Characteristics And Distribution Prediction Of Coal Reservoir Fractures In Complex Geological Structure Area

The Liupanshui area is positioned at the intersection of the Tethys and Coastal Pacific tectonic domains,and has undergone various tectonic movements.The intricate distribution of fractures within the coal reservoir has a significant impact on the conveyance of coalbed methane.With consideration to the regional tectonic processes,field geological surveys,geological data,literature analyses,and physical experiments were conducted to deeply examine the fracture development features within the Liupanshui area.This study predicts the distribution of fractures within the Longtan Formation coal reservoir in the Liupanshui area through finite element numerical simulation.The resulting analysis yields the following key findings:(1)The Liupanshui area has experienced significant tectonic impacts during both the Yanshan and Himalayan cycles,with the most prominent effects observed.Specifically,the Yanshan period precipitated the formation of fractures in the area,and the Himalayan period subsequently reformed these fractures.(2)The tectonics has a great impact on the gas content of coal reservoir,and the strong tectonics is easy to cause abnormal gas content of coal reservoir.In this region,the mercury pressure curve of coal can be divided into two categories.The characteristics of the first type of mercury injection curve are low inlet saturation and narrow pore hysteresis loop,while the characteristics of the second type of mercury injection curve are high inlet saturation and large volume difference between inlet and outlet mercury.The uniaxial compression test shows that the failure is divided into four different stages: pore crack compaction,elasticity,plasticity,and failure stage.The ancient tectonic stress test shows that the tectonic stress during the Yanshan period was 0.03 MPa,and the tectonic stress during the Himalayan period was 25.64 MPa.(3)Based on field geological investigation and analysis of logging data,the characteristics of macro fractures in the Longtan Formation are analyzed.The macro fractures are mainly distributed by structural shear fractures,with a small number of tensile fractures.The stratum dip angle is relatively large,and the through layer shear fractures are relatively developed.Through thin section identification and scanning electron microscope experiments,the characteristics of micro cracks in the coal rocks of the Longtan Formation were obtained.The study found that the micro cracks in the coal rocks were relatively developed,with a high degree of filling,and most of the cracks extended along the bedding direction.There are a large number of micro cracks and corrosion pores inside,which play a positive role in the adsorption and desorption of coalbed methane.The grey theory is used to calculate the correlation degree between each logging parameter and the development degree of coal seam fractures.The neural network is used to identify the development degree of coal reservoir fractures in the logging,and the effect is relatively ideal.(4)Through the establishment of a numerical model,the stress field distribution in the Liupanshui area during the Yanshan and Himalayan periods was simulated to quantitatively characterize the fractures using the Mohr Coulomb fracture criterion and the Griffith fracture criterion.The outcomes suggest that the impact of Yanshan movement in the Liupanshui area on creating fractures is significantly more substantial than that of the Himalayan period.