| Under the background of crude oil shortage and low-carbon economy, optimization andsimplification of oil-gas gathering&transportation plays a significant role in efficientdevelopment of oilfield. Recently, cooling gathering&transportation has been widely studied,designed, popularized and applied in oilfield surface engineering projects, as a effective wayto reduce investment and control cost, it has achieved obvious effects. However, thesimplified oil gathering processes represented by single-pipe concatenation process expose aseries of problems, which restrict normal production in the operation practice, such asdeposition in pipelines, blocking, high wellhead pressure and lack of scientific management.These problems are presented more seriously in severe cold areas and oilfields with variesdevelopment methods, as the oil-water two-phase system is a complex mixed system withunstable thermodynamics. Therefore, gelling characteristic temperature and gelling structuralstrength of non-Newtonian oil-water two-phase system are rheological measured in this article.Based on theoretical methods and laboratory simulation tests of thermodynamics tendencycoefficient and dynamics diffusion coefficient, the gelling law of oil-water two-phase systemcooling transportation is systematic studied and predicted. From energy transformationviewpoint, gelling deposition behavior and mechanism of oil-water two-phase systememulsification nucleation in low temperature in shear flow is also explained. Then, accordingto the study of single-pipe gathering process field tests, the problem of high wellhead pressurecaused by oil-water two-phase system cooling gathering gelling deposition behavior issystematic analyzed, and the control methods and schemes are proposed.By studying oil-water two-phase system gelling characteristic, it shows that rheologicalmeasurement technique is still suitable for description and explanation of gellingcharacteristic of crude oil with different water cuts. Based on the change law of viscoelasticityparameter, the gelling characteristic temperature and gelling structural strength can bemeasured correspondingly. Gelling temperature of oil-water two-phase system is lower thanfreezing point of crude oil, and the temperature would be further decreased as water cut rises,correspondingly, stress stability and the resistance to shear deformation in gelling areweakened and gelling strength is decreased. Under the constant stress, gelling characteristictemperature of crude oil with same water cut rises as the temperature drop rate increases, butthe gelling strength is weakened. Study of the effects of crude oil water cut and temperaturedrop rate on oil-water two-phase system gelling characteristic has important reference valueon surface cooling gathering design, parameter optimization and gathering&transportation safe operation of alpine region and low production wells under the new energy conservationsituation.In the study of pipeline transportation simulation tests, the effect of flow distortion onpressure drop test is considered. And aiming at non-Newtonian oil-water two-phase horizontalflow, differential pressure method to measure wax deposition thickness is improved, theeffects of oil-water two-phase system temperature, water cut, polymer containedconcentration, flow rate and system pressure on gelling law are studied. The main method andtarget in studying wax deposition of single-phase oil flow is to establish accuratethermodynamics or dynamics models to describe wax deposition process and to predict waxdeposition law. Therefore, based on typical single-phase flow wax deposition theoreticalmodel, relevant properties and flow characteristic parameters of two-phase system are asfunctions of water cut, thermodynamics tendency coefficient model and dynamics diffusioncoefficient model of describing oil-water two-phase flow wax deposition are regressiveestablished, and the deposition behavior of non-Newtonian oil-water two-phase system incooling transportation is predicted respectively. Under different working conditions, thechange rules of simulation tests are similar with the predict results of oil-water two-phasesystem deposition behavior, relative maximum deposition rate is achieved near the gellingtemperature and significant change in deposition behavior is happened near the phaseinversion point. Furthermore, deposition rate decreases as flow rate increases and depositionrate increases as polymer contained concentration of the system increases, and the depositionbehavior slows down in a certain degree when the pipeline transportation pressure rises up.Comparing with the thermodynamics tendency coefficient regression model, the dynamicsdiffusion coefficient regression model has better applicability to predict wax deposition inoil-water two-phase system, which combined diffusing effect, shear effect and depositionaging effect, and the average relative deviation is near10%between predicted values of rateof deposition thickness and test results. The dynamic behavior in deposition process isconsidered in the dynamics diffusion coefficient regression model and a diffusion coefficientis introduced to reflect the diffusion mechanism of wax molecule from flow to depositinterface in oil-water two-phase system.Due to the variety of system composition, instability of hydraulic and thermal workingconditions and the exist of emulsification conditions, the gelling deposition behavior ofoil-water two-phase system during cooling gathering&transportation is more complex thanthe wax deposition of crude oil. Through microscopic observation, gelling and aggregativenucleation are the most prominent physical states of the deposition. Melting temperature ofthe gelling deposition reaches more than60?, wax crystallization enthalpy is more than80J/g, and in the flow field mutational area, characteristic temperature value on DSC curve ismuch higher. Sand, scale and corrosion products of solid phase components analyzed bymorphology provide crystal nucleus for increase and growth of the deposition. At the sametime, the study of cooling flow structure reflect that the emulsification nucleation effect mustexist in shear flow field, and in the specific flow field area, shearing