Optimal Selection and Development Optimization Technology for Sweet Spots in Low Rank Thick Coal Seams—Taking the Gilgalangtu Area in the Erlian Basin as an Example

The thickness of the low rank coal reservoir in the Jiergalangtu Depression of the Erlian Basin is 80–140 m, which is a typical thick brown coal seam with a gas content of 1–2 m3/t and a reservoir permeability of 10−5−10−4 md. The vertical heterogeneity is strong, and it is necessary to select the vertical sweet spot section for effective development; By comprehensively studying the characteristics of low rank coal reservoirs in the Erlian Basin and the production characteristics of early evaluation wells, combined with nuclear magnetic resonance centrifugation experiments, it was confirmed that the proportion of movable water in low rank lignite and low rank coal accounted for 3.17–10.29%, and the porosity reached 40%. Compared with high rank coal, it has high porosity and low permeability characteristics. Using core description, conventional and nuclear magnetic resonance logging, four different coal types in lignite, namely woody coal, silky coal, detrital coal, and mineralized coal, were predicted; The 62 coal seam of the Saihantala Group IV coal seam is selected as the geological engineering double sweet spot section for development. Research has shown that low rank coal has high porosity and water content. With the large-scale production of water during the development process, the permeability of coal seams continues to increase, and is affected by the difference in gas and water content, showing a distribution pattern of upper gas and lower water; The main controlling factors affecting the production of low rank coal single wells have been identified as gas content, reservoir pore structure, and fracturing effect. Unlike high rank coal reservoirs, which experience a rapid increase in gas saturation after drainage, depressurization, and desorption, low rank coal reservoirs exhibit a high water production rate and a gradual increase in gas saturation after desorption. Therefore, a reasonable drainage system has been optimized; By promoting and applying the development experience of the pilot test area in the Jiergalangtu Depression of the Erlian Basin, it provides certain reference for the development of China’s coalbed methane industry.