Abstract
Understanding the distribution of multi-scale hydraulic fractures (HFs) is critical to improving coal bed methane (CBM) production. The HFs range from metres, centimetres to millimetres are revealed through coal mining face, X-ray CT, and stereoscope. The hydraulic fracturing curves can be categorised as descending, horizontal, ascending and fluctuating. While descending and horizontal types exhibited more effective hydraulic fracturing compared with ascending and fluctuating types. Macroscopic fractures are predominantly horizontal, vertical, X and T shaped. Closer to the CBM wellbore, the macroscopic fractures are more closely spaced and show greater connectivity with the bedding planes of various coal rock layers. During hydraulic fracturing, the fracturing fluid expands selectively along weak surfaces, such as joints in coal seams, leading to the formation of main fractures. Under high fracturing fluid pressure, HFs can penetrate various maceral specification layers and even propagate through coal gangue. Quartz sand embedded in the coal can trigger millimetre-scale HFs while remaining open. The development of multi-scale HFs is influenced by factors such as coal structure, the roof and floor strength, geo-stress, hydraulic fracturing design parameters, quartz sand and coal fines.