Research On Coal Migration And The Key Technology Of Coal Control For Sucker Rod Pumping System In CBM Wellbore

The coal migration and coal control are regarded as the key technology of makingcontinuous and steady production in coalbed methane (CBM) wells. The coal migration incoal channel and wellbore, opening condition of fixed valve, and coal discharge conditionwere analyzed based on the results of field investigation in Hancheng and Sanjiao oilfield ofOrdos basin. Then the system of coal control was developed by the combined action ofpredicting coal production, design screen and sucker rod pump of coal control, and optimumorganization of rod and tubing. And it is contributed to extending the period of pumpinspection, reducing the costs of production and management, and enhancing the CBMdeliverability and comprehensive development effectiveness.The formation mechanism and affecting factors of coal channel were studied on the basisof occurrence condition caused by stress alternation and failure in coal reservoirs. The criticalflow velocity of free coal migration was obtained with its mechanical model in coal channelnear wellbore. The predicting model of critical pressure drop and flow velocity was developedfor matrix coal particle due to the stress state around the wall of coal channel and coal failurecriterion. Consequently, the critical occurrence condition was gained for coal particle. Basedon the differential equation of fluid flow through porous medium and conduit flow throughchannel, coal concentration and carrying capacity in the flow were considered, and the modelof coal production was solved by iterative algorithm with length increments. Then the coalproduction was predicted with the results of pressure, velocity and concentration in coalchannel, which provided the basis for coal migration and coal control in the wellbore.The settling property for spherical coal particle was obtained with the settling state andmechanical model in static water of tubing-casing annulus. And the settling property forsingle and group coal particle with arbitrary shape was gained due to the correctioncoefficient of interference settlement by the theoretical analysis and experiment. Themathematical model of two-phase (solid+liquid) pipe flow was set up on the basis ofprincipal and constitutive equation for two-phase flow. And the coal migration in rod-tubingannulus was studied by the combination of numerical solution and simulation analysis whilethe rod and tubing were relatively static and initial velocity was given for the two-phase flow. It provides the basis for the analysis of coal carrying capacity in the flow.The differential equation of fluid flow through rod-tubing annulus and mathematicalmodel of overall flow rate were developed based on the two-phase (solid+liquid) pipe flow.Then the kinetic characteristic of fluid flow was analyzed. The simulated experimental devicefor the production was built up. And the coal settling property in the moving water ofrod-tubing annulus was obtained with the theoretical analysis and experimental results. Themaximum diameter of coal particle which can be carried out the CBM well by the well fluidwas determined after the coal carrying capacity in the flow was get. Therefore, the gap widthof screen for different sizes of pumping unit and pump and wire straightened screen used inCBM wells were designed in order to block the coal particle.The condition of pressure drop for opening the fixed valve was analyzed during thesucker rod pumping system. The kinetic characteristic of valve was gained due to thenumerical solution of differential equation for the motion of fixed valve. Then themathematical model of opening condition of fixed valve was developed on the basis of theanalysis of the hydraulic loss caused by the well flow through valve gap and draught losscaused by the inertia of fixed valve. And the critical submergence depth for different pumpingconditions was acquired by the combination of numerical solution and test results of openingthe valve. At the same time, rational submergence depth to maintain high efficiency wasdetermined for sucker rod pump. The mathematical model of two phase gas and water flowinto the pump barrel was developed on the basis of the two phase fluid flow in CBM wells.And the variation of dynamic fluid level and volumetric efficiency was gained under differentoperating conditions. The coal concentration and velocity for different initial velocity and coaldiameter were calculated by the simulation analysis and numerical solution about the modelof fluid flow and coal migration in pump barrel. Finally, the coal-control sucker rod pumpwas designed and its calculation was completed based on the results of test and theoreticalanalysis. The results of field application show that the application of pump with the sizes of20-125TH,25-150TH and25-175TH can effectively improve the efficiency and extend theperiod of pump inspection.The preliminary design of rod and tubing diameters and their combination werecompleted by considering the rod fatigue strength, equal intensity principle and matchingproblem of rod, tubing and pump. Then the sizes of rod and tubing were preferred based oneffect of annular cross-section area of rod and tubing on polished rod load and area of collarand tubing on coal discharge, and discharge condition of coal particle. Consequently, theappropriate selection criteria was established and the specific combination of parameters between rod and tubing was determined for the low water production during the period ofsteady operation in CBM wells. Finally, the small sucker rod and its collar were designed bytaking into account the conditions of flow cavitation, anti-trip of sucker rod and strength ofmaterial. The results of field application show that the application of sucker rod with the sizesof CYG16and CYG19can decrease the value of polished rod load and improve the coalcarrying capacity in the flow.