Research On Prediction Models Of Phase State And Dynamics For Well Bore Sulfur Deposition In Gas Reservoir With High Sulfur Content

During the production and exploitation of gas reservoirs with high sulfur content, there always exist sulfur deposition and sulfur blockage, which can lead to the reduction of gas production and even will force the gas well stop product when serious. Change of well bore pressure, temperature, gas composition, gas flow state all may lead to sulfur deposition, affecting the normal production of gas wells. Sulfur deposition of gas wells with high sulfur content is a very complex process; however, the present study of sulfur deposition is still not deep enough. Only by further studying on the gas-liquid solid equation of state and phase behavior, the physical dissolution and chemical reaction mechanism of elemental sulfur, deposition mechanism of sulfur on the wall of well bore, and then establishing the appropriate forecasting model, that the reservoir well bore sulfur deposition dynamics can be predict, and the law of the well bore sulfur deposition can be known and understanded, and then which can be used to guide production.In this Dissertation, for the fluid phase behavior in gas reservoir with high sulfur content and wellbore sulfur deposition characteristics, by a variety of research methods, such as the phase theory, crystallization thermodynamics, kinetics and fluid mechanics, research on the equation of state, phase change prediction method, wellbore sulfur deposition mechanism and prediction methode have been carried out. The new gas-fluid-solid three-state equation— TPR equation which can effectively describe material’s gas, liquid and solid properties and phase behaviore, the three state change prediction modle for high sulfur content gas reservoir fluid with a chemical reaction, and the dynamic prediction model of sulfur diffusion deposition in wellbore have been established. Sulfur deposition dynamic prediction for the field well has been done, to explore the fluid phase behavior and the law wellbore sulfur deposition during in the process of gas well production. The main results are as follows:(1) The unified analysis TPR equation which can describe of the material gas-liquid-solid three-phase properties meanwhile has been established. The functions of pure substances’ parameters with temperature in TPR equation are established. By adopting the van der waals mixing rules, the parameters of the mixtures are determined. Through the TPR equation the thermodynamic’s expression equation of the components fugacity, the residual enthalpy and residual entropy are determined. Comparation of the forecast results of TPR equation to the experimental data and forecast results of the PR equation have been done.The results show that this equation can accurately reproduce the material’s gas, liquid and solid PT diagram, and forecast results of saturated material’s gas and the liquid property are better than that of the PR state equation, and can accurately predict the solid volume and variation character. Forecast results of the fluid density under different temperature and pressure are close to the forecast results of the PR equation and W.wagner, etc.’s state equation established based on the experimental data.(2) The gas-liquid-solid three-phase prediction model considerring the phase equilibrium and chemical reaction equilibrium has been established, and the solution of the prediction model has been put forward through hierarchical iterative phase and chemical reaction equilibrium calculations. This new model can be applied to predict many parameters, such as the phase diagram of high sulfur gas reservoir fluid, the existential state and content of the sulfur and other components, the solubility of the sulfur, and other volume features and thermodynamics properties. With the established three-phase phase prediction model, the solubility of the sulfur-hydrogen sulfide system, sulfur-methane system, sulfur-gas mixture system have been predicted, and the results are compared with the corresponding experimental data, the average error is about 5%. The results show that the model can be used to accurately predict multiphase complex phases of high sulfur natural gas. There exist various forms of coexistence areas in sulfur, natural gas and hydrogen system.(3) The mechanism of sulfur crystallization, diffusion and mass transfer, deposition and the mechanism of interaction between the sulfur particles and the pipe wall have been studied, from the point of the crystallization thermodynamics, kinetics and fluid dynamics.The critical nucleation prediction of the element of sulfur in Puguang Gas Field indicate that with the decrease of the temperature and pressure, the nucleation radius of the elemental of sulfur decreases and the nucleation rate increases. The critical nucleation radius of wellbore sulfur element in Puguang Gas Field is very small, which is between 0.55nm and 19.5nm. The nucleation rate is lower than I(m3.s)"’, and the possibility of precipitated sulfur particles from the wellbore is very small, the sulfur deposition is mainly heterogeneous crystal deposition on the pipe wall. The prediction about the critical capacity of carrying sulfur in Puguang Gas Field indicate that the maximum sulfur particle diameter the gas flow can carry increases with the decrease of the pressure and the increase of the temperature. And the maximum sulfur particles radius which can be carried is much higher than the critical nucleation radius of elementary substance sulfur. It showed that the gas flow can still carry them out to wellhead, even though sulfur particles appear. The stripping forecast of different scales of sulfur particles on the wall of well bore reveals that the smaller the particle, the needed gas flow stripping velocity is greater. The stripping velocity needed for the sulfur particles lower than 100μm, is much higher than the gas well flow rate in Puguang Gas Field. It declared that once if the sulfur particles deposited on the wellbore wall, gas flow will not able to stripp them.(4) The wellbore sulfur deposition dynamic prediction model in high-sulfur gas well has been built, which based on the Law of conservation of mass, momentum, consindering the mass transfer theory, three-phase phase theory and multiphase flow theory,. The equations are discretized through a special semi-implicit treatment method, by which the spatial derivation term in the equation is processed as implicit difference; while the non-derivation parameter is dealed as explicit difference.Iterative solution of differential equations for each control body is done by using the Marquardt method. The solving method for high-sulfur gas wellbore sulfur deposition dynamic prediction model is ultimately determined,(5)The forecasting results of well bore phase behavior and sulfur nucleation indicated that, as the decline of pressure and temperature from bottom of the well to wellhead, the sulfur’s solubility in gas decline. Above a position, the sulfur in gas is at the state of supersaturation. The original depth of sulfur precipitation will increase in gas well bore when the gas productivity decreases. The percentage of chemically dissolved sulfur to the total sulfur in gas that is not saturated decreasese with the temperature’s fall.The percentages of chemically dissolved sulfur to toal content of sulful in actual wells are usually very low. The percentage of chemically dissolved sulfur in bulk sulfur dissolved in gas is about 4%-8% in Pugugang gas field. The radius of nucleation of sulfur at the state of supersaturation is small. The critical radius of nucleation in foreign actual well A is about 1.4-16nm, while 3-34nm in Pugugang gas field and the critical formation velocity of nucleation in which is nearly 0. It can be concluded that even the element sulfur is at the state of supersaturation, the sulfur particles still could not be easily formed in the fluid. The sulfur deposition is mainly crystal deposition occurred on the tube wall, which is controlled by the diffusion deposition.(6)The dynamic forecasting results of sulfur deposition indicated that, from the bottom of well to wellhead, as the increase of deposition distance, the change of sulfur deposition speed presents two kinds of tend in actual wells:firstly increase and then decrease, and always increase. After the deposition, the flow radius decrease, the pressure and temperature gradient increase, gas flow speed increase and the maximum radius of particles that the gas flow can carry increase, meanwhile the gas flow speed that lead to the stripping of the particles on the wall decline, the critical nucleation speed of supersaturation fluid increase. As the increase of the productivity time, the increasing speed of sulfur deposition layer depth will increase, The deposition plugging time in foreign actual well A is 24years and 3 months, and it’s respectively 11 years and 4 month,9 years and 10 months in Puguang 104-1 well and Puguang 302-1 well. Under the same condition of a well, with the productivity decline, the original deposition position drops, at the same time the plugging time is later.