超临界CO2缝内携砂机理研究

Supercritical carbon dioxide (SC-CO2) fracturing is considered to be an effective new method for non-aqueous fracturing in unconventional oil and gas production. Because of the SC-CO2 low viscosity, however, and physical parameters such as viscosity and density change with temperature and pressure is big, resulting in the construction process of the proppant carrying difficulties and sand plug at the crack tip and other issues frequently. Therefore, in order to solve the key scientific issue of sand carrying mechanism in fractures during SC-CO2 fracturing, the following researches are planned to perform: 1) Theoretically and experimentally study on the sedimentation of solid particles in SC-CO2 fluid, to reveal the effects of the key parameters such as solid particle density and sphericity on the sedimentation characteristics of the particles, and obtain the sedimentation model of particles in SC-CO2 fluid. 2) Considering the complex fracture morphology of SC-CO2 fracturing and the correlation characteristics of parameters such as temperature, pressure, density and viscosity of SC-CO2, to establish a physical model and mathematical model for sand carrying of SC-CO2 in fracturing cracks based on the above-mentioned sedimentation model, and using numerical simulation method to reveal the law of SC-CO2 sand carrying in the fractures; 3) Experimentally study on the influence of key parameters such as temperature, particle size and displacement on the sand carrying behavior of SC-CO2 in the fracture, by comparing the results of numerical models and experiments, to reveal the mechanism of sand-carrying in SC-CO2. The results can provide a theoretical basis for the efficient development of unconventional oil and gas resources technology for SC-CO2 fracturing.

超临界CO2压裂被认为是非常规油气增产的高效无水压裂新方法。但超临界CO2黏度低，黏度和密度等参数变化大，导致施工过程中缝内支撑剂携带困难以及缝端砂堵问题频出。针对超临界CO2缝内携砂机制这一关键科学问题，计划开展如下基础研究：1）理论和实验研究固相颗粒在超临界CO2流体中的沉降特性，揭示颗粒密度、圆球度等关键参数对其沉降特性的影响规律，得到固相颗粒在超临界CO2流体中的沉降模型；2）考虑超临界CO2压裂的复杂裂缝形态，结合超临界CO2温度、压力、密度、粘度等参数的关联特性，以上述沉降模型为基础，建立超临界CO2压裂缝内携砂物理和数学模型，并采用数值模拟的方法，研究揭示缝内超临界CO2携砂运移规律；3）实验研究温度、粒径、排量等关键参数对超临界CO2缝内携砂的影响规律，对比数模与实验结果，揭示超临界CO2缝内携砂机理。研究结果可为超临界CO2压裂高效开发非常规油气藏技术的形成奠定理论基础。

超临界CO2压裂是一种新型无水压裂技术，被认为是页岩油气、煤层气等非常规油气高效开发的重要手段之一。然而，由于超临界CO2流体黏度低、压裂裂缝形态复杂等因素，在加砂过程中易发生砂堵，难以有效支撑裂缝。本项目围绕超临界CO2压裂中支撑剂颗粒沉降规律、运移机理等基础问题，采用理论分析、室内实验和数值模拟相结合的方法展开了系统研究。首先，搭建了超临界CO2颗粒沉降实验装置，进行了超临界态CO2环境中颗粒沉降速度测试实验，揭示了颗粒在超临界CO2流体中沉降规律；随后，基于自主研制的超临界CO2缝内携砂实验系统，实验揭示了支撑剂颗粒的运移机制和砂堤形成的特点，明确了关键泵注参数对支撑剂运移的影响规律；然后，采用CFD-DEM方法建立了超临界CO2携砂运移模型，基于颗粒级的可视化分析从介观的角度揭示了支撑剂在平面裂缝中的运移机制；最后，基于超临界CO2致裂岩石裂缝面的粗糙特征生成粗糙裂缝模型，揭示了粗糙裂缝内支撑剂的运移和铺置特征，明确了关键参数对支撑剂在裂缝内输送和分布的影响规律。研究结果为油田现场压裂设计提供理论参考。相关成果在《Petroleum Science》、《International Journal of Heat and Mass Transfer》、《Journal of Petroleum Science and Engineering》、《Journal of Natural Gas Science and Engineering》、《石油学报》、《天然气工业》等期刊发表论文20篇，其中SCI检索13篇、EI检索6篇，行业TOP期刊6篇；合作出版专著1部；已授权发明专利7项，申请发明专利2项；在国内外学术会议公开报告6次；培养博士研究生6名、硕士研究生2名，其中与德州农工大学、路易斯安那州立大学、阿尔伯塔大学等知名高校联合培养博士生3名。基于项目研究成果，培养的博士生获蔡司显微技术奖学金1项、发表论文获《石油学报》2022年度十篇高影响力论文，个人获得绿色矿山科学技术一等奖、中国石油和化工自动化应用协会科学技术奖一等奖、中国石油和化学工业联合会青年科技奖各1项。