The core samples required for this study were collected from the Sa II reservoir of the Saertu Formation in the Naner Zone of the Saertu Oilfield, and the core depth is 930–943 m. A total of 10 core samples were taken, including five ASP flooding samples with an average air permeability of 0.638 μm² and an average porosity of 27.9% and five water flooding samples (before ASP flooding) with an average air permeability of 0.443 μm² and an average porosity of 26.9%. The lithologies are all oil-bearing siltstones with good reservoir properties.

Scanning electron microscopy (SEM) and X-ray diffraction methods were applied to the surface morphology, and whole-rock mineral and clay mineral analysis of the core samples was conducted to determine the pore structure and mineral composition of the cores and to explore the characteristics of changes in the pore structure and clay minerals of the cores before and after ASP flooding (Fang, et al., 2016; Zhu et al., 2021).

SEM images show that different types of clay minerals have different morphologies and distribution characteristics. Kaolinite is an alteration product of feldspar, and its morphologies are book-page-like, worm-like, and accordion-like. Kaolinite is mostly distributed in the intergranular pores in the form of pore filling (Figure 3A). Kaolinite is easy to move with fluids under the scouring of fluids because of the loose intracrystalline structure and then block and divide the pores and throats. Therefore, kaolinite is an important velocity-sensitive mineral. Illite is attached to the particle surface or filled in the intergranular pores in the form of leaves or silk hairs (Figure 3B). The leaf-like crystallites separate the pores into several fine pores, which increase the tortuosity of flow channels. The hair-like crystallites are easily washed away by water and then block pores and throats, which leads to the decrease of porosity and permeability of reservoirs. The illite smectite mixed layer is an intermediate product of montmorillonite transformed to illite, and its morphologies are burr and curl. It has three typical structures, namely, lamellar structure, honeycomb structure, and flocculent structure (Figure 3C). Compared with other clay minerals, the illite smectite mixed layer has very strong water sensitivity. Chlorite is often symbiotic with authigenic quartz, and its morphologies are needle-like, pompon-like, and rose-like. The chlorite is distributed in pores in the form of pore filling and pore backing. Generally, the needle-like chlorite is mostly filled with pore backing on the surface of the particles, while the pompon-like and rose-like chlorites are filled in the pores (Figure 3D). Chlorite can be transformed from minerals such as biotite, hornblende, and montmorillonite. However, the authigenic chlorite is generally rich in high-valence iron ions, which easily react with the acid solution (e.g., HCl) in the drilling fluid to cause precipitation and reservoir damage. Therefore, chlorite is an acid-sensitive mineral.

The 10 selected core samples were extracted and dried at first, and then the X-ray diffraction method was used for the quantitative analysis of whole-rock minerals and clay minerals. The analysis results are presented in Table 1. As shown in Table 1, the alkali in the ASP system affects the content of the rock-forming minerals and clay mineral rock. The total amount of quartz is reduced after ASP flooding, and the potassium feldspar content is also reduced, while the plagioclase (albite) content increases slightly. Furthermore, the total content of clay minerals decreases after ASP flooding; therein, the relative content of the illite decreases, but the relative content of the chlorite and illite smectite mixed layer increases. However, the variation of kaolinite content in different core samples is quite different. Kaolinite contents in some samples decrease, while they increase in others.

In addition, ASP flooding changes the chemical composition of the rock. The ASP system injected into the reservoir, especially the alkali, on the one hand, will dissolve the rock minerals (e.g., kaolinite and feldspar), thereby causing the elemental components (e.g., silicon and aluminum) in the rock minerals to be transferred from the solid phase to the formation water; on the other hand, the aluminum and silicon ions which are dissolved by the alkali will regenerate the crystalline precipitate of aluminosilicate in other places under certain conditions (Maggio et al., 2010; Wu et al., 2015a; Song et al., 2015).

The feldspar of rock-forming minerals and some kaolinites are mostly eroded by alkali in the ASP flooding system. As a non-expanded clay mineral, kaolinite is one of the most common authigenic clay minerals in sandstone reservoirs, and it is also an indicator mineral for feldspar dissolution (Xing, 1983). The feldspar can generate kaolinite under eluviation, and its chemical reaction is as follows: ( K , Na , Ca ) [ AlSi 3 O 8 ] + H + → Al 2 Si 2 O 5 ( OH ) 4 + ( K + ,  Na + , Ca 2 + ) +  Si 4 + .

