| Northwest Sichuan Gas Field Qiongxi block is in the mid-late stage of gas development. The produced fluids contain a large amount of water with a high concentration of scale-forming ions. During produced water and gas flow in the gathering and transportation pipeline, the changes of pipeline temperature, pressure, gas-liquid phase velocity and other parameters, break chemical equilibria between the scale-forming ions. The chemical reaction between the scale-forming ions takes place to form crystals which can gradually deposit and are adsorbed on the internal surface of pipe. As a result, scaling problems in the pipe arise, which seriously affect the safe and efficient operation of natural gas gathering pipe network.In order to solve the scaling problem for Northwest Sichuan Gas Field gas gathering pipelines, based on thermodynamic, crystallization kinetics and fluid dynamics theory, experimental, theoretical and numerical simulation methods are used to study the scaling mechanism and antiscaling methods. The field experiments are made to verify the correctness of the theory and methods. The research achievements in this paper provide a theoretical and technical support for the development of gas gathering pipeline antiscaling program.Specific research contents and main results are as follows:(1) Through investigating the distribution of pipe network on-site in Northwest Sichuan Gas Field Qiongxi block, Qiongxi 6 well,12 well,14 well and Whitehorse 8 well are selected as a typical object of study. The water samples of Qiongxi 6 well,12 well,14 well and Whitehorse 8 wells, and the scale samples of Qiongxi 14 well and Whitehorse 8 well are collected. The liquid ion chromatography is used to analyze the ionic components in water samples. Analysis results show that water samples contain high concentrations of the scale-forming ions (HCO3-, SO2-, Ca2+, Mg2+, Ba2+, Sr2+). The scanning electron microscopy, high temperature lost weight method, Energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction analysis (XRD) are applied to analyze the micro morphology and the composition of scale samples. The results illustrate that calcium carbonate scale is about 85%, and corrosion scaling is 10% in scale samples of Qiongxi 14 well. Furthermore, calcium carbonate scale is about 85% and corrosion scaling is about 6% in scale samples of Whitehorse 8 well.(2) Based on the experimental data, the principles of chemical reactions and crystallization kinetics are used to study the scaling mechanism of gas gathering pipeline of Northwest Sichuan Gas Field Northwest Sichuan Gas Field. The chemical reaction between scale-forming ions (HCO3-,SO2-,Ca2+,Mg2+,Ba2+,Sr2+), the variation of thermodynamic conditions in the process of pipeline transportation, the generation of crystal particles and the adsorption of crystal grains lead to the scaling of the natural gas gathering pipeline.The saturation index, stability index and BP neural network are used to predict the scaling trend of the pipeline. Stability index and the saturation index do not consider the effect of salinity and velocity on scaling. Thus, the prediction results are not accurate. The method of scaling tendency prediction by using BP neural network is studied. The concentration, temperature, pressure, pH value, salinity and flow rate of the scaling ions are considered as the input parameters of BP neural network. Scaling tendency is considered as output parameter. Through repeated training, the optimal structure of neural network is determined, including the number of hidden layer nodes, threshold and weight. Predicted results show that BP neural network considers the effect of the scaling factors, and has an accurate prediction of the scaling tendency.(3) Scale in gas gathering and transportation pipeline of Northwest Sichuan Gas Field is a complex process, which involves chemical reaction, crystallization and adsorption effect. Combined with finite rate/eddy dissipation theory of chemical reaction flow model, Euler Lagrange method of particle discrete mathematical model, the numerical simulation model for the scaling rate of natural gas gathering and transportation pipeline based on chemical reaction flow is established. According to actual scaling data of the Northwest Sichuan Gas Field, based on the principle of chemical reaction, the scaling rate is calculated and is a constant, which lays a foundation for the accurate analysis of scaling law in the pipeline.Based on the established model, the formation and scaling process of CaCO3 particles in the straight pipe and the horizontal pipe of the gathering pipeline were simulated with FLUENT software. The results show that the deposition rate of solid dirt particles is negative correlation with pressure and the flow velocity, and is positive correlation with pipe temperature and particle concentration and deposition rate are positively correlated. There is no significant link between CaCO3 particle diameter and the deposition rate. In the horizontal bend pipe, the pressure of the pipe and the velocity of the medium has a negative correlation with the deposition rate of the solid particles. The bending ratio is positively correlated with the deposition rate. The formation and deposition of CaCO3 particles are revealed, which can promote the development of antifouling technology.(4) According to the experimental and theoretical research results, the physical anti scaling device based on electronic descaling technology is used to make field tests. The results show that the electronic anti scaling device can only prevent scaling in a short period of time, and can not be used for a long time in the field of anti-scaling. The research of chemical anti scaling technology is further carried out to solve this problem. The composition of water samples and scale samples is taken into account.11 kinds of scale inhibitors are screened out. The anti-fouling performance of scale inhibitor is evaluated from six aspects, including pH values, calcium ion concentration, alkalinity, adaptability of oxidizing bactericid, thermal stability, and sedimentary scale morphology. The single anti-scaling agent owned good scale inhibition properties is screened polyacrylic acid (PAA), hydrolyzed poly maleic anhydride (HPMA), maleic acid-acrylic acid copolymer (MA-AA), poly-aspartate (PASP) and poly epoxy succinate acid (PESA). In order to achieve optimal antiscale result, researches on multiple antiscaling agents blending formulations antiscale performance were carried out. The anti-scaling rate of the best scale inhibitor complex solutions can be up to 85% which is applicable to Qiongxi 6 well, Qiongxi 12 well, Qiongxi 14 well.(5) According to the results of scale inhibitor compound, the experiment schemes for acid washing and filling antiscale were designed. Furthermore, field experiments have been carried out in Qiongxi 14 well and White horse 8 well. The results show that the actual application effect of the scale inhibitor (hydrolysis of poly maleic anhydride (HPMA) and polyacrylic acid (PAA) according to 2:3 mixture) is very good in Qiongxi 14 wells. The filling scale inhibition agent can prolong the service life of the sewage pipe network. Compared with the test results of filling anti scaling agent and non filling anti fouling agent in White horse 8 well, the scaling rate of the pipeline was significantly decreased after the anti-scaling agent was added. As a result, the correctness of the theory and method is verified. |
PDF Full Text Request