Study On Multiphase Flow Law And Support Technology Of Wellbore For Natural Gas Hydrate Depressurization Test Production

With the rapid development of the national economy,china's energy demand is increasing rapidly,and its external dependence is constantly improving.The supply of conventional energy has been unable to ensure the energy security of China.The development of unconventional energy,especially natural gas hydrates with huge reserves and energy density,is of great significance to meet the needs of domestic production and life,provide sufficient oil and gas supply,and ensure China's energy security.However,as an emerging energy source,natural gas hydrate extraction still faces many technical difficulties.Among them,flow assurance is the key to the commercial exploitation of natural gas hydrates.Multiple hydrate production tests in both offshore sea and permafrost around the world have shown that the hydrate re-forming and sand production is the main causes of wellbore blockage and flow difficulties.The re-forming of natural gas hydrate and its attachment on the pipeline will reduce the flow area and lead to pipe blockage,thus affecting the normal flow of natural gas,water and other fluids,seriously affect the progress of operations,and even endanger property and personnel safety.The sand particles produced from hydrate reservoir are micron-sized,which are mainly composed of clay?<4?m?and silt?4–63?m?.However,the existing sand exclusion system can only filter out sand particles larger than 44?m,particles smaller than 44?m will flow into the wellbore through the opening of the sand exclusion system.The deposition of these micron-sized sand in wellbore have led to premature termination of several hydrate production tests.Therefore,from the perspective of preventing wellbore blockage and ensuring flow safety,it is necessary to study the formation conditions of natural gas hydrate and the methods or mechanisms to suppress the hydrate re-forming,to study the laws of micron-sized sand migration,deposition and restart in depressurized mining wellbore,and to model the multiphase flow of hydrate-sand-water-gas,which are of great significance to ensure the flow safety of hydrate production wellbore.To deal with these problems,a variety of research methods such as literature research,laboratory experiments,theoretical modeling and solution,numerical simulation,etc.are used to carry out the research on multiphase flow law and support technology in the wellbore of gas hydrate depressurization test production.The specific research work is as follows:?1?According to the mathematical model of hydrate formation and inhibition,the effect of composition,temperature,pressure and water content on natural gas hydrate formation is calculated.The accuracy of the simulation is verified by the experimental data published by different scholars.The results show that ethane,propane,CO₂ and H₂S promote the formation of hydrates,and the promotion of propane is more obvious than ethane.N₂ has little effect on the phase equilibrium curve of hydrate and shows a slight inhibition effect.When the pressure is low,the pressure change has a great influence on the hydrate equilibrium temperature.When the temperature is high,the temperature change has a greater influence on the hydrate equilibrium pressure.There is a positive proportional relationship between free water content and hydrate formation content.Five kinds of single-component alcohol inhibitors and four kinds of salts and their compound two-components inhibitors all have an inhibitory effect on hydrate formation,and the inhibitory effect becomes more obvious as the content increases.With the increase of total content,the inhibitory effect of unit content of different types of single-component alcohol inhibitors show four laws:the performance is weakened,the performance is basically unchanged,the performance is first weakened and then enhanced,and the performance is enhanced;the inhibitory effect of unit content of different compound two-component alcohol inhibitors shows two laws:the performance first weakens and then strengthens,and the performance basically remains unchanged.Among the single-component inhibitors,triethylene glycol and ethylene glycol have a better inhibitory effect,and diethylene glycol and ethanol have a lower inhibitory effect.Among two-component inhibitors,ethylene glycol+triethylene glycol,methanol+triethylene glycol has a better inhibitory effect,and methanol+diethylene glycol,diethylene glycol+ethanol has a lower inhibitory effect.With the increase of total content,the inhibition effect of unit content of single component salt inhibitors shows two laws:performance enhancement and performance basically unchanged.The inhibitory effect of unit content of all compound two-component salts is enhanced.Ca Cl₂has the best inhibitory effect among single-component inorganic salts.The inhibitory effect of Ca Cl₂+Mg Cl₂ is the best in the two-component inhibitors.The performance evaluation results of single and two-component inhibitors show that the compounding can effectively improve the inhibition effect of some alcohols