Wang Wei, Li Xinyu, Wei Zhikun, Xin Yuandan, Xiao Rong, Yang Hongxin, Chen Xiaoliang
College of Chemistry and Chemical Engineering, Yulin University, Yulin 719000, Shaanxi, P. R. China.
No. 1 Oil Production Plant, Petrochina Changqing Oilfield Company, Yan'an 716000, Shaanxi, P. R. China.
ACS Omega. 2023 Jan 17;8(4):3998-4009. doi: 10.1021/acsomega.2c06805. eCollection 2023 Jan 31.
CO flooding is an effective technique to enhance oil recovery from tight sandstone reservoirs. The CO-rich fluid reacts with the in situ minerals and results in the corrosion and precipitation of minerals in the sandstone, affecting the connectivity and morphological features of pores. However, the diagenesis of the tight sandstone is complex, and the structural modification behavior and mechanism in different lithofacies of tight sandstone during CO flooding are still unclear. This study combined CO flooding, thin-section casting, scanning electron microscopy, nuclear magnetic resonance, X-ray diffraction, and fractal analysis methods to investigate the changes in the pore structure of tight sandstone after CO flooding. The results show that the cement of the tight sandstone is complex and diverse. The tight sandstone can be divided into two types of lithofacies: clay cementation (CL) and ferrocalcite cementation (CA). The mineralogical alterations occur differently in each lithofacies of tight sandstone. Alterations in the cement minerals affect the pores morphology of tight sandstone depending on their mineralogical structure and texture, and the CO flooding mainly changes the micromorphology and heterogeneity of large pores. Clay minerals dominate the cement in the CL lithofacies of tight sandstone. The dissolution mainly occurs in small pores because the precipitation of new minerals and exfoliation of skeleton particles partially block large pores and transform them into small pores. Thus, the number of small pores of CL lithofacies increases while the number of large pores decreases. Such a phenomenon hinders the increase in porosity and permeability. On the other side, in the CA lithofacies of tight sandstone, the cement is dominated by ferrocalcite, and the dissolution of ferrocalcite is the primary mechanism of mineralogical alteration. As the ferrocalcite dissolution expands, it creates new flow paths, improving the connectivity of pores. The petrophysical properties of CA lithofacies improve significantly after CO flooding. There is a crucial need to study the changes in diagenetic characteristics of tight sandstone due to CO flooding. Such type of study provides insights related to the improvement and evaluation of the development of tight sandstone reservoirs during CO flooding.
CO驱油是提高致密砂岩油藏采收率的有效技术。富含CO的流体与储层中的矿物发生反应，导致砂岩中矿物的腐蚀和沉淀，影响孔隙的连通性和形态特征。然而，致密砂岩的成岩作用复杂，CO驱油过程中致密砂岩不同岩相的结构改造行为及机制尚不清楚。本研究结合CO驱油、薄片铸体、扫描电子显微镜、核磁共振、X射线衍射和分形分析方法，研究了CO驱油后致密砂岩孔隙结构的变化。结果表明，致密砂岩的胶结物复杂多样。致密砂岩可分为粘土胶结（CL）和铁方解石胶结（CA）两种岩相。致密砂岩各岩相的矿物学变化不同。胶结矿物的变化根据其矿物结构和纹理影响致密砂岩的孔隙形态，CO驱油主要改变大孔隙的微观形态和非均质性。粘土矿物在致密砂岩CL岩相的胶结物中占主导地位。溶解主要发生在小孔隙中，因为新矿物的沉淀和骨架颗粒的剥落部分堵塞了大孔隙并将其转化为小孔隙。因此，CL岩相的小孔隙数量增加而大孔隙数量减少。这种现象阻碍了孔隙度和渗透率的增加。另一方面，在致密砂岩的CA岩相中，胶结物以铁方解石为主，铁方解石的溶解是矿物学变化的主要机制。随着铁方解石溶解的扩大，形成了新的流动通道，改善了孔隙的连通性。CO驱油后CA岩相的岩石物理性质显著改善。迫切需要研究CO驱油导致的致密砂岩成岩特征变化。这类研究为CO驱油过程中致密砂岩油藏开发的改进和评价提供了见解。
