## 2 Materials and Methods  
### Materials  
In this study, samples of consolidated sandstones (clastic rock) confined to the Visean stage of the Lower Carboniferous deposits are used as a medium through which the fluid is filtered (Figure 1). The rock was extracted from a depth of 2 km during drilling with the coring of production wells at an oil field in the Perm Territory (Russia). The field has low reservoir pressure near the wells (below the saturation pressure of oil with gas), as a result of which oil degassing already occurs in the reservoir. The consequence of this is also the release of paraffins and asphaltenes from the oil, which block the rock in the near-wellbore zone of the reservoir during oil filtration into the well. When preparing the plugs, the API [36] standard was used, according to which the samples were drilled perpendicular to the core, i.e., parallel to the bedding, extracted on a Soxhet apparatus, and dried. In order to evaluate the mineralogical composition of the rock under study, an ample sample from the same collection was used to prepare a powder sample and a thin section that were then used for an X-ray phase analysis performed on a D2 Phaser Bruker diffractometer and for a petrographic analysis performed on a polarizing microscope, the Olympus BX51, respectively. In terms of X-ray phase determination, in the bulk composition of the rock, quartz predominates, potassium feldspar is dominant among minor minerals, and kaolinite is present among clay minerals. In terms of petrography, the rock is fine- to medium-grained. The elastic part of the sandstone is dominated by well-sorted quartz grains of 0.2-0.5 mm; less common are rock fragments (quartzites and single fragments of clayey rocks) and pelitized feldspars. The cement is predominantly regeneration quartz with the formation of conformal structures. Kaolinite is developed unevenly in the pores, poorly crystallized, and with an admixture of hydroma  
Figure 1: Sandstone plugs used in this study as filtering materials: medium permeability (224-24-14), high permeability (224-14-14), low permeability (224-3-14). indentation cementation, which is a cementless contact joint of grains, and a regeneration quartz joint of grains, which is characterized by a conformal structure. The pore space of the sandstone is unevenly represented by intergranular isolated and partially open hollow pores, presumably of secondary origin (formed by partial dissolution of a part of elastic fragments) of irregular shape, 0.08-0.5 mm in size. The mineralogical composition of the rock is given in Table 1. The microstructure of the rock is given in Figure 2.  
The primary reservoir properties of the rock matrix were determined. Porosity was measured by weighing a dry and a water-saturated sample in air and water, and permeability was measured by gas using the Darcy method on a filtration system. The characteristics of the samples are shown in Table 2.  
Crude oil containing paraffins and asphaltenes, obtained from wells in the same field as the samples, was used as a filtered liquid (Figure 3). The oil was additionally mixed to release residual associated gas. The density and dynamic viscosity of oil, measured at room temperature at 22 \({}^{\circ}\)C, were 836 kg/m3 and 3.3 mPa \(\times\) s, respectively, which is generally regarded as relatively light. The black color of oil is a characteristic of the majority of mature oil and gas fields in the Volga-Ural oil-bearing province and is associated with the presence of oil resins in addition to paraffins and asphaltenes. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Parameter** & **224-24-14** & **224-14-14** & **224-3-14** \\ \hline Lithology & Sandstone & Sandstone & Sandstone \\ \hline Permeability, mD & 117 & 606 & 35 \\ \hline Porosity, \% & 16.9 & 17.5 & 16.7 \\ \hline Pore volume, cm\({}^{3}\) & 3.7 & 3.8 & 3.6 \\ \hline Plug size, mm & \(\diameter 30\,l30\) & \(\diameter 30\,l30\) & \(\diameter 30\,l30\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of the samples.