Study of the borehole hydraulic fracturing and the principle of gas seepage in the coal seam

Coal seam gas has become one of the important factors restricting the safe production of mine in China, so it is urgent to solve the methane problem existing in coal mines. Ground gas drainage method is an easy technology to reduce the gas content in coal seam effectively. However, in China, the coal seam generally possess some characteristics, such as low pressure, low permeability, low saturation, and so forth, which means the borehole hydraulic fracturing must be carried out to improve the permeability of coal seam. The study on the hydraulic fracturing in coal seam is still in an early stage, therefore, the initiation and propagation mechanism of the crack as well as the gas seepage in crack are needed to study deeply. In this paper, firstly, the initiation and propagation mechanism of the crack is theoretically analyzed; then, the principle of gas seepage in cracks and the hydraulic fracturing progress are analyzed synthetically through numerical simulation. The results show that three-dimensional model of fracture propagation for the hydraulic fracturing is established. The stress state of rock has a great effect on the crack propagation. When the two ground stress on horizontal plane is equal, the crack propagates randomly with bifurcation phenomenon; however, when this two stress are different, the direction of the initiation and propagation of the crack is vertical to the minimum main stress. The appropriate pattern shape of borehole is diamond, and the diagonal lines should separately parallel to the maximum principal stress and the minimum principal stress. After fracturing, the accumulative volume of gas production behaves a logarithmic relationship with time, while the volume of gas production represents a negative exponent relationship with time.