and inorganic salts.?2?The concept of sand deposition concentration ratio has been put forward.The values of the sand deposition concentration ratio of different sand particle sizes,different flow rates,different volumetric concentration of sand production?VCSP?,and different hole inclination are measured through laboratory experiments.It is found that the sand deposition concentration ratio increases with the increase of hole inclination.This variation trend is different with the cuttings transport in drilling.In addition,Results suggested that bigger sand-diameter,lower flow rate,smaller VCSP all lead to a higher sand deposition concentration ratio.By using single factor analysis method and nonlinear fitting technique,the experimental data are fitted and a model for predicting the sand deposition concentration ratio is proposed.In the process of modeling,many methods such as Akaike Information Criterion and Bayesian Information Criterion is introduced to optimize the model.Furthermore,the statistical error analysis of the model is carried out,and the statistical parameter analysis results verify the ability of the proposed model to accurately predict the sand deposition concentration ratio within the assumed conditions.The differences of solid transport during hydrate production,conventional oil&gas production and conventional rotary drilling are compared and discussed.?3?The sand bed height in wellbore is an important index to show the flow area and flow condition of wellbore.The fluid property parameters,solid property parameters and production parameters that affect the sand bed height are dealt with dimensionless method and composed into five dimensionless groups with practical physical meaning.The effect of these dimensionless groups on the dimensionless sand bed height is analyzed.The results show that dimensionless sand bed height increases with the increase of VCSP,dimensionless sand particle size and hole inclination,and decreases with the increase of Reynolds number,which are monotonous.A model for estimating the sand bed height is proposed based on Buckingham-?theorem and single factor analysis method.The predictions are in good agreement with the experimental data and the performance and accuracy of the proposed model are checked by statistical error analysis.The distribution and accumulation of micron sized sand particles after the formation of sand bed are analyzed.Considering the net weight,additional mass force,lift force,drag force,fluid pressure loss,adhesive force and electrostatic force acting on the particle,a critical velocity model for incipient micron-sized sand motion is established.?4?Taking the world's first offshore natural gas hydrate depressurization test production wellbore in 2013 as the background,CFD numerical simulation technology is used to simulate the micron-sized sand transport of C-spiral well sections and pipe diameter mutation section in the test wellbores.Micron-sized sand particles are mainly accumulated in the corner of the C tube section and helix tube section,and the deposition of sand particles decreases with the increase of flow rate.The sand particles deposited in the upper part of the helical tube section is more difficult to clean than the lower part.As far as pipe diameter mutation section is concerned,micron-sized sand particles are mainly accumulated in the location where the pipe diameter changes.On the slice with pipe diameter mutation section,along the radial direction,it shows that the volume fraction of micron-sized sand particles in the middle of the slice is low,and there is an area where the sand particles are heavily accumulated around the low volume fraction part,and the remaining part has a volume fraction between the above two.According to the distribution characteristics of sand volume concentration with flow rate in complex well section,a method for determining the critical non-deposition flow rate in engineering is proposed and the critical non-deposition flow rate of C-spiral well section and pipe diameter mutation section under three kinds of particle sizes and three kinds of concentrations is obtained.Based on the numerical simulation data,the models for predicting the volume concentration of micron-sized sand in C-shape spiral well section and pipe diameter mutation section are proposed respectively,which may provide a useful reference for judging the flow condition rapidly in complex pipeline.?5?Based on the conservation law,the mass transfer and energy transfer process among different components caused by gas phase,liquid phase and hydrate phase change are considered.A set of continuity,momentum and energy equations are established which can describe the complex multiphase flow in the pipe string during hydrate production.Combining with the hydrate test production conditions,the corresponding initial conditions and boundary conditions are given.The staggered grid scheme is used to discretize the grid of the pipe column channel.Based on the finite volume method,the numerical iteration format between different grid nodes in the channel is derived,and the complete solution process is given.