Hydrate Prediction Of Long Distance Gas Pipeline

During the process of extraction,processing and transportation of oil and gas,when to meet certain conditions of temperature and pressure,the natural gas will be in contact with liquid water in the pipeline, thus forming a cage-type snow-like solid,which is the so-called"Gas Hydrate". At present, most of the large gas fields discovered in China are disoersed in remote areas of Shaanxi and Gansu, Sichuan and so on. And from the economical point of view, using pipelines to transport natural gas seems to be a better choice. But the high-pressure, low temperature condition is a favorable element for the formation gas hydrates. Once the gas hydrate formed, they will either attach to joint section of pipeline or gather up at the bottom of pipeline. A small amount of gas hydrate wil reduce the transportation diameter, the flow rate of natural gas in the pipeline will also decrease. With hydrates continue gathering up, they will cause blockage of gas pipeline, and even can cause the pipeline to explode which brings huge loss of capital and casualty. Therefore, in order to prevent and reduce the high risk of gas hydrate formation in pipelines and facilities, the primary task is to develop a range of physical and analytical methods to detect and predict the gas hydrate formation before it starts to form. It is hoped to make well-done preparations and to reduce the adverse consequenses as a result of gas hydrate formation in pipelines.Currently,schloars from both domestic and abroad are mainly focused on the induction and classification of existing hydrate forecasting models, or analyzing the effect of theimodynamic inhibitors and kinetic inhibitors based on experiments. Few people come up with a reliable gas hydrate forecasting model for long-distance gas pipeline. Existing models for predicting gas hydrate in pipelines are simly using the inlet pressure of pipeline. However,pressure and temperature of gas pipeline varies with distance which makes it inaccurate to make hydrate prediction only based on the inlet pressure.In this paper, the existing kinetic models,thermodynamic models and empirical models are briefly introduced. At the meantime,choosing Chen-Guo, which has the highest precision among these models as the basic equilibrium model for gas hydrate prediction in pipeline. Secondly,calculate the temperature and pressure distribution along the pipeline with MATLAB, based on the hydraulic and thermodynamic characteritics of gas pipieline. And also draw the curves of temperature and pressure distribution. Ultimately, to analyse whether there is going to be gas hydrate formation and where is can form in the pipeline with systematic analysis of the critical temperature curve,pressure distribution curve long the pipeline and the measured dew point of natural gas.Then the accuracy of softmare is examined through numerical examples. Results show that the proposed model in predicting gas hydrate formation has a tolerable precision and it can be applied to engineering practice. In addition to gas hydrate prediction, is also discussed different temperature and pressure distribution under different flow rates、inlet pressures and pipeline diameters. Then discusses their influences on gas hydrate formation and comes up with measures to prevent gas hydrate formation theoretically.Secondly, the traditional way to prevent hydrate formation is to remove the environmental or chemical factors which support the formation og gas hydrate. For example, to eliminate the chance of gas pipeline operating at a low temperature by insulating and external heating. Ethylene glycol can also be used to dehydrate natural gas. Besides, reducing the operation pressure of production and transportation system can also reduce the possibility of gas hydrate formation. However, these conventional methods are not always feasible. For example, due to the limitied operating space in offshore and deep-water environment, the cost of insulation and heating methods is huge. It is usually $1000,000 per kilometer in deepwater insulation. Therefore, there are a lot of limitations for these conventional methods.Another common used method to prevent hydrate formation is to add thermodynamic inhibitors into gas. Thermodynamic inhibitor is a water-soluble kind chemical which can reduce the activity of water, improve the hydrate ohase boundary pressure or lowering the temperature. Among which, methanol and ethylene glycol are commonly used in industry. However, dur to the large amount of usage, they can cause serious environmental problems. So the thermodynamic inhibitors are of low feasibility from the perspective of sustainal development.In the last chapter, both kinetic and thermodynamic inhibitors are briefly discussed and the possible future directions are outlined.