蜡和沥青质协同作用下原油管道固相沉积机理与规律研究

Most crude oils produced in our country has a high wax content. Meanwhile, the production of such waxy crude oil in the other countries also keeps increaing. Consequenlty, wax deposition has always been regards as one of the major problems for crude oil pipeline flow assurance. In this reaserch, one solid evidence was presented that the pipeline deposit contains a certain amount of asphaltene and other heavy components except wax. However, the influence of asphaltenes on wax depositon has nvever been considered in the wax deposition predictive models developed previously. Meanwhile, the research on asphalte deposition also neglect the simultaneously happed wax deposition. According to a comprehensive understanding of this problem, this research project can be conducted through three procedures. First, we are going to explore the influence of precipitated asphaltenes on the wax precipitation kinetics and the gelation kinetics of model oils. This relationship between precipitated asphaltenes concentration and wax precipitation curve can then be used to characterize the wax precipitation in pipeline flow. Second, based on the cold finger experiments, the effect of precipitated asphaltene on the radial mass flow of wax molecules can be adequately identified. Hence the effective mass transport coefficient for wax molecules can be back calculated. Third, combining the knowledge of velocity and temperature profile in a pipe flow, wax precipitation kinetics and mass transfer coefficients affected by precipitated asphaltene, the solid deposition predictive model can be developed using a reliable mathematical method. This solid deposition predictive model will be a helpful tool for crude oil pipeline flow assurance.

我国所产原油大多含蜡量较高，国外含蜡原油的产量也在迅速上升。蜡沉积一直是原油管道流动保障的主要问题之一。一个基本事实是，管壁沉积物中除了蜡，还有不少的沥青质等重组分。然而，迄今为止对蜡沉积的研究及所建立的模型都没有明确地考虑沥青质的因素，而对沥青质沉积的研究又忽略了蜡的存在。本项目针对这一问题，研究蜡与沥青质协同作用条件下原油管道中蜡与沥青质沉积的机理与规律，包括沥青质颗粒对蜡分子结晶过程及原油胶凝过程的影响规律及其机理，蜡与沥青质协同作用条件下蜡分子的径向扩散过程；通过耦合管流条件下的速度分布、温度分布、蜡与沥青质的浓度分布、沉积层中固相含量的增长速率、以及管壁处剪切力对沉积层的冲刷作用，建立管流条件下蜡与沥青质沉积的预测模型，发展原油管道固相沉积理论，为原油管道流动保障提供支持。

蜡沉积一直是原油管道流动保障的主要问题之一。管壁沉积物中除了蜡，还有不少的沥青质等重组分。本项目研究了蜡结晶、原油胶凝和固相沉积过程中考虑沥青质的作用，提出了两者之间的协同作用方式，取得了以下成果：（1）采用离心分离和显微观察相结合的方法，以聚集态沥青质质量随时间的变化关系，定量表征了沥青质的聚集动力学过程。（2）明确了分散态沥青质和聚集态沥青质对蜡结晶及胶凝过程的影响。分散态沥青质抑制蜡分子析出，但作为均匀分散在空间中的蜡分子结晶核心，能够帮助蜡晶网络向整个空间扩展，加速原油的胶凝过程；聚集态沥青质能够作为蜡结晶核心，促进蜡分子析出，弱化蜡晶网状结构强度，延缓原油胶凝结构建立。（3）引入XRD测试技术，将蜡晶的研究深入到分子层面。采用蜡晶的当量平均碳数（由晶胞参数c换算得到）与实际平均碳数之差Δn表征晶体结构中分子构象的无序程度。蜡晶中正构烷烃多分散性越强，Δn越大，分子构象越无序，屈服应力越小。将该方法延伸到沥青质对于凝胶屈服特性影响的研究，发现随沥青质浓度的提高，颗粒由分散到聚集的变化过程中，蜡晶构象发生变化，凝胶屈服应力发生先增大后减小的变化趋势。（4）建立了能够准确测量分子扩散系数的泰勒分散法实验装置，定量研究了温度以及浓度对蜡分子扩散系数的重要影响，发现最常用的经验公式（H-M经验式）低估了蜡分子的扩散质量流。（5）明确了在蜡沉积过程中，沥青质浓度对沉积层增长速度的影响。随沥青质浓度的增加，分散态沥青质逐步转化为聚集态沥青质，改变过饱和蜡分子的结晶动力学常数，造成沥青质对蜡沉积速率的影响存在“临界浓度”，以此为界，蜡沉积速率呈先增大后减小的变化趋势。（6）将蜡沉积模型中过饱和蜡分子结晶动力学常数与沥青质浓度相联系，改进了固相沉积预测模型。并建立了连续釜冷指蜡沉积实验装置，模拟实际管流条件下的固相沉积过程，为模型验证提供数据。