Study on the Variation Characteristics of Rock Mechanical Parameters of Coal Seam Roof Under the Influence of Artificial Fractures
Authors: Zhou H. , Zhang L. , Shi G. H. , Liu L. , Zhou F. J. , Wang B. , Wang W. C. , Liu Y. Y. 
Published: August 2024

ABSTRACT:
Coal seam mining poses serious safety risks due to roof ground pressure. Hydraulic fracturing effectively mitigates this threat. This study focuses on understanding the mechanical characteristics of fracturing and softening the roof. We obtain sandstone roof cores through downhole coring and conduct true triaxial hydraulic fracturing experiments along with CT scanning to identify various types of artificial fractures. The main conclusions are as follows:(1)After hydraulic fracturing, low-cementation natural fractures are easily activated, inducing localized micro-fractures in sandstone with large grain areas;(2) Post-fracturing, fracture morphologies include localized micro-fractures, single artificial fractures, activation of natural fractures by single artificial fractures, single artificial fractures not extending to natural fractures, and intersecting fracture networks. (3) The tests of rock mechanical parameters show that the compressive strength and Young's modulus of multi-fracture intersecting fracture network specimens are reduced by more than 40%, the compressive strength and Young's modulus of single artificial fracture activated natural fracture specimens are reduced by 10%-30%, (4) Stress-strain curve results indicate that more complex artificial fractures lead to lower residual strength and greater expansion post-destabilization. This study contributes to the theoretical basis for efficient coal seam roof management and process optimization in the petroleum domain.

1. INTRODUCTION
With the increasing demand for coal and the increase in mining intensity year by year, shallow coal resources have been on the verge of depletion, and coal mines have successively shifted to deep mining (Xue et al.,2021). In recent years, coal mine safety accidents have been effectively controlled due to the continuous improvement of coal mining equipment and technology level, as well as the increasingly perfect safety supervision system. However, among the various types of coal mine disasters, the roof fall has many triggering factors and is easy to burst, and difficult to prevent and control, resulting in that it is still an important factor restricting the safe production of coal mines (Hirata et al.,2007). Due to the complex geological conditions and stressful environment in the deep mines, the number, frequency, and intensity of high-stress concentration mines have greatly increased, and at present, impact ground pressure has seriously affected the personal safety of workers and threatened the safe and efficient production of coal mines (Pan et al.,2021). Moreover, the greater the mining depth of the coal mine, the more complex the conditions of the coal seam deposition, and the thickness and complexity of the immediate roof gradually increase, and the proportion of the roof plate accounted for more than one-third of the strong or competent overburden roof (Wang et al.,2016), and the hard roof plate has high strength, Joints Hard roof plate has high strength, non-developed fractures, strong self-carrying capacity if it is difficult to collapse in time, it will store a large amount of elastic energy gathered in the roof plate when the area of the overhanging roof is too large and exceeds a certain limit, a large area will collapse, resulting in an instantaneous release of elastic energy of the roof plate, which will cause great safety risks to the production of the mine (Fredericks et al.,1985). Therefore, the effective prevention and control of roof disasters and the monitoring and forecasting of impact ground pressure and prevention technology and other safety means are still the technical difficulties that need to be studied in depth for the development of the coal industry (Pan et al.,2019). As a basic method to prevent and control large-scale roof collapse, hard roof weakening treatment can reduce the cycle of initial pressure and pressure step by destroying the integrity of the roof strata, weakening the pressure, and effectively preventing rock bursting (Wang et al.,2009). At present, the main hard top weakening treatment methods are blasting (Mazaira et al.,2015). and hydraulic fracturing, of which the blasting method is only applicable to low-gas mines, which can lead to serious underground air pollution, it is difficult to deal with explosion prevention and residual explosions in shallow burial situations, and it poses a certain threat to the safety of the ground and the surrounding environment (Huang et al.,2016). Hard roof hydraulic fracture weakening treatment method is mainly through the overlying roof drilling hole in the predetermined height of the groove, through the sealing of the hole and water injection to make high-pressure water fractured rock destruction along the grouting (Huang et al.,2013). Compared with the blasting method, hydraulic fracturing can effectively improve the weakening effect and cause less damage to the straight top, which makes up for the blasting control technology and has a wide range of applications. Based on this, mastering the bearing capacity of the surrounding rock structure and the weakening mechanism is crucial for maintaining the overall stability of the mine, and the hydraulic fracturing technology of the roof plate is the key to the weakening of the hard roof plate.