Asphaltene deposition is a common phenomenon during CO₂ flooding in ultralow permeability reservoirs. The deposited asphaltene occupies the pore volume and decreases permeability, resulting in serious formation damage and pore well productivity. It is urgent to investigate the asphaltene deposition mechanisms, adverse effects, and preventive measures. However, few asphaltene deposition investigations have been systematically conducted by now. In this research, the asphaltene precipitation mechanisms and adverse effects were comprehensively investigated by using experimental and numerical methods. To study the effects of pressure, asphaltene content, and temperature on asphaltene precipitation qualitatively and quantitatively, the microscope visible detection experiment and the PVT cell static experiment were firstly conducted. The adverse effects on porosity and permeability resulted from asphaltene deposition were also studied by the core flooding experiment. Secondly, simulation models of asphaltene precipitation and deposition were developed and validated by experimental data. Finally, a case study from Changqing oil field was presented to analyze the asphaltene deposition characteristic and preventive measures. The experimental results showed that the asphaltene precipitation increases with the increased pressure before reaching the minimum miscible pressure (MMP) and gets the peak value around the MMP, while decreases slowly. The asphaltene precipitation increases with the increased temperature and asphaltene content. The variation trend of adverse effects on porosity and permeability resulted from asphaltene deposition is similar to that of asphaltene precipitation under the influence of pressure, asphaltene content, and temperature. The case study shows that the water-altering-gas (WAG) with high injection rate suffers more serious asphaltene deposition compared with the WAG with low injection rate, for the asphaltene precipitation increases as the increased pressure before reaching the MMP. The CO₂ continuous injection with high injection rate is the worst choice, for low sweep efficiency and the most severe formation damage. Thus, the WAG with optimal injection rate was proposed to maintain well productivity and to reduce formation damage resulted from asphaltene deposition during developing ultralow permeability reservoirs.

中文翻译：

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超低渗透油藏CO2驱油过程中沥青质沉积——以长庆油田为例

沥青质沉积是 CO ₂过程中的常见现象超低渗透油藏注水。沉积的沥青质占据孔隙体积，降低渗透率，造成严重的地层破坏和孔隙井产能。迫切需要研究沥青质沉积机制、不良影响和预防措施。然而，到目前为止，很少有系统地进行沥青质沉积研究。本研究采用实验和数值方法对沥青质沉淀机理和不利影响进行了综合研究。为了定性和定量研究压力、沥青质含量和温度对沥青质沉淀的影响，首先进行了显微镜可见检测实验和PVT电池静态实验。还通过岩心驱替实验研究了沥青质沉积对孔隙度和渗透率的不利影响。其次，通过实验数据建立和验证了沥青质沉淀和沉积的模拟模型。最后，以长庆油田为例，分析沥青质沉积特征及预防措施。实验结果表明，在达到最小混相压力（MMP）之前，沥青质沉淀随着压力的增加而增加，并在MMP附近达到峰值，而缓慢下降。沥青质沉淀随着温度和沥青质含量的增加而增加。在压力、沥青质含量和温度的影响下，沥青质沉积对孔隙度和渗透率的不利影响变化趋势与沥青质沉淀相似。案例研究表明，与低注入量的 WAG 相比，高注入量的水变气 (WAG) 遭受更严重的沥青质沉积，因为在达到 MMP 之前，沥青质沉淀随着压力的增加而增加。公司₂高注入率的连续注入是最差的选择，因为波及效率低，地层破坏最严重。因此，提出了具有最佳注入速率的 WAG 以保持井产能并减少开发超低渗透油藏期间沥青质沉积造成的地层损害。