Clay mineral content and their impact on reservoir quality: Insight into the carbonate reservoirs oil fields, Central-Eastern Russia

Clay minerals are widespread in both sandstone and carbonate reservoirs. Previous studies have emphasized that clay minerals influence oil filtration in sandstones; however, very few studies to date have examined the influence of clay minerals in carbonates on oil production. To address this, scanning electron microscopy images, X-ray diffraction, nuclear magnetic resonance, and filtration experiments with low-salinity water flooding are used in this study to investigate the presence of clay minerals. Based on the XRD data, the mineralogy of cement in the carbonate reservoirs of the two fields (Perm Krai, Russia) is characterized by the presence of both non-clay minerals (quartz, feldspar, anhydrite, and halite) and clay minerals (illite/smectite, fibrous illite, and kaolinite). Illite/smectite flakes occur mainly as small platelets (<0.2 μm), which are distributed over the surface of the calcite crystals. However, individual illite/smectite flakes were identified, reaching 50 μm in width and ∼5 μm in thickness. The NMR results indicate a significant amount of clay-bound water is present in core samples containing illite/smectite flakes and isolated examples of fibrous illite. Kaolinite and fibrous illite readily disperse when interacting with low-salinity water, allowing clay particles to migrate into small pores after dispersion. The dispersion and migration of kaolinite and fibrous illite contribute to a significant decrease in permeability compared to rocks containing only illite/smectite. Thus, studying the mineralogy and chemical composition of clay minerals is an urgent task for hydrocarbon production from carbonate reservoirs. The presence of clay minerals must be determined when selecting process fluids during operations and water for injection in reservoir pressure maintenance systems.