Yan Jiwei, Song Xiaoxia
Key Laboratory of In-Situ Property-Improving for Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, People's Republic of China.
College of Mining Engineering, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, People's Republic of China.
Sci Rep. 2025 Jul 1;15(1):21440. doi: 10.1038/s41598-025-06745-9.
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.
了解多尺度水力压裂裂缝（HFs）的分布对于提高煤层气（CBM）产量至关重要。通过采煤工作面、X射线计算机断层扫描（CT）和立体显微镜揭示了从米、厘米到毫米范围的水力压裂裂缝。水力压裂曲线可分为下降型、水平型、上升型和波动型。与上升型和波动型相比，下降型和水平型表现出更有效的水力压裂效果。宏观裂缝主要为水平、垂直、X形和T形。靠近煤层气井眼处，宏观裂缝间距更近，且与各煤层的层面具有更强的连通性。在水力压裂过程中，压裂液沿煤层中的节理等薄弱面选择性扩张，从而形成主裂缝。在高压裂液压力下，水力压裂裂缝可穿透各种煤岩组分层，甚至穿过煤矸石。嵌入煤中的石英砂能引发毫米级的水力压裂裂缝并使其保持张开。多尺度水力压裂裂缝的发育受煤体结构、顶底板强度、地应力、水力压裂设计参数、石英砂和煤粉等因素影响。
