深水环境下含蜡油-气混输管线中蜡沉积特性及模型研究

Crude oil exploited from the offshore oil field in our country is mostly waxy. With the tendency of the offshore oil field exploitation to the deep sea, the intense heat transfer between oil-gas pipeline and the subsea environment in low temperature condition in the deep sea aggravates the wax deposit building up on the pipe wall. In order to avoid the wax block problem, at first the applicant carry out the wax precipitation characteristics in oil at different pressure and temperature, by the high pressure micro differential scanning calorimetry, and a thermodynamic model of the gas-liquid-solid three phases equilibrium for wax precipitation is developed. Then the flow, heat and mass transfer model is carried out to calculate the velocity, temperature and wax concentration distribution at axial, radius and circumferential direction under the oil-gas two-phase flow. Based on the wax deposit experiment in the oil-gas two-phase loop pipe flow, mechanism of wax deposition is indicated by exploring the quantitative effect of the complex flow, heat transfer and mass transfer on wax deposit. Based on the above research, a more accurate wax deposit kinetic model to predict the wax deposit thickness rate and wax content in deposit, considering the multiphase flow, thermodynamics gas-liquid-solid equilibrium, mass and heat transfer, crystallization kinetic, and molecular diffusion. It is considered that the theoretical bases and technical supports would be provided for the deep water oil and gas transportation, energy saving and reduction of consumption.

我国海上油田所产原油大多为含蜡原油。随着海上油田开采走向深海，深水低温、强换热环境使得含蜡油-气混输管道中蜡沉积问题十分突出，严重威胁着海底油-气输送管线的流动安全。本项目针对深水含蜡油-气混输管道中蜡沉积控制的需要，通过高压型差示扫描量热仪，揭示蜡在油-气体系中的析出特性，建立蜡析出的气-液-固相平衡模型；结合含蜡油-气混输管道的特点，建立相应的流动、传热、传质计算模型，得到流体轴向、径向和环向上的速度、温度及原油中蜡分子浓度的分布规律；借助含蜡油-气两相蜡沉积环道实验，得到蜡沉积层的分布特性，揭示复杂流动、传热和传质对蜡沉积影响的规律，阐明蜡沉积机理。基于上述研究，建立综合考虑含蜡油-气管流及传热传质特性、蜡析出特性、蜡分子结晶动力学及蜡分子扩散的耦合动力学模型，用以更加准确获得含蜡油-气混输管道中蜡沉积分布规律，为深水油气田开发中海底管道的流动安全及节能降耗提供科学依据。

随着海上油气资源的开发和海底含蜡原油的不断开采，多相流动中的蜡沉积问题已经成为多相混输技术面临的一个新问题。首先，基于经典的热力学相平衡理论，建立了预测蜡在原油中的析出特性的液-固相平衡计算模块。该计算模块采用状态方程计算液相的逸度，正规溶液理论模型计算固相活度系数，比较准确地预测出了模拟油和原油体系的析蜡点温度，同时计算了在不同温度下的蜡析出量的情况。.油-气两相流动过程中的流动、传热和传质特性密切相关，是一个非常复杂的过程，对蜡沉积的准确预测具有重要作用。本课题建立了双极坐标下油-气两相在管道中流动、传热和传质的控制方程，并给出了相应的流动、传热和传质的边界条件，流体湍流流动采用低Re数 两方程进行描述；采用有限差分方法对上述控制方程进行离散化处理，并对流动、传热和传质离散方程进行耦合迭代求解，得到了分层流中各相流体轴向、径向和环向上的速度、温度及原油中蜡分子浓度的分布规律。.最后，结合蜡分子扩散机理和蜡沉积过程中传热、传质现象的规律，建立了油-气两相分层流流型下蜡沉积预测模型，该模型能对蜡沉积层厚度的增长速率给出准确预测。此外，采用该蜡沉积模型对油-气分层流流型下的管道蜡沉积过程进行了分析，从影响蜡分子扩散质量流出发，给出了管壁截面蜡沉积层的厚度分布为新月形的根本原因，定量解释了蜡沉积量随着气相折算速度的增加而增大的趋势。