# Research on an Equivalent Algorithm for Predicting Gas Content in Deep Coal Seams
## 3 The Gas Content of the Equivalent Layer
### Discussion on Algorithm for Calculating the Basic Gas Content of Equivalent Coal Seams  
There exist six distinct categories of equivalent layers, with the prediction of gas emissions during excavation encompassing both surrounding rock and coal seam gas. It is not necessary to consider the influence of surrounding rock gas in isolation, whereas the prediction of coal seam gas content ought to strive to mitigate the impact of surrounding rock gas. Based on Formulas (3) and (5), it can be definitively stated that:  
\[Q_{mj}=Q_{jh}-Q_{yj} \tag{8}\]  
\[Q_{mj}\approx Q_{jh}-q_{yy} \tag{9}\]  
(I) If the surrounding rock is clay or limestone with strong sealing properties, it can be considered that:  
\[q_{yy}=0 \tag{10}\]  
From Equations (5), (9) and (10), we can obtain:  
\[Q_{mj}=Q_{jh}=q_{my}+q_{mc}=q_{dj}+q_{mc} \tag{11}\]  
In the region where the adjacent rock comprises clay, the permeation capability of clay slurry into the coal mass, under hydraulic action during coal formation, surpasses that of mortar. This enhanced permeation facilitates an increase in the resistance against gas diffusion and flow within the coal, effectively augmenting its sealing properties. Consequently, coal seams and strata with clay-rich adjacent rocks exhibit higher gas concentrations compared to those with sandstone-dominated surroundings. Furthermore, clay layers of a certain thickness demonstrate a commendable sealing efficacy against gas diffusion, with this efficacy intensifying in proportion to the clay's thickness. (II) In the scenario where the adjacent rock is composed of fine sandstone characterized by low porosity, the effective porosity of this fine (silty) sandstone, while slightly higher than coal, possesses an exceedingly underdeveloped system of small pores, hindering effective gas storage. Consequently, the fundamental gas content within fine sandstone is substantially lower than that of coal. Hence, the emission component of the fundamental gas content in fine sandstone can be disregarded, stated as follows:  
\[q_{yy}\approx 0 \tag{12}\]  
From Equations (5), (9) and (12), we can obtain:  
\[Q_{mj}=Q_{jh}=q_{my}+q_{mc}=q_{dj}+q_{mc} \tag{13}\]  
(III) In the case where the surrounding rock comprises medium-fine and medium-coarse sandstone with significant porosity, the gas emission emanating from its gas content cannot be overlooked. To ascertain the fundamental gas content of the equivalent seam, it is imperative to subtract the emission emanating from the pertinent sandstone. From Equations (6) and (9), we can obtain:  
\[Q_{mj}\approx\left(q_{yy}+q_{my}+q_{mc}\right)-q_{yy}=q_{my}+q_{mc} \tag{14}\]  
This is equivalent to the formula for the area where the surrounding rock is fine sandstone, therefore:  
\[Q_{mj}\approx q_{my}+q_{mc}=q_{dj}+q_{mc} \tag{15}\]In actuality, upon the computation of the fundamental gas content within the corresponding seam, and the subsequent deduction of the emissions emanating from medium-fine and medium-coarse sandstone, the resulting figure aligns precisely with the fundamental gas content of the corresponding seam of fine sandstone. This underscores the fact that, irrespective of the thickness of the sandstone, the baseline gas emissions of the equivalent coal seam layer can be deemed equivalent to those of the fine sandstone layer. Although this value is of an approximate nature, the predictive outcomes are deemed satisfactory in fulfilling the precision criteria for practical application. Thus, the area equivalent to predicting coal seam gas content can be categorized into sandstone and clay zones. By eliminating the impact of medium-coarse sandstone within the sandstone zone, the fundamental gas emission and its influencing factors are incorporated into the calculations using fine sandstone that has an insignificant gas content.