## 5 Summary and Conclusions  
Based on a variety of testing methods, this study studied the pore structure and permeability influencing factors of a large number of sandstone samples, and finally selected the five most representative permeability classification indicators to guide classified grouting. Based on this, the permeability of the three layers of sandstone aquifer that are the most widely distributed in the Southwest Shandong coalfield was classified and evaluated in detail, and suggestions for classified grouting were put forward. The main conclusions of this study are summarized as follows:  
1. Primary intergranular pores are common in the Jurassic sandstone (100% probability of appearance in the sandstone) and comprise 2.5-9.0% of the sandstone visual area. The Jurassic sandstone is dominated by necking pore throats (average pore throat radii of 0.74-4.93 um); thus, the sandstone has good connectivity, high porosity and permeability and a good correlation between porosity and permeability. In general, the Jurassic sandstone is a medium- to low-permeability sandstone. 2. The primary intergranular pores in the Permian sandstone are much lower in number (10-50% probability of appearance in the sandstone), comprise 0.1-3.5% of the sandstone visual area, and have a large deviation in their distribution. These pore features are an important reason for the low porosity and permeability of the Permian sandstone. The pore throats in the Permian sandstone are small and fine (average pore throat radii of 0.04-0.62 um) and feature laminated shapes and curved lamellar shapes. These pore throat features are another reason for the poor pore connectivity and low permeability. Therefore, the Permian sandstone is a tight sandstone with a low to ultralow permeability. 3. The direct factor affecting the permeability and groutability of sandstone is the pore structure of sandstone, and the main factors affecting the pore structure of sandstone are sedimentation and diagenesis. Sedimentation is an initial factor affecting the pore structure of deep-mine sandstone in the Southwest Shandong coalfield, and a comparative analysis of sandstone composition, structure and other characteristic indicators shows that the sorting coefficient is the most critical evaluation indicator reflecting the influence of sedimentation in this rock. diagenesis is a later but more important factor affecting the pore structure of deep-mine sandstone in the Southwest Shandong coalfield. Differences in the comprehensive diagenetic coefficient of two of the studied categories of sandstone are highly consistent with the differences in their pore structure and physical properties, which indicates that diagenesis has the most important impact on the pore structure and that the comprehensive diagenesis coefficient is the best evaluation indicator reflecting the intensity of diagenesis. 4. According to the five representative evaluation indicators, namely, permeability, porosity, average pore throat radius, comprehensive diagenetic coefficient and sorting coefficient, and using the multi-indicator comprehensive evaluation method based on permeability, the pore structure of deep-mine sandstone in the Southwest Shandong coalfield is divided into four subcategories (I\({}_{1}\), I\({}_{2}\), II\({}_{1}\), and II\({}_{2}\)) belonging to two categories (I and II); of these subcategories, I\({}_{1}\) has medium pores and pore throats that are suitable for grouting with microfine cement based slurry; I\({}_{2}\) has medium-small pores and fine pore throats; II\({}_{1}\) has medium-small pores and fine pore throats or small pores and microthroats; both I\({}_{2}\) and II\({}_{1}\) are suitable for grouting with chemical slurry; and II\({}_{2}\) has micropores and microthroats that are unable to be grouted. 5. Unlike fractures, sandstone pores have a complex structure and poor connectivity, which results in a large evaluation error by a single permeability indicator and great difficulties in pore permeation grouting.