The reaction equation for the dissolution of kaolinite by alkali is as follows: Al 4 [ Si 4 O 10 ] ( OH ) 8 + 8 OH − = 4 Al ( OH ) 3 ↓ + 4 SiO 3 2 − + 2 H 2 O .

Kaolinite can be transformed into albite under the action of alkali, and the reaction equation is as follows: Al 2 [ Si 2 O 5 ] ( OH ) 4 + 2 Na + + 2 OH − + 4 Si ( OH ) 4 = 2 NaAlSi 3 O 3 + 11 H 2 O .

It can be concluded that the increase of anorthose content after ASP flooding is due to the formation of albite from kaolinite under the action of alkali. The content of the illite smectite mixed layer is increased, which is the aluminosilicate scale formed by mineral recrystallization (Lu and Guan, 1999). There are two reasons for the increase in kaolinite content. One is the newly formed kaolinite by feldspar under eluviation, and the other is the precipitation and accumulation of kaolinite particles in formation water caused by the adsorption flocculation of the polymer in the ASP system (Liu et al., 2014; Mansa et al., 2017). The increase in chlorite content is an aluminosilicate precipitate formed by mineral recrystallization in an alkaline environment.

It can be seen from the SEM images of core samples before ASP flooding that the core intergranular pores include the primary intergranular pores and the secondary intergranular pores. The main cements in the core are clay minerals. Among the clay mineral cements, it can be seen from Figure 4A that the kaolinite cements in the pores in the form of pore-filling and blocks the pore throats; parts of illite and chlorite are cemented on the surface of the particles in the form of pore-backing to decrease the pore space, and the other part is cemented in the pores in the form of pore-filling and blocks the pore throats (Figure 4B).

It can be seen from the SEM images of core samples after ASP flooding that the clay minerals block the pore throat, and the pore structure coefficient and the tortuosity of pore throat increase after ASP flooding. The kaolinite and illite in the clay minerals adhere to the surface of the rock particles and extend into the pore space, thereby causing the shrinkage of the primary intergranular pore throat (Figure 5A). In addition, the local pore throat is enlarged due to the dissolution of clay minerals and rock particles by alkali (Figure 5B).

In addition, according to the analysis of the 3D nondestructive core sample pore structure obtained by CT scanning, the reservoir pore–throat ratio (ratio of pore radius and throat radius) and pore throat coordination number (throat number connecting each pore) decreased significantly after ASP flooding (Figure 6 and Figure 7). The reason for this phenomenon is that the alkali component in the ASP system erodes the clay minerals and rock particles (feldspar) in the reservoir during ASP flooding. The clay minerals and rock particles are chemically unstable, which are then dissolved by alkali migrating with the composite system, thereby increasing the diameter of the local pore throat (Loucks et al., 2012). In addition, the fine particles in reservoir fluids are easily adsorbed, trapped, and retained in the narrow throat by the polymer, causing throat blockage and reducing the effective communication in throats.

Figure 8 and Figure 9 show the physical properties of core samples before and after ASP flooding; we can conclude that the changing trends of porosity and permeability are consistent after ASP flooding. The average porosity of the core samples increases, which is 0.97% higher than the value before ASP flooding. Sixty percent of the core porosity increases by more than 28%, and the maximum value is 28.66%. The core permeability after ASP flooding is mainly distributed between 663.19 and 882.88 mD. Compared with that of the cores before ASP flooding, the permeability increases significantly, and the increment is 195.09 mD. The influence of ASP flooding on the physical properties of cores is mainly caused by the dissolution of rock skeleton, clay minerals, and cuttings by alkali components in the ASP system (Olajire, 2014). Part of the pore throats become large because of alkali dissolution and fine particle migration; therefore, the physical properties become better. However, some parts of pore throats become smaller, and the tortuosity of the pore throat increases because of the accumulation and precipitation of fine particles in the pore fluids, thereby leading to a decrease in porosity and permeability. The porosity and permeability of the reservoir are polarized and the heterogeneity is enhanced after ASP flooding.