energy is defined as a function of pressure drop of circle pipe flow along with one characteristic length, which is theenergy produced by shearing stress working in the flow field. Then, according to the energyconservation equation, when there is kinetic energy in oil-water two-phase system in shearingflow field, shearing energy is an effective form to reflect the contribution of kinetic energy ofoil-water two-phase system to oil-water interface Gibbs free energy during emulsificationnucleation. By the correlation between flow pressure drop and shearing energy in typicalpoints, shearing energy analytical approach is used to quantitatively describe the contributionand distinction of flow field shearing action in gathering&transportation system on oil-wateremulsification nucleation. The instance calculation and analysis results show that in the sameoil-water two-phase system, shearing energy of elbow and valve bank are obviously higherthan that of common gathering pipeline under the same temperature, this indicates in flowfield mutational area, Gibbs free energy which the oil-water system required to overcomeduring emulsification at interface is relative lower, the probability of oil-water two-phasesystem emulsification aggregation and nucleation is higher in low temperature, then, it ismore obvious to action mechanism of gelling deposition behavior and it is mainly in localarea. Pores are filled with gelling deposition structure in cooling gathering&transportation,the deposit is with high density and viscosity, and except for the more than40%wax content,there are also lots of heavy constituents such as colloid, asphaltene and solid impurities. Themain existence forms of water phase in deposit are free water, capillary water, absorbed waterand internal emulsification water, it also propagates large amount of bacteria, and presentsspongy elastic gelling state characteristic in certain strength. The deposit is the result of thejoint effect of wax deposition and emulsification nucleation during temperature drop process.According to single-pipe oil gathering adaptation field tests, forming reasons of highwellhead pressure and production characteristics of corresponding wells in oil gatheringprocess are analyzed. It is considered that gelling process characteristic of oil-water two-phasesystem is the internal mechanism for forming high wellhead pressure in single-pipeconcatenation oil gathering process. Low water cut, production is less than minimum securityflow limit of gathering&transportation pipeline and wellhead temperature is below minimumtemperature limit of pipeline origin are the external performances of the formation of highwellhead pressure. Classification and management thought of wellhead pressure in coolinggathering&transportation is proposed, as level A:>1.5MPa, level B:0.8?1.5MPa, level C:0.5?0.8MPa, level D:?0.5MPa. Field test results indicate that majority wells with highwater cut and production are better adapted to single-pipe concatenation oil gathering process.However, to some well groups with low production and long oil gathering radius, especiallythe production of terminal well is far lower than concatenation well's, the gelling anddeposition are serious and the wellhead pressure are rising fast, it is necessary to combineother field controls and effective managements. Besides, there are better conformances ofsolidification oil formation law, oil-water flow pattern and the transition in field tests,laboratory experiments and theoretical studies. By systematic study of high wellhead pressurecontrol in single-pipe oil gathering process, the correlation of pigging period varied with transportation volume and polymer contained concentration near the gelling temperature isestablished. The effects of electric heater installed at wellhead on gelling structure formingtime extension, oil solidification rate mitigation and wellhead pressure control areinvestigated. The improvements of gathering&transportation process are proposed such asperfect pigging supporting facility, install constant pressure regulating valve at wellhead,change pipe material and extend assistant water mixing process. The study of feasibility ofoil-water two-phase system cooling transportation with low viscosity shows that effectivesurfactant type viscosity reducer could adapt to deposit dissolution, deposit elimination andfrictional pressure drop reduction. And it is more applicable to guarantee the transportation ofoil-water two-phase system in main pipe of valve group room after pigging, extend washperiod of the main pipe, and control the rise of wellhead pressure, the drag reductionefficiency achieve more than20%. In addition, considered the background of gathering&transportation system optimization design and low-energy operation during high water cutstage, investment and operation cost of various management technologies are compared,sufficient bases are provided to select practical engineering application scheme in thecombination of technicality and economy.Combined experiments and theoretical study we found that, in cooling gathering&transportation process, basic condition of deposition behavior occurred in oil-water two-phasesystem is the gelling characteristic and keys of gelling are formation of heterogeneous systemand effects in phases. Besides, fundamental of gelling in the system is the combined actionmechanism of wax deposition and emulsification nucleation during temperature drop. Andthis deposition behavior leads to part (or whole) effective flow area of gathering&transportation pipeline decrease, flow resistance increase and wellhead pressure rise. Thestudy has significant value on mastering gelling mechanism in oil-water two-phase system,the cause of high wellhead pressure and potential of energy-saving and cost-reducing ingathering&transportation system. In addition, it will provide experimental and theoreticalsupport for further optimization and simplification of oil gathering process and operatingparameters, and it can also supply technical support scheme for maintaining the efficient, lowconsumption, security, coordination and stable operating status in oilfield surface engineering. |
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