sucker rod pumping system are reviewed. Then developments on direct-driven electro-hydraulic technology are also investigated. At last the main contents of the research are presented.In chapter 2, the design, simulation analysis and prototype functional test of the direct-driven electro-hydraulic pumping unit are presented. Firstly, the load capacity, the working stroke and stroke frequency of the designed pumping unit are presented. Then the lifting motion,different hydraulic drive and power balance schemes are discussed. The detail design procedure for the direct-drive electro-hydraulic pumping unit with gas balance is carried out. The performance of the hydraulic pumping unit is verified by simulation. At last, the functional test of the prototype is conducted and the transmission efficiency is discussed.In chapter 3, the polished rod motion control of the direct-driven electro-hydraulic pumping unit is discussed. Firstly, the state-space model for the direct-driven electro-hydraulic pumping unit is deduced. Then an adaptive switching gain sliding mode positon tracking controller is designed considering the hydraulic pumping unit suffers from parameter uncertainties and unknown disturbances. The effects of sliding mode observer, tracking differentiator and Kalman extended state observer on state and disturbance estimation are compared. At last, the proposed control strategy is verified by simulation and experiments.In chapter 4, the sampled discrete-time sliding mode control for the motion control of the direct-driven electro-hydraulic pumping unit is discussed. Firstly, the discrete-time reaching law and the related width of quasi-sliding mode band are analyzed with different disturbance compensation. Then the design of sliding surface considering the properties of the reduced dimension system is presented. The stability criterions for the sliding mode control with one step delayed disturbance estimation and input filter are discussed. Lastly, the proposed discrete-time sliding mode control strategy is verified by simulation and experiments.In chapter 5, the downhole system is modelled and analyzed. Firstly, the dynamic model of the rod string in 3D well bore is discussed, including the application and effect of buoyancy on rod string dynamics and the viscous damping coefficient. The finite-differential numerical method to solve the rod string dynamics is presented. Then the downhole system is simulated with different oil well gas ratio and different polished rod motion, and the pump conditions as well as the polished rod loads are compared under different simulation settings.In chapter 6, the matching between well production and well inflow is studied and an energy-saving operation strategy is proposed. Combining downhole system model and well inflow dynamics, the balanced states of the sucker rod pumping system are simulated under different stroke frequencies. Finally, an energy-saving operation strategy based on downhole pump fillage control is proposed, and the method to calculate pump fillage is presented.In chapter 7, the research work and conclusion of this dissertation are summarized. The main contributions are highlighted. Future research proposals are presented in the end.